Selected Papers

This page contains information and links to some selected LPI papers, articles, and publications that have been written by the LPI Team. Some of these articles are available for purchase from the various publishers, and the link in the preview box may take you to the article on the publisher’s website.

XR-Köhler devices

CPV and illumination systems based on XR-Köhler devices”, Proc. SPIE 7785, Nonimaging Optics: Efficient Design for Illumination and Solar Concentration VII, 77850A (August 18, 2010); doi:10.1117/12.861968

IODC Illumination Design Problem

In response to a problem offered before the 2010 International Optical Design Conference (IODC) in Jackson Hole, Wyoming, USA, LPI Associate Scientist, Pablo Benitez and his PhD student Wang Lin presented this impressive solution. It implemented several state-of-the-art patented LPI technologies: SMS 2D XX optics, free-form grooved mirrors, and SMS 3D free-form microlens array optics with etendue-squeezing. One presenter commented that the etendue squeezing microlens was a “wonderful device” and the solution “combines some of the state-of-the-art work being done in nonimaging optics.”

High performance Fresnel-based photovoltaic concentrator

In order to achieve competitive system costs in mass-production, it is essential that CPV concentrators incorporate sufficient manufacturing tolerances. This paper presents an advanced concentrator optic comprising a Fresnel lens and a refractive secondary element, both with broken rotational symmetry, an optic producing both the desired light concentration with high tolerance (high acceptance angle) as well as an excellent light homogenization by Köhler integration. This concentrator compares well with conventional Fresnel-based CPV concentrators. This article was originally published in Optics Express Volume 18 Issue 9.

SMS Image FM 080808 final

The simultaneous multiple surface (SMS) method was first invented for nonimaging optics but this paper shows how it can be extended into imaging. The ease with which the SMS method calculates higher-order aspheric surfaces offers great advantages in imaging design. An ultra short throw distance video projection system is shown using two highly aspheric mirrors.

Solar concentration LPI overview

The presentation gives an overview about concentrating photovoltaics (CPV). Basic concepts like concentration and acceptance angle and the trade off between those two are introduced. The most important companies in the CPV field are listed and their concentrating systems analyzed.

Kohler XR photovoltaic Concentrator

An on-axis XR concept is combined with Kohler integrating optics. The design approach maintains high efficiency and acceptance angle and achieves excellent uniformity across the solar cell.

Free form XR photovoltaic Concentrator

A novel photovoltaic concentrator is presented, that combines high concentration and high efficiency that operates close to the theoretical acceptance angle limit. The system is based on 1 cm2 multi-junction solar cells and a so called XR concentrator, shown here in a freeform off-axis configuration. The large acceptance angle relaxes the manufacturing tolerances of optical and mechanical components of the system.

Köhler Integrators Embedded into Illumination Optics Add Functionality (2008)

This work reviews the Köhler concept and its application to different kind of optics, ranging from photovoltaic concentrators to automotive LED headlights. In the former, it is shown how irradiance peaks on the solar cell can be avoided, while preserving high aiming tolerance (acceptance) of the solar concentrator. In the latter, it is to shown how to overcome the drawbacks associated with large source image sizes, as well as source misalignments, ill defined source edges, and how low source radiance can be compensated for.

Optic basics (2008)

This document introduces basic optical concepts including: refraction and total internal reflection, intensity, illuminance and luminance, nonimaging optics and etendue.

LED Backlighting technology (2005)

A novel LCD backlight concept suitable for LEDs has been designed using the flow line method. This method does not rely on multiple stochastic scattering as conventional backlight but on controlled injection of light into a lightguide and controlled ejection to be emitted into narrow desired angular ranges with very high efficiencies.

Optical Design Considerations for Automotive Rear Lighting with LEDs

One of the most challenging applications for high brightness LEDs is in automotive headlights. Optical designs for a low or high beam headlights are plagued by the low flux and luminance of LEDs compared to HID or incandescent sources, by mechanical chip placement tolerances and by color and flux variations between different LEDs. Furthermore the creation of a sharp cutoff is very difficult without baffles or other lossy devices. Basic calculations for intensities and flux are presented. A novel LED headlight design is introduced and analyzed.

OFF-AXIS TIR Lens For Conformal Luminaires

A novel family of TIR lenses is shown that can be used to create collimating optics that follow the contour of exterior automotive lights. The lens geometry is a derivative of “normal” TIR lenses but it can produce an off-axis collimated beam from an off-center source.

SMS design method in 3D geometry: examples and applications (2003)

This presentation provides an overview of the SMS 3D design method. It introduces the concepts of wave front coupling, SMS chains, surface creation and wave front control in 3 dimensions. Some academic examples are investigated and the freeform RXI® in 3D is introduced.

SMS Design Examples in 2D: RX and RXI® for Optical Wireless Communications

This article contains examples of the SMS 2D method as well as basic calculations to determine the angular spread of the optics. Optical architectures like the CPC and the FLC are revised and RXI® and RX are introduced. Although focused on wireless communication, the same concepts are valid for solar concentration and illumination.

Experimental measurements of a prototype high concentration Fresnel lens CPV module for the harvesting of diffuse solar radiation (November 18, 2013)

Noboru Yamada and Kazuya Okamoto

A prototype concentrator photovoltaic (CPV) module with high solar concentration, an added low-cost solar cell, and an adjoining multi-junction solar cell is fabricated and experimentally demonstrated. In the present CPV module, the low cost solar cell captures diffuse solar radiation penetrating the concentrator lens and the multi-junction cell captures concentrated direct solar radiation. On-sun test results show that the electricity generated by a Fresnel lens-based CPV module with an additional crystalline silicon solar cell is greater than that for a conventional CPV module by a factor of 1.44 when the mean ratio of diffuse normal irradiation to global normal irradiation at the module aperture is 0.4. Several fundamental optical characteristics are presented for the present module.

Newer optics efficiently mix, dim, and color-tune LED light (February 25, 2013)

Juan C. Miñano, Pablo Benítez, Rubén Mohedano and Roberto Alvarez

Low-cost, plastic-injected optics mix light from different color LED dies without a significant decrease in average brightness, simplifying luminaire design both optically and electronically.

New generation of advanced optics for photovoltaic concentration (Spanish) (November 1, 2010)

Aleksandra Cvetkovic,PhD

The solar concentrating photovoltaic (CPV in the Anglo-Saxon terminology) has always been a promising field of action that would reduce the cost of photovoltaic energy. The high cost of solar cells can be decreased by the concentration technique, capable of producing at least the same power with fewer solar cells, and therefore potentially, at lower cost. (Text is in Spanish)

Aspheric V-groove reflector design with the SMS method in two dimensions (January 22, 2010)

Dejan Grabovickic, Pablo Benítez, and Juan C. Miñano

The Simultaneous Multiple Surface design method in two dimensions (SMS2D) is applied to the design of aspheric V-groove reflectors. The general design problem is to achieve perfect coupling of two wavefronts after two reflections at the groove, no matter which side of the groove the rays hit first. Two types of configurations are identified, and several symmetric and asymmetric design examples are given. Computer simulations with a commercial simulation package are also shown.

High performance concentrating photovoltaic module designs for utility scale power generation (August 24, 2009)

Pablo Benitez, Juan C. Minano, Aleksandra Cvetkovic, Maikel Hernandez, Oliver Dross and Roberto Alvarez et al

The Boeing Company Phantom Works has developed three different prototype photovoltaic concentrator arrays since March 2007. Identified as Prototype A, B and C, the experimentally proven technical characteristics of each design are presented. The concentrator designs utilize a 1 cm2 multi-junction solar cell assembly in conjunction with SMS non-imaging optical designs [1, 2] manufactured with low-cost mass-producible technologies. Prototype A is an on-axis XR optical concentrator with a 733x geometrical concentration demonstrating a ± 1.73° acceptance angle and 23.7% conversion efficiency. Prototype B is an off-axis free-form XR optical concentrator with a 810x geometrical concentration demonstrating a ± 1.32° acceptance angle and 25.3% conversion efficiency. Prototype C is the most recent off-axis free-form XR optical concentrator with a 801x geometrical concentration and a theoretical ±1.80° acceptance angle demonstrating a conversion efficiency greater than 27.0%. Prototype C is also the basis for the Boeing Proof of Design (POD) module, demonstrating an acceptance angle of ±1.48° and a conversion efficiency of 29.4% (as of May 8, 2009). Manufacturability has been paramount during the design process, resulting in high performance concentrating photovoltaic modules using production quality components.

Overview of the SMS design method applied to imaging optics (August 12, 2009)

J. C. Minano, Pablot Benitez, Wang Lin, Fernando Munoz, Jose Infante, and Asuncion Santamaria

The Simultaneous Multiple Surfaces (SMS) was developed as a design method in Nonimaging Optics during the 90s. Later, the method was extended for designing Imaging Optics. We present an overview of the method applied to imaging optics in planar (2D) geometry and compare the results with more classical designs based on achieving aplanatism of different orders. These classical designs are also viewed as particular cases of SMS designs. Systems with up to 4 aspheric surfaces are shown. The SMS design strategy is shown to perform always better than the classical design (in terms of image quality). Moreover, the SMS method is a direct method, i.e., it is not based in multi-parametric optimization techniques. This gives the SMS method an additional interest since it can be used for exploring solutions where the multiparameter techniques can get lost because of the multiple local minima.

