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LPI's innovative optical designs are founded on its numerous technology innovations, as summarized here. This versatile technology-suite has been detailed in published papers, extensive patent applications, and the two patents so far issued (U.S. 6,639,733 and 6,646,813).
1. SIMULTANEOUS MULTIPLE SURFACE (SMS) METHOD Traditional illumination optics uses variants of parabolic reflectors and Fresnel lenses, and in general only a single surface is designed at a time. Trial-and-error iteration of separately designed surfaces is the typical design method for luminaires with several optically active surfaces. Conventional imaging optics for multiple lens elements is inappropriate for illumination optics, which is basically non-imaging (or Anidolic), a different branch of optics, recently founded and less developed than imaging optics. To meet the need for more sophisticated non-imaging optical design methods, LPI scientists have conceived, mathematically developed, and numerically implemented an entirely new way of designing complex illumination optics. Beginning over a decade ago, the Simultaneous Multiple Surface (SMS) method was originally developed as a way of generating a pair of two-dimensional profiles that would form three-dimensional surfaces by linear or rotational sweeping. For the more general case of a free-form surface, a new, three-dimensional SMS method has also been developed, and a patent applied for. Non-imaging optics lies at the heart of illumination engineering, which is concerned with the redirection and distribution of light itself, not of images. Mathematically, non-imaging optics is founded on the edge-ray theorem, which states that an optical system designed to deal with the bounding rays of the light going in and the light going out will be able to attain the thermodynamic limit. This limit, measured by the geometric concept of etendue, specifies the narrowest-angle beam attainable by a given sized output aperture. Etendue is the product of aperture and the sine of the angle of the outermost ray of the input beam. An ideal optical system conserves etendue, so that an output aperture wider than the input results in a narrower beam, a process called collimation. An output aperture with a wide-angle output (such as in the case of the focal point of a magnifying glass) is much smaller than the input aperture (the lens diameter), a process called concentration. In the past, the compound parabolic concentrator (CPC) and its variants were the only nonimaging optics capable of ideal performance. Their disadvantage is that their depth is many times their width, while usually the opposite is desired instead. There's only so much you can do with just a single surface. LPI scientists have gone beyond this paradigm by inventing the capability of designing two or even three surface-profiles at once, making it possible to synthesize completely new times of ideal, or etendue-limited, nonimaging illumination systems. This approach utilizes particular, highly diagnostic groups of edge rays, known as caustics, to conjoin corresponding short segments of the two surface-profiles being designed. The boundary conditions of the problem locate the first pair of segments, and both surface-profiles are then generated segment-by-segment until done. The SMS method has been used to design solar concentrators and, later, LED collimators. In general, the two or three surfaces are designated by the abbreviations 'R' for Refraction, 'X' for Reflection, as by a mirror, and 'I' for Total Internal Reflection (incident angles steep enough for the Fresnel secondary ray to actually capture all the energy of the incident ray, so there is no transmitted ray). A conventional refracting lens would be an 'RR', while the SMS photovoltaic solar concentrators were 'RX', and the SMS LED collimators are 'RXI'. Although light in an LED lens is going the reverse direction than in a solar lens, both use the same acronym as the solar case, else the LED collimator would confusingly be called an IXR. The particular SMS design applications so far fielded by LPI have utilized a further design principle, folding of the optical path, as described below. The 2-D SMS method designs surface profiles, from which the surfaces themselves are generated. Free-form surfaces are defined as those not generated by a profile sweep, so a new design method is needed. The philosophy of the 2D SMS method has been adapted to work with full three-dimensional wavefronts rather than groups of two-dimensional rays. It has been known for a century that for any two wavefronts, a unique surface exists (refractive or reflective) that transforms one into the other. The 3D SMS method is based on the corresponding idea that for any two pairs of wavefronts a unique pair of surfaces exist that transform one pair into the other. The 3D SMS method begins with a process resembling the lacing of a shoe, whereby two profiles are generated by alternately using rays from the two input wavefronts and joining them with corresponding rays from the two output wavefronts. These profiles are the seed ribs of the input and output surfaces. After further seed ribs are generated, a surface is installed upon them by a process known in numerical surface mathematics as 'skinning', the opposite of the removal process usually associated with the word. The 3D SMS method is more numerically involved than the 2D SMS method. Its practical use hinges on the selection of the pairs of wavefronts to be used to generate the pairs of surfaces. These input and output wavefront-pairs are selected in accordance with the particular illumination problem to be solved. The LED automotive headlamp described below was designed this way.
