Abstract:
Optical systems are described that have at least one source of a beam of blue light with divergence under 15°. A phosphor emits yellow light when excited by the blue light. A collimator is disposed with the phosphor and forms a yellow beam with divergence under 15°. A dichroic filter is positioned to transmit the beam of blue light to the phosphor and to reflect the beam of yellow light to an exit aperture. In different embodiments, the beams of blue and yellow light are incident upon said filter with central angles of 15°, 22°, and 45°. The filter may reflect all of one polarization and part of the other polarization, and a polarization rotating retroreflector may then be provided to return the unreflected light to the filter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is filed concurrently with U.S. patent application Ser. No. ______, (docket No. 047654-0016-00-US) of Falicoff for “Wideband Dichroic Filter Design for LED-Phosphor Beam-Combining,” the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY FUNDED RESEARCH AND DEVELOPMENT 
       [0002]    This invention was supported in part by the National Energy Technology Laboratory Award No. DE-FC26-05NT42341. The Government may have certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates generally to optical systems using a phosphor to convert light of one color to another color. 
         [0005]    2. Description of Related Art 
         [0006]    Light emitting diodes (LEDs) are a widely available, inexpensive, and efficient light source. However, a typical LED provides light of a single dominant color, with a very narrow spectral range. 
         [0007]    It has previously been proposed to use a blue LED to excite a white phosphor, thus combining the simplicity of an LED as an electrically-powered light source with the better spectral output of a phosphor, or to use a blue LED to excite a yellow phosphor and combine the yellow light with unconverted blue light to produce a substantially white output beam. However, the LED prior art is also less than satisfactory regarding the geometry of phosphor utilization in LEDs, such as for LEDs that generate white light. Commonly-assigned U.S. Pat. No. 7,286,296 of Chaves et al. proposes novel configurations of manifolds for combining LED and phosphor light, but there is still room for further improvement. 
         [0008]    The above-referenced concurrently filed application discloses dichroic filters for combining the blue light of an LED and the yellow light from a photostimulated patch of phosphor into a single white output beam. Because both the blue and yellow light as they emerge from the LED and the phosphor are uncollimated, no presently-available filter can efficiently combine their light until the range of incidence angles has been restricted. The most geometrically convenient beam-combination angle is 45°, but unless the light is polarized only a narrow angular range can be accommodated without loss of efficiency, typically 42-48°, whereas a realistic illumination system will exhibit a 30-60° range of angles into the filter. This ±15° range, however, is relatively narrow as collimation range goes, and cannot be expected to be smaller. While a 45° angle of beam combination is well known from RGB displays, it has not been possible to overcome the severe polarization effects that occur at this angle. In order to overcome this, several RGB display systems were developed where the band-pass operates at less than 45°. 
         [0009]    A well known system that operates at a nominal 30° is the so-called Philips color TV camera. This system is described on pg. 1-79 of the book “Optical Coating Technology” by P. W. Baumeister, SPIE Press 2004. According to Baumeister (pg. 1-79) this “30°” design “has become an industry standard” and is based on U.S. Pat. No. 3,202,039, which describes the incidence angles as “smaller than 30°” without specifying the angle more precisely. Another approach is given in U.S. Pat. No. 3,905,684. In this prior art the band-pass filter operates at a nominal angle of 16° according to Baumeister (pg. 1-84) and therefore is an improvement over U.S. Pat. No. 3,202,039, as it is much easier to design a short or long pass filter that operates about this incidence angle. Baumeister states (pp. 1-84) that “The aforementioned Philips system has been used in a projection system . . . In a modification [Cook 75],  FIGS. 1-189  shows that an additional wedge prism is added to the entrance face, which permits the angle of incidence upon the blue dichroic (that follows) to be 16° for an axial ray. This is compared with the 30° angle for the system in §1.8.4.2.1 [the Philips system of U.S. Pat. No. 3,202,684]. It follows from the discussion . . . that this relatively small angle reduces the polarization splitting.” 
         [0010]    The above-referenced concurrently-filed U.S. patent application of Falicoff discloses short-pass filters operating at a nominal incidence angle of 15°, and methods of designing such filters. The present application discloses manifold systems for combining un-polarized yellow and blue light based on a short-pass filter operating at a nominal incidence angle of 15°, for which the novel filters of the concurrently-filed application, while not essential, are especially suitable. 