Free form V-groove reflector design with the SMS method (August 20, 2009)

Dejan Grabovickic, Juan C. Minano, and Pablo Benitez

The Simultaneous Multiple Surface design method (SMS) is applied to the design of free-form V-groove reflectors. The general design problem is to achieve perfect coupling of two wavefronts after two reflections at the groove, no matter which side of the groove the rays hit first. All possible 2D designs are listed and used as a basis for the 3D free-form reflector analysis. The extension to the 3D case is given, and illustrative particular canonical examples are developed. Besides the theory and design work, first prototypes were produced of a mirrorless TIR device using V-grooves that emulates a parabolic reflector. The experimental measurements of the TIR reflector show reflectance up to 98%.

Design of a novel free-form condenser overcoming rotational symmetry limitations (September 27, 2008)

Juan C. Miñano; Pablo Benítez; José Blen; Asunción Santamaría

The importance of condenser optics is the fact that it is the bottleneck limiting efficiency in commercially available projection systems. Efficiency is a key parameter of projector performance, since it augments screen luminance, enabling the system to perform well under increasing levels of ambient light. Conventional condensers use rotational symmetric devices, most of them being elliptic or parabolic mirrors. They perform very far from the theoretical limits for sources such as arc lamps or halogen bulbs. Typical small displays in the 5-15 mm2 etendue range have geometrical efficiencies about 40-50% for the best condensers; although theory allows about 100% (no reflection nor absorption losses are considered). Two basic facts are underlying this effect: The coma aberration of the reflectors and the rotational symmetric image of the source making the source projected image to unfit with the target. Thus, the only way to improve this performance is to generate a free form design that is able to control the shape and rotation of the source projected images. As yet, this can only be done with the SMS3D design method. We present here one of such designs achieving a geometrical efficiency that is 1.8 times that of an elliptical condenser for a 4:1 target aspect ratio and for the range of target etendue with practical interest and 1.5 for 16:9 target. This design uses only 1 additional reflection, i.e., uses a total of 2 reflections from the source to the target. A prototype of this free form condenser has already been built.

High-efficiency free-form nonimaging condenser overcoming rotational symmetry limitations (September 27, 2008)

Juan C. Miñano; Pablo Benítez; José Blen; Asunción Santamaría

The importance of condenser optics is the fact that it is the bottleneck limiting efficiency in commercially available projection systems. Conventional condensers use rotational symmetric devices, most of them being elliptic or parabolic mirrors. They perform very far from the theoretical limits for sources such as arc lamps or halogen bulbs. Typical small displays in the 5-15 mm2 etendue range have geometrical efficiencies about 40-50% for the best condensers; although theory allows about 100% (no reflection nor absorption losses are considered). The problem is in the coma aberration of the reflectors and the rotational symmetric image of the source making the source projected image to unfit with the target. Thus, the only way to improve this performance is to generate a free form design that is able to control the shape and rotation of the source projected images. As yet, this can only be done with the SMS3D design method. We present here one of such designs achieving a collection efficiency 1.8 times that of an elliptical condenser for a 4:1 target aspect ratio and for the range of target etendue with practical interest and 1.5 for 16:9 target. These designs use only 1 additional reflection, i.e., use a total of 2 reflections from the source to the target. A prototype of one type of free form condenser has already been built.

High-order aspherics: the SMS nonimaging design method applied to imaging optics (September 27, 2008)

Fernando Muñoz; Pablo Benítez; Juan C. Miñano

The simultaneous multiple surface (SMS) method has been used to design nonimaging devices, such as solar concentrators and collimators, which work near the thermodynamic limit at highest efficiencies. The very high compactness of these devices is obtained through the simultaneous design of two high-order (above 30th) aspheric surfaces. In imaging optics, low-order aspheric surfaces were introduced to correct Seidel aberrations. The ease with which the SMS method calculates higher-order aspheric surfaces offers great advantages in imaging design. The SMS method can design N rotationally-symmetric surfaces that, by definition, form sharp images of N one-parameter subsets of rays. The design strategy consists in finding the best configuration of these subsets of rays in phase-space, one that ensures that image-quality specifications will be met by all non-design rays. As a first example of an SMS imaging device, a new video projection optics system is presented, featuring extremely short throw distance, high compactness and wide angle projection.

Köhler integrators embedded into illumination optics add functionality (September 24, 2008)

O. Dross; R. Mohedano; M. Hernández; A. Cvetkovic; J. C. Miñano; P. Benítez

The Köhler illumination concept was originally invented to achieve uniform illumination in microscopy1. Köhler integrators can also be formed by arrays of lenticulations that can be any combination of reflective and/or refractive surfaces, organized in corresponding pairs. Arrays of integrating facets can be arranged not only on flat surfaces but on rotationally symmetric and even freeform surfaces6. Currently flat lenslet arrays are widely applied as homogenizing optics2 for lithography, machine vision illumination, and projection. Adding Köhler facets onto already designed surfaces can improve the optical system performance, while respecting its original function. In general, the optics output can be made somewhat independent of the source characteristics, although at the expense of a slight ètendue dilution or efficiency losses. This work revises the Köhler concept and its application to different kind of optics, ranging from photovoltaic concentrators to automotive LED headlights. In the former, irradiance peaks on the solar cell can be avoided, while preserving high aiming tolerance (acceptance) of the solar concentrator. In the latter, LEDs drawbacks like large source image sizes, source misalignments, ill defined source edges, and low source radiance can be compensated.

High-order aspherics: the SMS nonimaging design method applied to imaging optics (September 17, 2008)

Fernando Muñoz; Pablo Benítez; Juan C. Miñano

The simultaneous multiple surface (SMS) method has been used to design nonimaging devices, such as solar concentrators and collimators, which work near the thermodynamic limit at highest efficiencies. The very high compactness of these devices is obtained through the simultaneous design of two high-order (above 30th) aspheric surfaces. In imaging optics, low-order aspheric surfaces were introduced to correct Seidel aberrations. The ease with which the SMS method calculates higher-order aspheric surfaces offers great advantages in imaging design. The SMS method can design N rotationally-symmetric surfaces that, by definition, form sharp images of N one-parameter subsets of rays. The design strategy consists in finding the best configuration of these subsets of rays in phase-space, one that ensures that image-quality specifications will be met by all non-design rays. As a first example of an SMS imaging device, a new video projection optics system is presented, featuring extremely short throw distance, high compactness and wide angle projection.

Design of a novel free-form condenser overcoming rotational symmetry limitations (September 11, 2008)

Juan C. Miñano; Pablo Benítez; José Blen; Asunción Santamaría

The importance of condenser optics is the fact that it is the bottleneck limiting efficiency in commercially available projection systems. Efficiency is a key parameter of projector performance, since it augments screen luminance, enabling the system to perform well under increasing levels of ambient light. Conventional condensers use rotational symmetric devices, most of them being elliptic or parabolic mirrors. They perform very far from the theoretical limits for sources such as arc lamps or halogen bulbs. Typical small displays in the 5-15 mm2 etendue range have geometrical efficiencies about 40-50% for the best condensers; although theory allows about 100% (no reflection nor absorption losses are considered). Two basic facts are underlying this effect: The coma aberration of the reflectors and the rotational symmetric image of the source making the source projected image to unfit with the target. Thus, the only way to improve this performance is to generate a free form design that is able to control the shape and rotation of the source projected images. As yet, this can only be done with the SMS3D design method. We present here one of such designs achieving a geometrical efficiency that is 1.8 times that of an elliptical condenser for a 4:1 target aspect ratio and for the range of target etendue with practical interest and 1.5 for 16:9 target. This design uses only 1 additional reflection, i.e., uses a total of 2 reflections from the source to the target. A prototype of this free form condenser has already been built.

The free form XR photovoltaic concentrator: a high performance SMS3D design (September 9, 2008)

Aleksandra Cvetkovic; Maikel Hernandez; Pablo Benítez; Juan C. Miñano; Joel Schwartz; Adam Plesniak; Russ Jones; David Whelan

A novel photovoltaic concentrator is presented. The goal is to achieve high concentration design with high efficiency and high acceptance angle that in the same time is compact and convenient for thermal and mechanical management. This photovoltaic system is based on 1 cm2 multi-junction tandem solar cells and an XR concentrator. The XR concentrator in this system is an SMS 3D design formed by one reflective (X) and one refractive (R) free-form surfaces (i.e., without rotational or linear symmetry) and has been chosen for its excellent aspect ratio and for its ability to perform near the thermodynamic limit. It is a mirror-lens device that has no shadowing elements and has square entry aperture (the whole system aperture area is used for collecting light). This large acceptance angle relaxes the manufacturing tolerances of all the optical and mechanical components of the system included the concentrator itself and is one of the keys to get a cost competitive photovoltaic generator. For the geometrical concentration of 1000x the simulation results show the acceptance angle of ±1.8 deg. The irradiance distribution on the cell is achieved with ultra-short homogenizing prism, whose size is optimised to keep the maximum values under the ones that the cell can accept.