LPI's scientists have originated optical configurations wherein light travels back and forth multiple times (as in an astronomical telescope), but where the resulting path is equivalent to a relatively long path that would be obtained without such reflections. Known in imaging optics as folded optics, this approach has been adapted to illumination optics by LPI scientists, using the 2D SMS method. To appreciate this fully, consider that mainstay of conventional illumination, the parabolic reflector. The original luminaire is the parabolic reflector. The first searchlights were merely large paraboloids focused on a small bright carbon arc. To make the beam's angular width smaller, the mirror diameter must be correspondingly enlarged, but nothing can be done about the inherent nonuniformity of the beam, with the narrowest angles from the edge and the widest from near the center of the mirror. The result is a much wider beam than the thermodynamic limit, because different parts of the paraboloid are at different distances from the central light source, and thus reflect different angular widths. In imaging optics this condition is known as coma, and it results in a halo of wasted light around the useful main beam output by a paraboloid, such as a flashlight or searchlight. This is in addition to the light paraboloids waste that shines straight out the reflector aperture. 2.1 THE RX SOLAR CONCENTRATOR Heat removal is a primary limiting factor for high-power LEDs. It turns out that cooling techniques developed for LEDs are equally applicable to high-concentration solar photovoltaics, using mass-produced silicon cells much smaller (1mm) than typical. Thus LPI has developed the 1" RX lens for 1000X concentration, with only 0.2" thickness. This circular lens is cut into a square for rectangular arrays to be installed on thin lightweight panels for two-axis tracking. The light is refracted ® by the entry aperture, and reflected (X) by the back surface to focus onto the target. The RX lens must be optically bonded to the silicon cell, which is not difficult in mass production. LEDs, however, are a different matter, since they must be produced in their own epoxy packages, so that a lens would have to be glued to it in a secondary operation, a mass-production disadvantage, which also makes difficult the swapping out of defective LEDs. Also, the RX lens has its target cell well above the bottom of the lens, whereas LEDs need to be mounted on a circuit board. Thus LPI has modified the RX lens by making the top 'R' surface act also as an internal reflector, further concentrating light from the coated rear surface of the lens. Also, a refracting surface surrounds the LED package, hence the 'airgap' part of the name. This thin plastic lens has 85% efficiency and a highly uniform beam of the narrowest possible output angle, an unprecedented ±3o, expanding only 10% of the distance projected. Besides this LED version, other RXI designs have been implemented in glass for automotive headlamps. LPI's proprietary LED headlamp lens is the first one capable of attaining all performance specifications using currently available luminance levels of high-power white LEDs. In spite of their unprecedented solid-state brightness, these LEDs can only work in headlamps having the most efficient optics possible, with etendue-limited performance. LPI's SMS method was used to generate this radically novel lens. When LPI's customer permission is granted by the release of the headlamp product, this lens will be exhibited at this site.
There is a further novel optical principle incorporated in LPI's LED headlamp. Special reflectors installed next to the LED package cause its output pattern to be altered for a better fit with the headlamp angular prescription, which is wider than it is high. This principle of squeezing a square LED into a rectangular pattern has been dubbed etendue squeezing, and is the subject of a recent LPI patent application. Besides this matching, etendue squeezing also makes it possible for the headlamp lens to attain a very rapid vertical cutoff, for unprecedented glare reduction, making LPI's LED headlamp superior to conventional HID headlamps, which have become notorious for their high near-beam glare levels when seen in rear-view mirrors.
