US 20050117125 A1
A white beam with high color-rendering index is formed by uniting a collimated red beam and a collimated blue−green beam, the latter having as its source a blue LED with a green phosphor. The white beam is formed by a prism with an amber low-pass dichroic filter. The prism cross-section can be either a square or an equilateral triangle. The triangular prism can have one third of its mass reduced by stepped facets. Both types of prism can be elongated to accommodate multiple collimators. Switching between white, red, and blue−green allows application to police vehicles.
1. A white-beam combining system comprising a dichroic prism, a source of red collimated light, and a source of blue−green collimated light, said collimated light inputs situated so as to properly enter said prism and be combined therein into a common output beam.
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10. A white-beam combining system comprising an integrated dichroic mirror, a source of red collimated light, and a source of blue−green collimated light, said collimated light inputs situated so as to properly perform spectrum merging based on the integrated dichroic mirror being transparent for low energy photons emitted by the red collimated light source and reflecting the high energy photons emitted by the blue−green collimated light source.
This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/520,513 filed Nov. 14, 2003, of Minano, et. al., for DICHROIC BEAM COMBINER UTILIZING BLUE LED WITH GREEN PHOSPHER, which U.S. Provisional Patent Application is hereby fully incorporated herein by reference.
One the several ways of using light-emitting diodes (LEDs) to make white light is separately generating three differently colored wavebands and subsequently combining them. This has the advantage of each color source being separately cooled, so that the combined luminous output of the three LEDs is potentially higher than that of one. One example of a single LED producing white light is a blue LED coated with a yellow phosphor, such as those produced by Lumileds and by Nichia.
Dichroic beam combining systems are found in television, particularly those utilizing dichroic prisms and even crossed dichroic prisms, containing two dichroic mirrors. These systems, however, typically involve mixing the light of three different colors. In the case of machine vision, particularly for such tasks as fruit sorting, it would be three spectral-waveband channels.
Illumination systems utilizing dichroic mirrors can be found in commercially available lamps such as the MR series with built-in dichroic-coated glass reflectors, called cold reflectors because they do not put heat into the light beam. Dichroic reflectors are also utilized as hot mirrors, which return longer wavelengths to a source in order to boost its shorter-wavelength emissions.
The present invention, in contrast, utilizes a dichroic prism for combining only two wavebands in the generation of white light. This is a more suitable function of a dichroic reflector, which is a nearly lossless cutoff filter designed for 45° incidence. It thereby divides white light into longer wavelengths, which are transmitted, and the shorter wavelengths, which are reflected. Thus a dichroic reflector for beam combining could be made for any wavelength that suitably divides the visible spectrum. The cutoff wavelength can be chosen according to the thickness and composition of the component layers of the coating. Because the exact cutoff wavelength of a dichroic coating is somewhat angle sensitive, it is best for the beam-combining function if neither beam contained wavelengths very near the cutoff.
When using two colors to produce a white combination, it is only necessary that each component's location on the chromaticity diagram be opposite the other from a white center point, and that the two beams have the proper relative fluxes that balance to that white. This will suffice for such direct-view functions as a white-light navigation beacon or accent lighting. For illumination, however, two wavelengths give poor color rendition, making it difficult to distinguish red vs. green objects, in the case of a yellow-blue pair of LEDs combined by a green dichroic reflector, or green vs. magenta objects, in the case of a red-cyan pair combined by an amber dichroic reflector. In the latter case of an amber reflector, good color rendition would require the prior art of LED-beam combining to use blue and green LEDs with a cyan dichroic filter.
The color-rendition problem of a single dichroic filter is solved by the present invention by a novel component of the present invention, a blue LED with a green phosphor. Conventional white LEDs, such as those by Nichia or Lumileds, utilized blue LED chips coated with a yellow phosphor, to produce white outputs with good color rendition. Such LEDs will soon be applied to automobile headlamps, but their peak luminance sets the peak candlepower limit for a given headlamp size.
Instead, the blue LED with green phosphor of the present invention has its light efficiently collected and collimated, thence to be combined with a similarly collimated red beam, by an amber dichroic prism. A novel alternative is disclosed to the 45° prisms of the prior art.
The present invention relates generally to the production of superior levels of luminance by dichroicly combining a collimated red beam with the collimated beam of a blue LED with a green phosphor. Dichroic prisms of the prior art form a cube consisting of a pair of 45° prisms joined hypotenuse-to-hypotenuse on either side of a dichroicly reflective interface, preferably a dielectric stack of quarter-wave layers. The two collimated light sources are located on different faces of the cube and lie 90° to each other. Disclosed herein is a novel configuration comprising an equilateral-triangle prism with the two collimated light sources adjacently positioned. These two collimated sources can be advantageously mounted on the same circuit board, so that their output beams enter the beam-combining prism in parallel.
The green phosphor of the present invention operates near the 555 nm wavelength of peak luminous efficacy (683 lumens/watt). It necessarily produces more lumens than a yellow phosphor of the same quantum efficiency. A green phosphor coating on a high-power blue LED can be given the requisite thickness for proper color balance with a high-power red LED. The present invention advantageously combines these two wavebands into a white resultant with high color-rendering index as well as higher luminance than possible solely with a white LED based on the same blue chip. This superior performance is because of the higher luminance of a blue LED with green phosphor being efficiently combined with the additional luminance of a red LED with separate cooling means. The linear nature of the prism allows long arrays of such red-blue LED pairs to be utilized for very powerful vehicular lighting. In the case of police vehicles, switching off either set of LEDs would produce either a red or a cyan light, the latter being close to the conventional blue of police emergency lights. Such a long prism is easily swiveled to vertically steer the high-intensity output beam.
The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:
Corresponding reference characters indicate corresponding components throughout the several views of the drawings.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings, which set forth illustrative embodiments in which the principles of the invention are utilized.
The following description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.
Referring next to
Shown is a modified RXI 71 containing two LEDs 73, 75 (one of high bandgap (blue+green) 73 and another one of low bandgap energy (red) 75), and a dichroic mirror 77.
The spectrum merging RXI is formed by, at least, 3 optical surfaces: two of them are the surfaces of a conventional RXI designed for a single emitter, which can be one of the two emitters 70, 75. This emitter is the red LED 75 in the example
This dichroic mirror 77 is designed as follows: First trace the red rays emanating from a central point of the red emitter until the RXI aperture. Calculate a wavefront Wo′ of these rays once they have exited the RXI aperture. Calculate the wavefront Wi obtained when a set of blue rays emanating from a central point of the blue LED are reflected by the RXI aperture surface. Calculate the wavefront Wo such that when Wo is refracted at the RXI aperture it becomes the wavefront Wo′. Calculate the dichroic mirror profile as the Cartesian oval reflector that couples the wavefronts Wi and Wo′. There is only 1 degree of freedom in calculating the dichroic mirror: This is the optical path length from the wavefront Wi to the wavefront Wo′. This optical path length is chosen so the dicroich mirror 77 does not intersect any of the 2 other optical surfaces and the resulting dichroic surface is inside the RXI.
The dichroic mirror 77 can be made as an interferential filter: by vacuum evaporation of its components or with the GBO technology of 3M. This last technology provides polymer sheets which can be vacuum molded if the deformation is not too big. This feature is particularly interesting for this invention. The most important advantage of the integrated dichroic mirror 77 is volume saving when compared to the other solutions. The dichroic mirror 77 must be, in general, a non flat surface (more expensive) and the rays are not as collimated as they are in the non integrated case when they reach the dichroic mirror. This means that the dichoric mirror 77 must keep its wavelength selective characteristics for a wider range of incidence angles (the importance of this effect depends on the particular application).