The SMS3D photovoltaic concentrator (September 9, 2008)

Aleksandra Cvetkovic; Maikel Hernandez; Pablo Benítez; Juan Carlos Miñano; Joel Schwartz; Adam Plesniak; Russ Jones; David Whelan

A novel photovoltaic concentrator is presented. The goal is to achieve high concentration design with high efficiency and high acceptance angle that in the same time is compact and convenient for thermal and mechanical management [1]. This photovoltaic system is based on 1 cm2 multi-junction tandem solar cells and an XR concentrator. The XR concentrator in this system is an SMS 3D design formed by one reflective (X) and one refractive (R) free-form surfaces (i.e., without rotational or linear symmetry) and has been chosen for its excellent aspect ratio and for its ability to perform near the thermodynamic limit. It is a mirror-lens device that has no shadowing elements and has square entry aperture (the whole system aperture area is used for collecting light). This large acceptance angle relaxes the manufacturing tolerances of all the optical and mechanical components of the system included the concentrator itself and is one of the keys to get a cost competitive photovoltaic generator. For the geometrical concentration of 1000x the simulation results show the acceptance angle of ±1.8 deg. The irradiance distribution on the cell is achieved with ultra-short homogenizing prism, whose size is optimised to keep the maximum values under the ones that the cell can accept. The application of the XR optics to high-concentration is being developed in a consortium leaded by The Boeing Company, which has been awarded a project by US DOE in the framework of the Solar America Initiative.

Increased brightness by light recirculation through an LED source (September 2, 2008)

Julio C. Chaves; Waqidi Falicoff; Bill Parkyn; Pablo Benítez; Juan C. Miñano

LED light sources are finding ever increasing application in illumination. LEDs have many advantages, such as high efficiency, long life, compactness, directional light emission, mechanical resistance, low-temperature operation, light color control and low UV or IR emissions. These and other advantages make them very well suited for general illumination applications as well as flashlights, car headlights, backlights, or frontlights. In most applications, LEDs are combined with optics to direct their light output. Brighter LEDs have a smaller emission area and, therefore, may be coupled to smaller optics. This is very important in many applications where compactness is crucial, particularly automotive headlamps. When LED brightness is insufficient, it can be augmented by recirculating part of the emitted light back to the LED’s emitting surface. This increase in brightness comes at the expense of a reduced flux-emission. As an example, the brightness of an LED with a diffuse reflectivity of 70% may be increased by nearly that much if it is coupled to a high-efficiency recirculating optic. Such augmentation, however, comes at the expense of a flux reduction, as much as 50%. Several optical geometries are explored in this paper to achieve that recirculation together with raytracing results using on a simple model of an LED. Also a number of optical architectures will be shown that escape the classical nonimaging etendue limit associated with traditional optics.

High-performance Kohler concentrators with uniform irradiance on solar cell (September 2, 2008)

Maikel Hernández; Aleksandra Cvetkovic; Pablo Benítez; Juan C. Miñano

A new free-form XR Kohler concentrator is presented that combines high geometric concentration, high acceptance angle and high irradiance uniformity on the solar cell. This is achieved by modifying the optical surfaces to produce Kohler integration. Although the new optical surfaces (that is, the ones including Kohler integration) behave optically quite different from the ones that do not integrate, but from the macroscopic point of view they are very similar to them. This means that they can be manufactured with the same techniques (typically plastic injection molding or glass molding) and that their production cost is the same i.e., with a high potential for low cost and high optical efficiency. The present approach is completely new and allows keeping the acceptance angle at high values and the concentration factor without increasing the number of optical elements. The simulated optical performance of a Kohler integrating solar concentrator is presented. This concept is the first design combining non flat array of Kohler integrators with concentration optics.

High-efficiency free-form nonimaging condenser overcoming rotational symmetry limitations (August 27, 2008)

Juan C. Miñano; Pablo Benítez; José Blen; Asunción Santamaría

The importance of condenser optics is the fact that it is the bottleneck limiting efficiency in commercially available projection systems. Efficiency is a key parameter of projector performance, since it augments screen luminance, enabling the system to perform well under increasing levels of ambient light. Conventional condensers use rotational symmetric devices, most of them being elliptic or parabolic mirrors. They perform very far from the theoretical limits for sources such as arc lamps or halogen bulbs. Typical small displays in the 5-15 mm2 etendue range have geometrical efficiencies about 40-50% for the best condensers; although theory allows about 100% (no reflection nor absorption losses are considered). The problem is in the coma aberration of the reflectors and the rotational symmetric image of the source making the source projected image to unfit with the target. Thus, the only way to improve this performance is to generate a free form design that is able to control the shape and rotation of the source projected images. As yet, this can only be done with the SMS3D design method. We present here two of such designs types; one of them achieving a geometrical efficiency that is 1.8 times that of an elliptical condenser for a 4:1 target aspect ratio and for the range of target etendue with practical interest and 1.5 for 16:9 target. The other design type is more adequate for circular targets and gets a geometrical efficiency up to 1.4 that of an elliptical condenser. These designs use only 1 additional reflection, i.e., use a total of 2 reflections from the source to the target. A prototype of one type of free form condenser has already been built.

The XR nonimaging photovoltaic concentrator (September 21, 2007)

M. Hernández; P. Benítez; J. C. Miñano; A. Cvetkovic; R. Mohedano; O. Dross; R. Jones; D. Whelan; G. S. Kinsey; R. Alvarez

The performance of the XR solar concentrator, using a high efficiency multi-junction solar cell developed recently by Spectrolab, is presented. The XR concentrator is an ultra-compact Nonimaging optical design composed of a primary mirror and a secondary lens, which can perform close to the thermodynamic limit of concentration (maximum acceptance angle for a given geometrical concentration). The expected acceptance angle of the concentrator is about ±2 deg for a geometrical concentration of 800x (a Fresnel lens and secondary system typically has ±0.6 deg of acceptance for 300x of geometrical concentration).

XR: a high-performance PV concentrator (September 11, 2007)

M. Hernández; P. Benítez; J. C. Miñano; A. Cvetkovic; R. Mohedano; O. Dross; R. Jones; D. Whelan; G. S. Kinsey; R. Alvarez

The performance of the XR solar concentrator, using a high efficiency multi-junction solar cell developed recently by Spectrolab, is presented. The XR concentrator is an ultra-compact Nonimaging optical design composed of a primary mirror and a secondary lens, which can perform close to the thermodynamic limit of concentration (maximum acceptance angle for a given geometrical concentration). The expected acceptance angle of the concentrator is about ±2 deg for a geometrical concentration of 800x (a Fresnel lens and secondary system typically has ±0.6 deg of acceptance for 300x of geometrical concentration).

Geodesic lenses applied to nonimaging optics (September 11, 2006)

Juan C. Miñano, Pablo Benítez, Bill Parkyn, Dejan Grabovickic, Fernando García, José Blen, Asunción Santamaría, Julio Chaves, and Waqidi Falicoff

A novel waveguide-optical integrator is introduced for applications to LEDs. The concept is based upon a Kohler illuminator made of Luneburg lenses. Typical Kohler illuminators are formed by pairs of thin lenses, and perform badly when the paraxial approximation is rough, i.e., when the angular span of the incoming rays is wide. In contrast, the new illuminator performs ideally for angular spans up to 90o (±45o), and has only a 3% loss for a 180o angular span. In general such an illuminator cannot be made in 3D, because adjacent Luneburg lenses overlap. It can, however, be implemented in planar optics, by using Rinehart geodesic lenses, which moreover do not use gradient index material. This waveguide device has application in illumination engineering as a light mixer, particularly for LEDs. Another light mixer using a combination of two kaleidoscopes with a geodesic lens is also presented. Irradiance at the exit of a kaleidoscope has good light mixing if the kaleidoscope is long enough, but the intensity is never well mixed, irrespective of the length. Inserting a Rinehart geodesic lens produces a 90-degree phase-space rotation of the rays, i.e., it exchanges irradiance and intensity. A further kaleidoscope assures complete mixing in both irradiance and intensity.

A superior architecture of brightness enhancement for display backlighting (September 11, 2006)

Oliver Dross, William A Parkyn, Julio Chaves, Waqidi Falicoff, Juan C. Minano, Pablo Benitez & Roberto Alvarez

Brightness enhancement of backlighting for displays is typically achieved via crossed micro prismatic films that are introduced between a backlight unit and a transmissive (LCD) display. This design offers no free parameters to influence the resulting light distribution and suffers from insufficient stray light control. A novel strategy of light recycling is introduced, using a microlens array in conjunction with a hole array in a reflective surface, that can provide higher luminance, superior stray light control, and can be designed to meet almost any desired emission pattern. Similar strategies can be applied to mix light from different colored LEDs being mounted upside down to shine into a backlight unit.