Incandescent lamps have been around so long that thousands of products have been designed around them, making light-bulb substitutes a prime market for LEDs, which have longer life and higher efficiency (especially for colored lights) than incandescents. The problem is that LEDs only emit into a hemisphere, so simply putting one in a light bulb's place will not do. Also, an LED's light emits right out of the package base, whereas light-bulb filaments are normally spaced above the base. In order to close these differences, LPI scientists have developed a family of optical devices that take an LED's light and transforms it into a variety of outputs, some with a more light-bulb-like distribution, others with output that shines only on a flashlight reflector but not out its front, saving the normally spilled light for the output beam.
Frame projectors are lights that illuminate within the frame of a mounted photograph or painting. Most commercial versions are mere spotlights that deliver a round beam onto a rectangular target. There are a few expensive lensed versions that actually image a rectangle, but only for low slant angles, with the target squarely facing the light. Instead, LPI has designed a frame-projector lens for such high-slant situations as illuminating a wall-mounted target from a ceiling location near the wall (such as a painting in a museum or living room). In spite of such unprecedented obliquity (70o off-normal), uniform illumination is achieved over a well-delimited rectangular zone. LPI plans to market a family of these lenses to museums and art shops, including a battery-operated LED version for mounting street-lamp style on the frame itself.
Many illumination needs involve producing a vertically narrow but horizontally broad pattern, like the 360o pattern of a lighthouse. Masthead navigation lights for boats and aircraft warning lights for antennas are two prominent examples of 360o lights. Bow lights for boats and wingtip lights for airplanes are examples of 180o lights. LPI has originated miniaturized TIR drum-lenses that can produce 180o or 360o patterns. Besides these vehicular applications, these powerful LED lamps are also being developed for signage, where light must be cast sideways into the sign's white-painted interior without producing hot spots with light sent directly out of the sign.
The Total Internal Reflection (TIR) lens is a relative of the conventional Fresnel lens. While Fresnel lens facets just refract light the same as a thick lens, the TIR lens also has facets that bend light much more than refractive facets ever could, past 90o. The first TIR lens patent was filed twenty years ago by one of LPI's senior scientists, who applied it to the Diamond Light flashlight line, still sold at Sears. LPI has applied for a patent covering the ability to use a TIR lens to form a beam that comes out of the lens at unprecedented slant angles from its outer surface. When the profile of the facets of a TIR lens forms linear grooves, a linear TIR lens is formed, with a focal line rather than a focal point. Linear TIR lenses were originally developed for such line sources as fluorescent lamps, and in the longitudinal direction such a lens has a very wide pattern. When LEDs are placed on the focal line, however, an exterior set of pillow lenses can focus the light from each LED to make the longitudinal pattern as narrow as the transverse pattern of the TIR lens. Such a lens has been developed for an automotive daylight running lamp, with amber LEDs powering a beam that horizontally exits a slanted surface. LPI has designed circular off-normal TIR lenses that emit a tight beam at 45o from its flat exit surface. Such a lens is useful for ground-covering spotlights that can be flush-mounted in building walls when uninterrupted surfaces are desired, while also offering better protection from weather and easier personnel access.
Fresnel reflection occurs every time a lens surface transmits a primary ray, so that a dimmer, secondary ray bounces back off the surface, typically with at least 4% reflectance (normal incidence). Its strength was first mathematically derived by Fresnel, who showed how the two different planes of polarization are differently reflected. Traditionally it has been considered an annoying source of stray light and a parasitic drag on throughput. LPI, however, has harnessed this phenomenon to produce unprecedented types of output beams.
LPI's issued patent, U.S. 6,639,733, covers a family of polarization-control devices that utilize an unpolarized collimated input beam to produce highly polarized outputs in a variety of radial and linear polarized states. Some configurations have these polarization states under high-frequency electronic control, offering a jam-proof method of light-beam communication.
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