       SUMMARY OF THE INVENTION 
       [0011]    Embodiments of optical manifolds are described herein that provide the ability to efficiently convert a beam of light with a phosphor, and especially (but not exclusively) to collimate the blue light output of one or more blue LEDs, concentrate that light onto a yellow phosphor, and then combine and collimate the yellow light from the phosphor with the same or different blue light into a single substantially white output that is substantially homogeneous. 
         [0012]    According to one embodiment of the invention, there is provided an optical system comprising at least one source of a beam of blue light with divergence under 15°, a phosphor that emits yellow light when excited by said blue light, a collimator disposed with said phosphor and forming a yellow beam therefrom with divergence under 15°, and a dichroic filter positioned to transmit said beam of blue light to the phosphor and to reflect said beam of yellow light to an exit aperture, said beams of blue and yellow light being incident upon said filter with a central angle (the angle of incidence of the center of the beam) of 15° and a distribution of incidence angles thereabout between 0° and 30°. 
         [0013]    According to another embodiment of the invention, there is provided an optical system comprising at least one source of a beam of blue light with divergence under 15°, a phosphor that emits yellow light when excited by said blue light, a collimator disposed with said phosphor and forming a yellow beam therefrom with divergence under 15°, and a dichroic filter positioned to transmit said beam of blue light to the phosphor and to reflect said beam of yellow light to an exit aperture, wherein said optical system is so aligned that said beams of blue and yellow light are incident upon said filter with a central angle of 30° and a distribution of incidence angles thereabout between 15° and 45°. 
         [0014]    According to a further embodiment of the invention, there is provided an optical system comprising at least one source of a beam of blue light with divergence under 15°, a phosphor that emits yellow light when excited by said blue light, a collimator disposed with said phosphor and forming a yellow beam therefrom with divergence under 15°, and a dichroic filter inclined 45° and positioned to reflect all of one polarization and part of the other polarization of said beam of blue light to the phosphor and to reflect said beam of yellow light to an exit aperture, said beams of blue and yellow light being incident upon said filter with a central angle of 45° and a distribution of incidence angles thereabout between 30° and 60°, said system further comprising a polarization rotating retroreflector which returns said polarized blue light to said filter, which reflects it out said exit aperture in parallel alignment and overlap with said yellow beam. 
         [0015]    According to a further embodiment of the invention, there is provided an optical system comprising at least one source of a beam of blue light with divergence under 15°, a phosphor that emits yellow light when excited by said blue light, a collimator disposed with said phosphor and forming a yellow beam therefrom with divergence under 15°, and a dichroic filter in air inclined 45° and positioned to reflect all of one polarization and part of the other polarization of said beam of blue light to the phosphor and to reflect said beam of yellow light to an exit aperture, said beams of blue and yellow light being incident upon said filter with a central angle of 45° and a distribution of incidence angles thereabout between 30° and 60°. 
         [0016]    According to a further embodiment of he invention, there is provided an optical manifold according to the present invention in which the dichroic filter is a filter as disclosed and/or claimed in the above-mentioned concurrently filed application. 
         [0017]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
           [0019]      FIG. 1  is a cutaway perspective view of a T-shaped optical manifold with its dichroic mirror in air. 
           [0020]      FIG. 2  is another cutaway view of the manifold shown in  FIG. 1 . 
           [0021]      FIG. 3  shows a L-shaped optical manifold with a phase-rotating reflector. 
           [0022]      FIG. 4  shows a collimating white-light optical manifold based on a 22°-incident dichroic filter 
           [0023]      FIG. 5  shows the manifold of  FIG. 4  passing photostimulative blue rays through to the phosphor. 
           [0024]      FIG. 6  shows the manifold of  FIG. 4  passing yellow phosphor light into an output beam. 
           [0025]      FIG. 7  shows the manifold of  FIG. 4  passing auxiliary blue light into the same output beam. 
           [0026]      FIG. 8  shows a collimating white-light optical manifold based on a 15°-incident dichroic filter. 
           [0027]      FIG. 9  shows the manifold of  FIG. 8  passing photostimulative blue rays through to the phosphor. 
           [0028]      FIG. 10  shows the manifold of  FIG. 8  passing yellow phosphor light into an output beam. 
           [0029]      FIG. 11  shows the manifold of  FIG. 8  passing auxiliary blue light into the same output beam. 