Luminance enhancement of LED light sources for etendue-limited applications (September 11, 2006)

Oliver Dross, William A Parkyn, Julio Chaves, Waqidi Falicoff, Juan C. Minano, % Pablo Benitez

Many existing approaches to increase source luminance achieve it at the expense of a considerable drop in efficacy. In this paper we present four novel and different ways (patent pending) to considerably enhance LED luminance and offering the possibility of having relatively high efficacy.

Combination of light sources and light distribution using manifold optics (September 11, 2006)

Oliver Dross, William A Parkyn, Julio Chaves, Waqidi Falicoff, Juan C. Minano, % Pablo Benitez

Light from several LEDs or other light sources may be combined using light guides shaped as manifolds. These manifolds are composed of smaller elements such as CPCs, angle transformers, angle rotators, light shifters, light guides or others. Although some components, such as CPCs or angle transformers, have all-optical surfaces, other devices may be designed with non-optical surfaces. These may be used to place the injection gate in the case of injection-molded optics, to attach handles or holders and other non-optical components to the manifold without affecting the optical performance. Also, in some of these devices, the geometry can be changed by simple changes in the position of the curves that compose the optic profile. These optics may be applied in efficiently combining light from several LEDs into one single large source, changing the aspect ratio of a light source or in distributing light from one (or more) sources onto several targets.

Design and modeling of a measuring device for a TIR-R concentrator (September 8, 2006)

Daniel Perez Calero, Juan Carlos Minano, Pablo Benitez, Maikel Hernandez, and Aleksandra Cvetkovic

One of the most usual procedures to measure a concentrator optical efficiency is by direct comparison between the photocurrent generated by the compound concentrator/solar cell and photocurrent that single cell would generate under identical radiation conditions. Unfortunately, such procedure can give a good idea of the generator final performance, but can not indicate the real amount of radiation that will impinge over the cell. This apparent contradiction is based on the fact that once the cell is coupled with the concentrator, rays incidence is not perpendicular, but highly oblique, with an angle that can reach 70o or even greater for high concentration devices. The antireflective coating of the cell does not perform well enough for the whole incidence angle and frequency ranges because low cost is other important requirement for the solar cells. In consequence, the generated photocurrent drops for large incidence angles. In our case, a 70% incidence angle could, in the worst case, mean a 34% loss on generated photocurrent. With the aim of correcting such problem a special device has been designed in the framework of a EU funded project called HAMLET. The concept of the device is to substitute the concentrator receptor by a system formed by an optical collimator that would reduce concentration and incidence angle, and a characterized solar cell. The paper gives the results of this measuring procedure.

Geodesic lens: new designs for illumination engineering (August 3, 2006)

Juan C. Miñano, Pablo Benítez, Bill Parkyn, Dejan Grabovickic, Fernando García, Asunción Santamaría, José Blen, Julio Chaves, & Waqidi Falicoff

A novel waveguide-optical integrator is introduced for applications to LEDs. The concept is based upon a Kohler illuminator made of Luneburg lenses. Typical Kohler illuminators are formed by pairs of thin lenses, and perform badly when the paraxial approximation is rough, i.e., when the angular span of the incoming rays is wide. In contrast, the new illuminator performs ideally for angular spans up to 90° (±45°), and has only a 3% loss for a 180° angular span. In general such an illuminator cannot be made in 3D, because adjacent Luneburg lenses overlap. It can, however, be implemented in planar optics, by using Rinehart geodesic lenses. This waveguide device has application in illumination engineering as a light mixer, particularly for LEDs.

The past, present, and future of optical design (August 3, 2006)

Warren J. Smith; Ellis Betensky; David Williamson; Juan C. Miñano; R. John Koshel

The field of optical design has a rich history and a rewarding future. Four leaders in the field will start the International Optical Design Conference with a plenary panel to discuss the history, the current trends, and what the outlook is for this exciting field. Topics will range from lens design in previous years to zoom lens to microlithography to free-form surfaces for illumination to imaging applications.

Etendue preserving mixing and projection optics for high brightness LEDs applied to automotive headlamps (July 18, 2006)

Aleksandra Cvetkovic, Oliver Dross, Julio Chaves, Pablo Benítez, Juan C. Miñano, & Rubén Mohedano

A novel LED light extraction and mixing optic and two free form SMS surfaces are employed in a high efficiency projection optic. By combining the light of several high brightness LEDs with a single optical element, an ultrabright light source can be formed, whose shape and emission characteristics can be adapted to almost many kinds of illumination problems. A LED combiner forms a virtual source that is tailored for application. The illuminance distribution of this virtual source facilitates the generation of the desired intensity pattern by projecting it into the far field. The projection is accomplished by one refractive and one reflective freeform surface calculated by the 3D SMS method. The method is demonstrated for an LED automotive headlamp. A high quality intensity pattern shape and a very sharp cutoff are created tolerant to LED to optics misalignment and illuminance variations across the LED surface. A low and high beam design with more than 75% total optical efficiency have been achieved.

Design, manufacturing and measurement of a PV miniconcentrator for front point-contact silicon solar cells (September 3, 2005)

D. Pérez; J. C. Miñano; P. Benítez; F. Muñoz; R. Mohedano

A novel photovoltaic concentrator has been developed in the framework of the European project “High efficiency silicon solar cells concentrator”. In this project, front-contacted silicon solar cell have also been designed and manufactured by the project leader (the French LETI). This silicon cell concept is potentially capable to perform well (24% efficiency has been predicted) for much higher concentration levels than the back-contacted cells (and, of course, than the two-side contacted cells). The concentrator is formed by one lens of squared contour with flat entry surface and large-facet Fresnel exit surface, and a secondary that encapsulates the solar cell. On the contrary to the conventional Fresnel lens plus nonimaging secondary concentrators, the primary and secondary are designed simultaneously, leading to better concentration-acceptance angle product without compromise with the compactness. The grid lines in the front-contacted cells are aluminium prisms (which contact the p+ and n+ emitters, alternatively), acting as a linear cone concentrator that concentrates Cg =1.52× in the cross sectional dimension of the prisms. The secondary concentrator has a refractive rotational symmetric top surface that is crossed with two linear flow-line TIR mirror. Then, in the cross section normal to the aluminium prisms, the secondary provides a 2D concentration of Cg =12×, while in the cross section parallel to the prisms it provides a 2D concentration of Cg =24.16× as the grid lines in this dimension. Therefore, the cell is rectangular (1:2.08 aspect ratio), being the grid lines parallel to the shorter rectangle side. The total 3D geometrical concentration is 24.16×(12×1.52) = 455× for the square aperture and rectangular cell, and gets a design acceptance angle α=±1.8 degrees. Injection moulded prototypes are have been manufactured and measured, proving an optical efficiency of 79%. Computer modelling of the concentrator performance will also be presented.

Free-form integrator array optics (September 2, 2005)

J. C. Miñano; M. Hernández; P. Benítez; J. Blen; O. Dross; R. Mohedano; A. Santamaría

A new design method of free-form Kohler integrator array optics is presented. Only few solutions to the integrator design problem are known, which apply for specific and simple source and targets (for instance, flat integrator lenslet arrays when the source and target are squares located at infinity). The method presented here find more general solutions and the resulting optics is formed by two arrays of free-form optical surfaces (which can be either reflective of refractive). The contour curves of the array units are also obtained from the design. Two types of Kholer integrators will be defined, depending if they integrate only along one direction across the source (one-directional integrators) or in the two directions (two-directional integrators). This design method has been applied for an ultra-compact high efficiency LED low beam head lamp producing a legal pattern independently of the chip luminance variation and permitting LED position tolerances of ±200 microns. The ray tracing proves that the high gradient (0.32) and its vertical position in the pattern remain invariable when chip is moved.

LED headlight architecture that creates a high quality beam pattern independent of LED shortcomings (August 22, 2005)

Oliver Dross; Aleksandra Cvetkovic; Julio Chaves; Pablo Benitez; Juan C. Minano

One of the most challenging applications for high brightness LEDs is in automotive headlights. Optical designs for a low or high beam headlights are plagued by the low flux and luminance of LEDs compared to HID or incandescent sources, by mechanical chip placement tolerances and by color and flux variations between different LEDs. Furthermore the creation of a sharp cutoff is very difficult without baffles or other lossy devices. We present a novel LED headlight design that addresses all of the above problems by mixing the light of several LEDs first in a tailored light guide called LED combiner, thereby reducing color and flux variations between different LEDs and illuminance and color variations across the LED surfaces. The LED combiner forms a virtual source tailored to the application. The illuminance distribution of this virtual source facilitates the generation of the desired intensity pattern by projecting it into the far field. The projection is accomplished by one refractive and one reflective freeform surface calculated by the 3D SMS method. A high quality intensity pattern shape and a very sharp cutoff are created tolerant to LED to optics misalignment and illuminance variations across the LED surface. A low and high beam design with more than 75% total optical efficiency (without cover lens) and performance as latest HID headlights have been achieved. Furthermore it is shown that the architecture has similar tolerance requirements as conventional mass produced headlights.

Afocal video-pixel lens for tricolor LEDs (August 20, 2005)

Julio Chaves; Juan C. Minano; Pablo Benitez

An afocal system keeps parallel any two parallel rays emitted by a pair of point sources. If they have the same intensity at identical angles, this design will ensure that the far-field intensity will be higher than that of the bare sources as well as giving same the intensity to both, so that color balance is preserved. If these sources are of different color, it is possible to mix them in the far field while increasing the intensity in a prescribed way. If a third LED is placed at the midpoint between the other two, its intensity pattern will still be close to the one created for the other two souces. This enables the pixel to color-mix red, green and blue LEDs while increasing their apparent intensity and preserving the color-balance of the video signal across the far field. This enables large active-video screens to redirect and intensify their light towards the audience, instead of just spreading it out uselessly.

High-efficiency LED backlight optics designed with the flow-line method (August 20, 2005)

Juan C. Minano; Pablo Benitez; Julio Chaves; Maikel Hernandez; Oliver Dross; Asuncion Santamaria

A novel backlight concept suitable for LED’s has been designed using the flow-line design method, which allows controlling both the illumination uniformity and light extraction without scattering the light. This contrasts with conventional LED backlight optical designs, which are based on the use of a light guide with Lambertian scattering features that break the guidance and extract the light. Since most of Lambertian scattered light is re-guided, the average ray path in conventional backlights is long and multiple bounces are needed, which may lead to low efficiency. On the other hand, the new design presented here is not only efficient but also provide a relatively high collimation of the output beam (an output beam within a 10 degrees half-angle cone, with total theoretical efficiency over 80% including Fresnel and absorption losses). Wider beams can be controlled by design or obtained by adding a holographic diffuser at the exit. The new design offers other very interesting practical features: it can be very thin, can be made transparent (which widens its applications, including front lighting), can mix the colors from several LED’s or recover reflected polarization for LCD illumination.

Offence against the Edge Ray Theorem? (November 29, 2004)

Pablo Benitez; Juan Carlos Minano

The purpose of this paper is to present an overview of the Edge-Ray Theorem in 2D geometry, covering the different optical systems treated up today, including some cases (as the refraction/reflection of sequential optical surfaces) which have not been formally discussed previously, and also analyzing the role of slope discontinuities in the creation/annihilation on edge rays. Also illustrative novel examples are given. In section 2, a simple device that seems to beat either the Edge Ray Theorem or the Second Law of Thermodynamics is presented. In section 6, it is proven that there exits perfect solution to the theoretical problem of achieving maximum concentration on a circular receiver from a source at infinity with a single slope discontinuity and a sizeable gap between optics and receiver. At the end (section 7), the explanation of the device of section 2 offending the Edge Ray Theorem will be given.

On the analysis of microstructured surfaces (September 29, 2004)

Pablo Benitez; Juan Carlos Minano; Maikel Hernandez

The study of general microstructures in 2D geometry and rotational 3D microstructures is presented. The study is based on infinitesimal microstructures for some calculations and the macro-profile of the surface can be treated as a new type of optical surface with a certain deflection law, which will be different of the reflection law or the Snell law. In two dimensions, we discuss the propagation of wavefronts by general microstructured surfaces (which do not fulfill the Fermat principle) and the discontinuity of the eikonal function at the microstructure. Naturally, a classification of the microstructures is obtained (regular and anomalous) and the concept of 2D ideal microstructures is also introduced, as those that perfectly couple two macroscopic extended bundles in 2D geometry. In 3D, after classifying the rotational optical systems into point-spot and ring-spot types, the first-order properties of both regular and anomalous rotational microstructured surfaces are discussed. Finally, an application of anomalous rotational microstructured surfaces to the problem of mixing the light from three RGB LED chips is introduced.

Fermat’s principle and conservation of 2D etendue (September 29, 2004)

Juan Carlos Minano; Pablo Benitez

Application of the Stokes theorem to the conservation of the 2D etendue of a one-parameter bundle of rays leads to the Lagrange’s integral invariant, one consequence of which establishes that the integral &sh;p∙dx between any two points is independent of the path of integration (p is the ray vector field and x is the vector position), and more generally, the integral &sh;p∙dx between two wavefronts is independent of the path of integration. This integral is called the optical path length. This is another way to see Fermat’s principle. The conservation of 2D etendue is a property of any Hamiltonian system. Using the Hamiltonian formulation, there is no difference between the configuration variables x and their conjugates p. Thus an integral invariant &sh;x∙dp can also be established similar to the Lagrange invariant. We show how its application to simple cases leads to Cartesian-oval designs through an unconventional method. The 2D etendue conservation is connected with Levi-Civita’s anormalita function and with the ray equation. In this connection we found that the equation p×(∇×p)=0 suffices for a vector field to be a ray vector field.

Review of SMS design methods and real-world applications (September 29, 2004)

Oliver Dross; Ruben Mohedano; Pablo Benitez; Juan Carlos Minano; Julio Chaves; Jose Blen; Maikel Hernandez; Fernando Munoz

The Simultaneous Multiple Surfaces design method (SMS), proprietary technology of Light Prescription Innovators (LPI), was developed in the early 1990’s as a two dimensional method. The first embodiments had either linear or rotational symmetry and found applications in photovoltaic concentrators, illumination optics and optical communications. SMS designed devices perform close to the thermodynamic limit and are compact and simple; features that are especially beneficial in applications with today’s high brightness LEDs. The method was extended to 3D “free form” geometries in 1999 that perfectly couple two incoming with two outgoing wavefronts. SMS 3D controls the light emitted by an extended light source much better than single free form surface designs, while reaching very high efficiencies. This has enabled the SMS method to be applied to automotive head lamps, one of the toughest lighting tasks in any application, where high efficiency and small size are required. This article will briefly review the characteristics of both the 2D and 3D methods and will present novel optical solutions that have been developed and manufactured to meet real world problems. These include various ultra compact LED collimators, solar concentrators and highly efficient LED low and high beam headlamp designs.

Virtual filaments that mimic conventional light bulb filaments (September 29, 2004)

Julio Chaves; Fernando Munoz; Juan Carlos Minano; Pablo Benitez; Bill Parkyn; Waqidi Falicoff; Yupin Sun

Conventional incandescent light bulbs have a wire filament acting as an extended light source with nearly constant intensity throughout its quasi-spherical emission pattern. Here we present a novel family of optical devices that make use of commercially available Lambertian or near-Lambertian LED light sources, in conjunction with tailored optical element bonded to the top surface of the LED. These hybrid devices can emulate the output of traditional incandescent filaments, or can be designed to produce a wide range of light output beam patterns. We call these new devices Virtual Filaments, as they can be designed to appear the same as an incandescent filament, with a similar light output pattern, and having a similar focal position above the base. These new lamps can then be used in the same applications as those they replace, thus eliminating the need to redesign or replace the original luminaire. We present several possible optical designs that can be used with a number of standard LEDs to replace standard incandescent bulbs. In one example we show a design that provides an output with near-uniform intensity across a full beam angle of 300 degrees, from a focal position 20 mm above an LED. Other major advantages of these new devices include their ability to be given sharp cutoffs, to homogenize non-uniform LED light sources and to color-mix the output of RGB LEDs.

Simultaneous multiple surface optical design method in three dimensions (July 1, 2004)

Pablo Gimenez-Benitez; Juan Carlos Miñano; Jose Blen; Rubén Mohedano Arroyo; Julio Chaves; Oliver Dross; Maikel Hernandez; Waqidi Falicoff

The simultaneous multiple surface (SMS) method in 3-D geometry is presented. Given two orthotomic input ray bundles and another two orthotomic output ray bundles, the method provides an optical system with two free-form surfaces that deflects the rays of the input bundles into the rays of the corresponding output bundles and vice versa. In nonimaging applications, the method enables controlling the light emitted by an extended light source much better than single free-form-surface designs, and also enables the optics contour to be shaped without efficiency losses. The method is also expected to find applications in imaging optics.

Simultaneous multiple surface design of compact air-gap collimators for light-emitting diodes (July 1, 2004)

Fernando Munoz; Pablo Gimenez-Benitez; Oliver Dross; Juan Carlos Miñano; William A. Parkyn

The simultaneous multiple surface (SMS) method is used to design air-gap RXI-type lenses, which efficiently produce a very narrow beam from high-powered LEDs. These designs are compact, with both front surface and reflecting back surface calculated simultaneously from periphery inward, as profiles of circular symmetry, via applying the edge-ray principle to the chip geometry. The light source is a Lambertian-glowing cube 1.2 mm square and 0.15 mm high, as viewed through its clear (n = 1.54) package dome, with emission down to 95 deg from the symmetry axis. A given exit-aperture diameter defines a minimum, étendue-limited collimation angle, α = arc sin (chip-width/diameter). At the center of the back surface there is a cavity surrounding the source. The front surface reflects that source flux to the back surface, which reflects it back forward again, accomplishing the optical folding thereby. The back surface is shaped so that the light it reflects forward will be refracted out the front surface to become the collimated output beam.