           [0030]      FIG. 12  is a perspective view of the manifold of  FIG. 8 , also showing a mounting. 
           [0031]      FIG. 13  shows an alternative configuration of a white-light optical manifold based on a 15°-incident dichroic filter. 
           [0032]      FIG. 14  is a graph of the angular distribution of auxiliary blue light incident on the filter. 
           [0033]      FIG. 15  is a graph of the angular distribution of photoluminescent blue light incident on the filter. 
           [0034]      FIG. 16  is a graph of the angular distribution of yellow light incident on the filter. 
           [0035]      FIG. 17  is a graph of the spectral performance of the filter at various incidence angles, along with the cumulative spectra of LED and phosphor. 
           [0036]      FIG. 18  is a graph of total filter performance in the context of LED and phosphor spectra. 
           [0037]      FIG. 19  shows an asymmetric intensity pattern of a single manifold. 
           [0038]      FIG. 20  shows a well-defined flat-top intensity pattern produced by two manifolds oppositely mounted side-to-side. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    Reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The embodiments are described by way of explanation, and not by way of limitation. 
         [0040]    Numerous embodiments in the above-mentioned U.S. Pat. No. 7,286,296 of Chaves et al. use a spectrum-splitting mirror operating at 45° to the flow of light. While such devices are common in color-combining LCD projectors, they typically are effective only for one polarization. Normal-incidence dichroic mirrors have no such problem, since both polarizations behave identically. Also, at angles near normal incidence, the wavelength shift of the mirror&#39;s reflectance spectrum as the angle changes is less than at higher incidence angles, because the cosine function is changing slowly: cos 0°/cos 10°=0.985 while cos 55°/cos 45°=0.811. Thus a 10° range of incidence between 0° and 10° causes a 1.5% shift, or 7.5 nm at a nominal wavelength of 500 nm, whereas the same range between 45° and 55° causes a 19% shift; or 95 nm at the same wavelength, precluding all but the narrowest beams (±3°). This cosine relationship is approximate but is representative of the performance of commercially available short-pass filters. 
         [0041]    At this time there are no short-pass or long-pass dichroic mirrors or filters that can effectively handle a wide range of incidence angles centered at a 45 degree nominal angle of incidence. So-called omni-directional band-pass filters are not yet possible using traditional thin film coatings. Even a very sophisticated technology such as rugate filters cannot overcome the limitation of existing thin film technology. The 3M Company of Minnesota claims to have the capability of doing this by manufacturing films containing as many as a thousand layers. Each layer in the film can also be anisotropic, thereby providing an extra degree of freedom to the design. However, no such product has been released by this company and it therefore remains to be seen if this technology is in fact capable of achieving the goal. In summary there are two fundamental problems with systems that employ a band-pass filter operating around 45 degrees. First, they exhibit strong angle shift from one angle of incidence to the next. Secondly, they cannot handle properly both states of polarization when the angle of incidence is close to 45 degrees, or more specifically when the angle of incidence gets close to the Brewster angle. 
         [0042]    One reason for the second problem is that the previously proposed 45° filters operate within a dielectric, which greatly reduces the differential refractive index when light enters the filter.  FIG. 1  discloses a preferred embodiment with the 45° mirror in air. The filter  8105  shown in  FIG. 1  has a thin, flat structure bounded by air/dielectric interfaces parallel to the plane of the filter, and thus at 45° to the central axes of the incident, transmitted, and reflected beams in the air. Not only does this supply a large differential refractive index, but also it reduces the 45° incidence angle to 28° inside the filter, and a 55° angle to 33°. The above-mentioned cosine effect is reduced to cos 33°/cos 28°=0.95, for a spectral shift of 25 nm, causing only mild losses. Other small losses will accrue from Fresnel reflections when light goes into air, and from geometric losses due to beams overly expanding. The Fresnel losses can be reduced by designing the filter to include an anti-reflective coating. This must, however, be optimized so that the differential refractive index is not reduced. Such filters are presently available from a number of commercial sources such as JDSU of Santa Rosa, Calif. 