SMS design method in 3D geometry: examples and applications (January 8, 2004)

Pablo Benitez; Juan C. Minano; Jose Blen; Ruben Mohedano; Julio Chaves; Oliver Dross; Maikel Hernandez; Jose L. Alvarez; Waqidi Falicoff

The Simultaneous Multiple Surface (SMS) method in 3D geometry is presented. Giving two orthotomic input ray bundles and other two orthotomic output ray bundles, the method provides an optical system with two free-form surfaces that deflects the rays of the input bundles into the rays of the corresponding output bundles and vice versa. In nonimaging applications, the method allows controlling the light emitted by an extended light source much better than single free-form surfaces designs, and also enables the optics contour to be shaped without efficiency losses. The method is expected to find also applications in imaging optics.

Edge-ray design of compact etendue-limited folded-optic collimators (January 8, 2004)

William A. Parkyn; Fernando Munoz; Juan C. Minano; Pablo Benitez

The air-gap RXI lens efficiently produces a very narrow beam from high-powered LEDs. This design has an aspect ratio of 5, with both front surface and reflecting back surface designed simultaneously from periphery inwards, as profiles of circular symmetry, via applying the edge-ray principle to the chip geometry. The light source is a Lambertian-glowing cube 1.2 mm square and 0.15 mm high, as viewed through its clear (n=1.54) package dome, with emission down to 95° from the symmetry axis. A given exit-aperture diameter defines a minimum, etendue-limited collimation angle, α=arcsin(chip-width/diameter). At the center of the back surface is a tailored quasi-hemispheric cavity surrounding the source and serving to uniformly distribute the source flux over the front surface. The front surface reflects that source flux to the back surface, which reflects it back forward again, accomplishing the optical folding thereby. The back surface is shaped so that the light it reflects forward will be refracted out the front surface to become the collimated output beam.

Polarization-controlled luminaires utilizing multiple Fresnel reflections (November 12, 2003)

Waqidi Falicoff; Juan C. Minano; H. John Caulfield; Roberto Alvarez

A novel luminaire utilizes repeated Fresnel reflections by angled surfaces to transform a small collimated input beam into a controlled output pattern with a high degree of polarization, either linear or radial. Applications to backlighting, front-lighting, optical communications and automotive lighting will be discussed.

Off-axis TIR lens for conformal luminaires (November 12, 2003)

William A. Parkyn; Waqidi Falicoff; Juan C. Minano; Pablo Benitez; Julio Chavez; Yupin Sun

For reasons both fluid-dynamic and stylistic, volumetric constraints on vehicular luminaires grow more exacting. For full design-freedom of luminaire placement and shape, new designs are needed that have shallow depth and are capable of emitting a beam that makes a net angle with the local surface normal. Automotive headlamps, fog-lamps, and daylight-running lamps may need to project their illumination patterns onto the road from a position on sloped front surfaces. A conventional paraboloid, however, must be recessed behind a sloped window, thus using up space inside the vehicle-skin. A conventional TIR lens, with its output beam centered on its axis of circular symmetry, will also have to intrude into the vehicle interior, and shine through a sloped window. Instead, the luminaire should be thin enough to mount on a vehicle’s skin without needing a hole to be cut into it, a luminaire also capable of emitting its beam substantially off the local normal. To this end, two new TIR lenses are introduced here that generate off-normal beams. In one, a circular TIR lens takes on an internal tilt of its symmetry axis to produce a collimated output beam with high tilt, nearly 45° from the surface normal of the lens exterior. In the other, an off-axis linear TIR lens can be made with an internal tilt to the reflected rays. When used with LEDs, this new linear lens can be combined with exterior transverse lenslets, tailored to meet an intensity prescription.

Eye-safe collimated laser emitter for optical wireless communications (December 6, 2002)

Pablo Benitez; Juan C. Minano; Francisco J. Lopez; Dioen Biosca; Ruben Mohedano; Mabel Labrador; Fernando Munoz; Kazutoshi Hirohashi; Masahisa Sakai

In this work we present laser-based novel devices that maximize the emitted power for constant eye safety level and beam divergence angle, i.e., without affecting the eye safety classification or the necessary tracking accuracy. This is achieved by breaking the spatial coherence of the beam, which allows the system to be considered as an extended light source. The system comprises a laser, a diffuser, a collimator and, sometimes, other optical elements. As an example, one of the devices is composed of a laser, a Lambertian reflective-type diffuser, and a single-piece reflective-refractive collimator of 20 mm aperture and ultra-high numerical aperture (NA = 1.43), which re-collimates the radiation into 3.5 deg. (full angle). According to the IEC 60825-1:1993 (amendment 2, 2001-01), the Accessible Emission Limit (AEL) (Class 1, wavelength λ = 780 nm, exposure T = 30000 s) for this device is 35.9 times greater than that of a laser with the same divergence angle (15.6 dB), i.e., this device is allowed to emit 35.9 times more power than that of the laser alone with the same divergence angle. The switching time, the beam divergence and the eye safety classification remain the same. This power gain varies with the design conditions. In the cases analyzed it goes from = 8.4 (9.24dB) to 551.3 (27.4 dB).

Elliptic bundles in homogeneous refractive index media: toward the general solution (November 19, 2001)

Pablo Benitez; Juan C. Minano; Manuel Gutierrez

The first 3D ideal concentrators that were found composed solely of mirrors had the property of transmitting elliptic bundles. These are the flow line concentrator (FLC) and the cone concentrator (CC), designed in the late 1970s. More recently, the Lorentz geometry formalism was applied to the problem of finding elliptic bundles in a medium of homogeneous refractive index. In this approach, the edge rays of the elliptic bundle were identified with lightlike curves in Einstein’s gravitational theory. A restrictive condition was imposed in this approach: the edge rays were forced to be geodesics of the Euclidean metric and of the Lorentz metric. This restriction provided a tool for getting results, and new elliptic bundles were found. Later, by application of a series expansion from the bundle defined at a reference surface ,it was proven that other solutions exist, and thus the condition imposed in the Lorentz geometry approach was shown to be too restrictive. However, it was not demonstrated that the reference surface approach is general either. A subset of the reference surface solutions was also recently found using the Poisson Bracket design method in curvilinear coordinates which provided a deeper insight in the properties of these bundles. In this paper we present a new approach that leads to the equations which must be fulfilled by all the possible elliptic bundles in an homogeneous medium. This approach is based on the application of the Poisson Bracket design method in Cartesian coordinates. The already-known elliptic bundles are identified as particular solutions of the general equations. The search of new solutions is open, and the condition that must be fulfilled by them is given.

Application of the 2D etendue conservation to the design of achromatic aplanatic aspheric doublets (November 19, 2001)

Juan C. Minano; Pablo Benitez; Fernando Munoz

Conservation of etendue or phase space volume has been a useful tool in nonimaging design and analysis. It is one of the Poincare’s invariants associated to any Hamiltonian system. It expresses that the phase space volume of a region representing a bundle of rays do not vary when the rays proceed along the optical system. Another of these invariants is the 2D etendue conservation in 3D optical systems. This invariant can be expressed as the conservation along the ray trajectories of the differential form: dxdp + dydq + dzdr where x, y, z are position coordinates and p, q, r are the conjugate variables in the Hamiltonian formulation. When the optical system is frequency dependent (through the dependence of the refractive index of w) or it is time dependent, then the Hamiltonian formulation must include two new variables: t (time) and its conjugate variable -w. The application of the 2D etendue conservation to this new set of variables allows formulating the conditions for achromatic designs in a simple way. The results are coincident with Conrady’s formula and its simplicity permits a direct application to the design of achromatic lenses. We have applied these concepts to the design of achromatic aplanatic aspherical doublets, where the aplanatic condition means free of spherical aberration and circular coma of all orders and the achromatic condition means that the doublet is aplanatic for wavelengths in a neighborhood of the design wavelength. Several examples of these designs are given.

Design, construction, and measurement of a dielectric single-mirror two-stage (DSMTS) photovoltaic concentrator (November 19, 2001)

Ruben Mohedano Arroyo; Pablo Benitez; Juan C. Minano; Francisco Bercero; Pablo Lobato

The 30 X DSMTS is a trough-like photovoltaic concentrator, meant to track the sun in one axis, which has a mirror allocating two concentration stages and a secondary lens that increases its acceptance up to +/- 2.3 degrees. Provided that the sun subtends an angle of +/- 0.256 degrees, such acceptance seems excessive. However, thanks to it, we can relax requirements that often demand accuracy in systems of the kind. For instance, the shape of the mirror can be achieved by simply bending an aluminum sheet. To foresee the results we may expect of this strategy, we carried out some mechanical calculations, whose results are the boundary conditions that lead to a minimum standard deviation on the local slope of the elastic mirror with regards to the theoretical value. We checked by ray tracing that such an error actually provoked a small decrease on the acceptance. This fact persuaded us to carry out the manufacture of two elastic prototypes. In the test that have been performed so far with them we achieved an acceptance angle of +/- 1.63 degrees and a collection efficiency of 98 percent at a geometrical concentration of 30 times, results that can be considered as outstanding in the photovoltaics framework.

TIR-R concentrator: a new compact high-gain SMS design (November 19, 2001)

Jose L. Alvarez; Maikel Hernandez; Pablo Benitez; Juan C. Minano

In this work it is presented a new design of a TIR lens-mushrooms lens device developed with the Simultaneous Multiple Surfaces (SMS) method. In SMS nomenclature, it is named TIR-R. In contrast to previous TIR-mushroom designs, application of the SMS method to this configuration consists in the simultaneous design of both TIR (total internal reflection) and R (refraction) optical surfaces using extended ray-bundles and the edge-ray theorem. In this paper is presented a basic approach to do the design. In this basic approach, first it is considered the TIR lens as a microstructured surface with infinitesimal flat facets. Afterwards, it is generated a TIR lens with finite size facets from the already designed one. In an advanced approach could be considered the TIR lens with finite facet size and designed simultaneously each facet with a portion of the outer surface of the mushroom lens. With respect to others SMS high-gain devices (as the RXI), the TIR-R concentrator has the following advantages: is a mirror-less device, there is not shadowing elements, and the receiver/emitter element’s placement is more favorable for encapsulation and electrical connection. As it is common in the SMS devices, the TIR-R concentrator achieves wide acceptance angle and high efficiency with a low aspect ratio (thickness to entry aperture diameter ratio). For example, a 1256X concentration device has a theoretical efficiency of 100 percent (without optical losses) with an acceptance angle of +/- 1.7 decgrees, and an aspect ratio of 0.34.

Novel nonimaging lens for photodiode receivers with a prescribed angular response and maximum integrated sensitivity (February 6, 2001)

Pablo Benitez; Juan C. Minano; Maikel Hernandez; Kazutoshi Hirohashi; Satoru Toguchi; Masahisa Sakai

12 In this work we present a novel optical lens that can be designed to provide any specified angular sensitivity to a receiver, illuminating the sensitive area almost isotropically. This lens, which has been designed in the framework of nonimaging optics, consists in a single dielectric piece that encapsulates the receiver (as conventional epoxy packages of photodiodes), whose interface with air is an aspheric refractive surface. Several trial products have been manufactured with different angular sensitivities (linear response, cos-1(theta) and cos-2(theta) ). The experimental results have shown that the trial devices have the specified angular sensitivity with +/- 5% accuracy.

Ultracompact nonimaging devices for optical wireless communications (October 1, 2000)

Ruben Mohedano Arroyo; Juan C. Minano; Pablo Benitez; Jose L. Alvarez; Maikel Hernandez; Juan-Carlos Gonzalez; Kazutoshi Hirohashi; Satoru Toguchi

Ultracompact optics for optical wireless communications (December 13, 1999)

Juan C. Minano; Pablo Benitez; Ruben Mohedano Arroyo; Jose L. Alvarez; Maikel Hernandez; Juan-Carlos Gonzalez; Kazutoshi Hirohashi; Satoru Toguchi

Advanced optical design methods using the keys of nonimaging optics lead to some ultra compact designs which combine the concentrating (or collimating) capabilities of conventional long focal length systems with a high collection efficiency. One of those designs is the so-called RXI. Its aspect ratio (thickness/aperture diameter) is less than 1/3. Used as a receiver, i.e. placing a photodiode at the proper position, it gets an irradiance concentration of the 95% of the theoretical thermodynamic limit (this means for example, a concentration of 1600 times with an acceptance angle of +/- 2.14 degrees). When used as an emitter (replacing the aforementioned photodiode by an LED, for instance), similar intensity gains may be obtained within an angle cone almost as wide as the 95% of the thermodynamic limit. In a real device these irradiance(and intensity)gains are reduced by the optical efficiency. This combination of high concentration factors, relatively wide angles, simplicity and compactness make the optical device almost unique. This work will show the results of the measurements done with several RXI prototypes of 40-mm aperture diameter, all of them made of PMMA (by injection process).

Poisson bracket design method review: application to the elliptic bundles (October 6, 1999)

Juan C. Minano; Pablo Benitez

Poisson bracket design method is, as yet, one of the few known 3D nonimaging concentrator design methods. In general, this method provides concentrators requiring variable refractive index media, which become quite impractical. Another fruitful 3D-concentrator study was done by application of the Lorentz geometry to the nonimaging theory. In this study some new 3D ideal concentrators (with constant refractive index) were found by the analysis of the certain sets of edge rays with some particular symmetries. These bundles were called elliptic bundles of rays. On the other hand, a new approach based on the concept of reference surface has been recently used to find new bundles of this type. In this paper we review Poisson bracket design method, reformulate it and apply it to the analysis of elliptic bundles. The basic equations have become fully symmetric after reformulation. Moreover, the expression of the Hamiltonian function is of the same type as the expression of the first integral that contains the elliptic bundle. In this way one cannot distinguish, from the mathematical point of view, which is the Hamiltonian function and which is the first integral function. The formulation must be done with some selected coordinates in which one of the coordinate lines are the flow lines of the elliptic bundle and the other two ones are normal to the flow lines and tangent to the symmetry planes of the cone of edge rays. All the elliptic bundles found with the Lorentz geometry method and the reference surface method are again found here excepting those that aren’t symmetric with respect the coordinate surfaces. No new elliptic bundle has yet been found. Nevertheless the new analysis procedure is more adapted to arbitrary orthogonal coordinate systems and to variable refractive index, and so it is expected it will provide new bundles.

Design in 3D geometry with the simultaneous multiple surface design method of nonimaging optics (October 6, 1999)

Pablo Benitez; Ruben Mohedano Arroyo; Juan C. Minano

The Simultaneous Multiple Surfaces of Nonimaging Optics has been used successfully in the past for the synthesis of concentrators in two dimensions. In this paper we present a first approach to extend this design method to 3D geometry. As a first result, an aspheric lens without rotational symmetry that focus sharply two plane wavefronts at two points in 3D geometry is found, feature which cannot be obtained with an axisymmetric optical system with a finite number of optical surfaces.

New nonimaging static concentrators for bifacial photovoltaic solar cells (October 6, 1999)

Pablo Benitez; Maikel Hernandez; Ruben Mohedano Arroyo; Juan C. Minano; Fernando Munoz

Two new static nonimaging designs for bifacial solar cells are presented. These concentrators have been obtained with the Simultaneous Multiple Surface design method of Nonimaging Optics. The main characteristics of these concentrators are: (1) high compactness, (2) linear symmetry (in order to be made by low cost extrusion), (3) performance close to the thermodynamic limit, and (4) a non-shading sizable gap between at least one of the cell edges and the optically active surfaces. This last feature is interesting because this gap can be used to allocate the interconnections between cells, with no additional optical losses. As an example of the results, one design for an acceptable angle of +/- 30 degrees gets a geometrical concentration of 5.5X, with an average thickness to entry aperture width ratio of 0.24. The 3D ray-tracing analysis of the concentrators is also presented.

Application of variational calculus to nonimaging optics: design for small sources in three dimensions (October 6, 1999)

Ruben Mohedano Arroyo; Pablo Benitez; Juan C. Minano

In this work the variational calculus will be applied to the design with axisymmetric sequential optical surfaces for small sources directly in 3D. This method is proven to be useful even in the case in which the skewness distributions of output bundle and emitter do not fit. The tool provides both the optimum 3D ray bundles at the exit aperture of a rotational collimator and the best 2D assignation of rays allowing the collimation reach its limit. Once this best assignation has been obtained, the optical profiles can be designed easily using only the central ray of the bundles. Afterwards, the actual device is obtained by rotation. As a consequence of the kind of method aforementioned, the angular performance of the collimators can be nearly error- free predicted before ray tracing.

RXI concentrator for 1000X photovoltaic energy conversion (October 6, 1999)

J.C. Minano, Pablo Benitez, J. L. Alvarez, and Maikel Hernandez

The purpose of this work is to present the measurements of the main characteristics of a series of RXI concentrators developed: angular transmission, acceptable angle, optical efficiency, and optical concentration. The RXI concentrator has been designed with the Simultaneous Multiple Surfaces method developed by Minano et al. at the Instituto de Energia Solar–Universidad Politecnica de Madrid. The design characteristics are: geometric concentration 1256X, acceptance angle 1.8 degree(s).

Manufacturing tolerances for nonimaging concentrators (October 3, 1997)

Pablo Benitez; Ruben Mohedano Arroyo; Juan C. Minano

A 2D-error model for nonimaging concentrators composed by multiple optical surfaces is presented. The concentrator surfaces can be either dioptrics, sequential mirrors (as a conventional parabola) or non-sequential mirrors (i.e. CPC- like mirrors). Under the hypothesis of the model, the slope errors of all the surfaces can be transferred to entry aperture and combined there, and the effect of errors can be studied with a single probability density function as in the case of one-mirror concentrators. A single number, the concentrator error multiplier, is defined to characterize the concentrator tolerance to errors. This number and the concentrator acceptance angle are the key to analyze the error sensitivity of concentrators. Finally the model is used to quantify the maximum tolerance on the concentrators surfaces to guarantee a specified quality.

Contactless two-stage solar concentrators for tubular absorber (October 3, 1997)

Pablo Benitez; Juan C. Minano; Raphael Garcia; Ruben Mohedano Arroyo

Two new types of two-mirror solar concentrator for tubular receiver, the snail concentrator and the helmet concentrator , are presented. The main feature of these concentrators is that they have a sizable gap between the secondary mirror and the absorber, and they still achieve concentrations close to the thermodynamic limit with high collection efficiencies. This characteristic makes them unique and, on the contrary to the present two-stage designs, allows for the location of the secondary outside the evacuated tube. One of the differences between the snail and the helmet concentrators is that the last is symmetric (as the conventional parabolic trough) but the first is not. For an acceptance angle of (alpha) equals +/- 0.73 degs and a collection efficiency of 96.8% (i.e. 3.2% of the rays incident on the primary mirror within the acceptance angle are rejected), the snail concentrator and the helmet concentrator achieve an average flux concentration of 91.1% and 72.8% of the thermodynamic limit, respectively. The gap between the absorber and the secondary mirror is 6.8 and 12.1 times the absorber radius for each concentrator. Moreover, both concentrators have also high rim angles of the primary mirror: +/- 86.2 degs (helmet) and 3.1 – 98.8 degs (snail). This is of interest for a good mechanical stability of the collector.

Design of CPC-like reflectors within the simultaneous multiple-surface design method (October 3, 1997)

Pablo Benitez; Ruben Mohedano Arroyo; Juan C. Minano; Raphael Garcia; Juan-Carlos Gonzalez

The design of non-sequential mirrors within the Simultaneous Multiple Surface design method of nonimaging concentrators is presented. The formulation of the edge ray theorem for this problem is stated, which defines the rays to be considered in the design. As a result three new nonimaging concentrators are developed, which work efficiently and close to the thermodynamic limit of concentration in two dimensions.

Application of Lorentz geometry to nonimaging optics: new 3D ideal concentrators (August 21, 1995)

Manuel Gutierrez; Juan C. Minano; Carlos Vega; Pablo Benitez

A new family of 3D ideal nonimaging concentrators with rotational symmetry is presented. The formulation has been done using Lorentz geometry. The flow line concentrator and cone concentrator appear as particular cases with this approach.

Experimental measurements of RXI concentrators (August 21, 1995)

Juan-Carlos Gonzalez; Pablo Benitez; Juan C. Minano; Jose L. Alvarez; M. R. Ochoa

A small series of nonimaging RXI concentrators designed for concentrating radiation on a 1 mm2 photodiode have been manufactured by injection molding of PMMA (polymethyl methacrylate). Their entry aperture diameter is 40 mm and the design acceptance angle is +/- 2.24 degrees. Measurements done at (lambda) equals 890 nm show an optical efficiency of 33.8% and the relative transmission drops from 90% to 10% when the incidence angle varies from 1.2 to 3.4 degrees. A computer model of the concentrator which takes into account the different optical losses shows that optical efficiency is increased up to 57.1 % using silver metallization and (lambda) equals 800 nm. Moreover if photodiode position error is corrected, then the efficiency grows up to 76.2%, and if profile errors are also corrected, then the efficiency reach 88.2%. A more stepped angular transmission requires a most accurate manufacture process.

Analysis of the image formation capability of RX concentrators (August 21, 1995)

Pablo Benitez; Juan C. Minano

The analysis of the RX concentrator as an imaging optical system is presented. When the field of view is small (less than 6 degrees full angle), even for concentrations up to 95% of the theoretical maximum, its imaging performance is similar (in MTF terms) to that for normal incidence of an f/3.7 planoconvex spherical lens with optimum defocusing. This imaging capability allows us to use the same RXc with receivers/emitters different from the one of design with no loss of nonimaging quality. Also this feature combined with its capability to concentrate radiation close to the thermodynamical upper bound suggest that the RXc may be an interesting optical device for focal plane array applications.

Poisson brackets method of design of nonimaging concentrators: a review (November 1, 1993)

Juan C. Minano

A review of the nonimaging concentrator method of design called Poisson brackets method is presented. The method is valuable for 2D and 3D concentrators. The 2D version of the method has provided an interesting concentrator: the CTC (Compound Triangular Concentrator). Unfortunately, so far only impractical 3D concentrators (with variable index of refraction) have been obtained with the 3D version of the method. Nevertheless, these 3D concentrators prove that there are optimal and ideal 3D concentrators besides the trumpet concentrator and the trivial cases.

Design of optical systems which transform one bundle of incoherent light into another (November 1, 1993)

Juan-Carlos Gonzalez; Juan C. Minano

A new method of design of optical systems are presented in this paper. This method is a generalization of one recently developed at Instituto de Energia Solar (I.E.S.) of Univ. Polit. de Madrid. The method referenced has been used on the design of non-imaging concentrators for finite and infinite source. Outstanding results at the 3D performance have been achieved. The method now presented has also been used in the concentrators design, getting the same results as former’s. The aim of this method is the design of two surfaces optically active (lens, two mirror or lens-mirror combination) to transform one incoherent radiation of light arriving at the system’s aperture into a different radiation at the system’s outlet. A computer program has been built for the method application, and some examples have been included.

New nonimaging designs: the RX and RXI concentrators (November 1, 1993)

Juan C. Minano; Juan-Carlos Gonzalez; Pablo Benitez

Two new nonimaging concentrators called RX and RXI are presented. Both of them have been designed with a method of design recently developed. The concentrators, formed from a single dielectric piece, achieve the optimum relation between concentration and acceptance angle in 2D. Their performance in 3D is very good when the acceptance angle of the concentrators is small (less than 5 degrees for a source at infinity): total transmissions above 95% are usually achieved (total transmission is the ratio of the etendue of those rays impinging on the entry aperture, within the acceptance angle, which reach the receiver, over the etendue of the bundle formed by all the rays impinging the receiver). These values are similar or even higher than those achieved by an equivalent CPC (same acceptance angle and same refractive index). The RX shown here have been designed for a finite source and the RXI for a source at infinity.

Combined optics for concentration and light trapping in photovoltaics (November 25, 1992)

Philip A. Davies; Juan C. Minano

Light trapping by means of external cavities theoretically offers the same potential to improve the efficiency of solar converters as does concentration. The ideal efficiency of 86% could be reached by any combination of concentration and light trapping, provided all the etendue of the cell is coupled either to itself (giving light trapping) or to the sun (giving concentration). But with real solar cells, there are optimum conditions of concentration and light trapping which maximize the efficiency. The possibility of achieving these conditions is compared between two optical systems: (1) axisymmetric Fresnel lens with plane upper surface, and ellipsoidal cavity, (2) similar Fresnel lens plus secondary lens using total internal reflections to trap light. With regard to a deep-emitter cell, of moderate technology, either system can attain near optimum conditions, giving an efficiency about 4% points above 1-sun efficiency. With a more speculative cell model assuming back mirroring and cell thinning to reduce series resistance, the maximum efficiency (predicted 8% points above 1-sun efficiency) cannot be obtained due to limitations of the optical system, and about 5% points above 1-sun efficiency appears feasible.

Angular confining cavities for photovoltaics (October 24, 1991)

Juan C. Minano; Antonio Luque

Using anidolic or nonimaging optics principles, we present a study of the role of light confinement, with special application to photovoltaics. In particular, we study how and to what extent the reflectance of an absorbing surface can be reduced. Also, we study how and to what extent the absorption of light in a volume can be increased. In both cases the use of light confining cavities is the key for these improvements. Cavities different from the Helmholtz sphere, but rather based in an angular-spatial limitation of the escaping beam, are presented as a key tool for practical designs.

Design of nonimaging lenses and lens-mirror combinations (October 24, 1991)

Juan C. Minano; Juan P. P. Gonzalez

A new method of design of nonimaging concentrators is presented and two new types of concentrators are developed. The first one is an aspheric lens, and the second one is a lens- mirror combination. A ray tracing of 3-D concentrators (with rotational symmetry) has also been done, showing that the lens-mirror combination has a total transmission as high as that of the full compound parabolic concentrators, while their depth is much smaller than the classical parabolic mirror-nonimaging concentrator combinations (8 times smaller or more). Another important feature of this concentrator is that the optical surfaces are not in contact with the receiver, as occurs in other nonimaging concentrators.