         [0043]      FIG. 1  shows a cutaway view of remote phosphor system  8100 , comprising four input solid dielectric square CPCs  8102  each coupled to a blue LED (not shown in detail), auxiliary blue LEDs  8103  and their 15° solid dielectric square CPC  8104 , spectrum-splitting mirror  8105 , large 15° CPC  8106  and transparent phosphor  8107 , and dielectric hollow tube  8108  shown cutaway to reveal spectrum-splitting mirror  8105 , designed to operate in air, not a dielectric. The walls of tube  8108  transfer light via total internal refection (TIR). Optionally the side in  FIG. 1  that is cutaway from dielectric tube  8108  and the side opposite it can be replaced with mirrors. 
         [0044]      FIG. 2  shows a perspective view of same, cutaway to show the interior of dielectric hollow tube  8108 , optically bonded to the face of CPC  8106 . CPC  8104  can be seen to be optically bonded to the outside of tube  8108 , which has the basic function of containing the spread of light coming out of CPCs  8102 , ensuring that most of it goes into CPC  8106 . 
         [0045]      FIG. 3  shows an alternative preferred embodiment that uses a 45° dielectric-immersed spectrum-splitting mirror that is optimized for one polarization, while allowing part of the other polarization to be transmitted. This transmitted polarized light is then reflected by a 180° phase rotator, causing this light to be reflected out of the system.  FIG. 3  shows remote phosphor system  8200 , comprising four input CPCs  8202  each coupled to a blue LED (not shown), prism block  8204 , polarized spectrum-splitting mirror  8205 , large CPC  8206  for a transparent phosphor that is not shown, and phase rotator  8208 . Mirror  8205  reflects all blue light of one polarization and only part of the other. Rotator  8208  rotates that other polarization into the plane of the first, causing it to be reflected through output port  8209 . Proper design of mirror  8205  will result in that output blue light balancing the yellow photostimulated light of the phosphor to white 
         [0046]    As the principal reflection angle of spectrum-splitting mirrors decreases, their design becomes easier to implement for both polarizations. Instead of 45°, prism configurations are possible that utilize incidence angles of 30° and 15°. 
         [0047]    The prisms shown hereafter have a refractive index of 1.64 to ensure against TIR failure. Practical considerations formerly dictated a lower index, the 1.57 value of the glass BAK-4, because a wide variety of shapes is commercially available for prisms in this material. Newer materials are now available at n=1.64, for the small wedge-prisms required for these embodiments. For example Ohara Corporation of Japan produces a suitable high transmittance glass PBM2Y. This higher index material has an added advantage in that it reduces the beam angle that strikes the dichroic filter. For example if the maximum beam angle in the dielectric collimator (with an index of 1.5) is 15°, this angle will be reduced to approximately 13.7° in the high-index prism. 
         [0048]      FIG. 4  shows a remote phosphor system  8300 , comprising photostimulative blue LEDs  8301 , 10° dielectric square CPCs  8302  (two of the four are shown), 12° wedge prism  8303  optically coupled thereto, air gap  8304 , scalene triangular prism  8305  having angles 34° (top as seen in  FIG. 4 ), 68° (left bottom) and 78° (right bottom), dichroic filter  8306 , 34° isosceles triangular prism  8307 , two oppositely oriented 22° right-angle triangular prisms  8309  with low-index gap  8310  between them and low-index gaps  8308  above each, scalene triangular prism  8311  having angles 102°, 44° (left bottom) and 34° (right bottom) and disposed across low-index gap  8312  from prism  8305 , large 15° dielectric square CPCs  8313  and  8314 , transparent-phosphor box  8315 , and auxiliary-blue LED-array  8316 . (Alternatively, the transparent phosphor can be replaced by a standard phosphor patch. The phosphor material in this case should have low absorptance to its emission.) The low-index gaps confine collimated rays that are at glancing angles, guiding them via total internal reflection (TIR) so that they stay with their beam. For example, a silicone adhesive with refractive index of 1.4 would confine rays with an angle of incidence higher than sin −1  (1.45/1.64)=62°, far less than the 75° minimum expected if the light emanates from a 15° collimator with its axis parallel to the gap. Thus, the entire effective beam from such a collimator can be confined by such a low-index gap. 
         [0049]      FIG. 5  shows the photostimulative operation of remote phosphor system  8300 . The system angle β of 34° is also shown. Square CPCs  8302  collect light from blue LEDs  8301  to form a collimated output beam shown as rays  8317  proceeding down to the left through wedge prism  8303  and across air gap  8304 , after which some of its rays glance off low-index gap  8312 . Because of its short wavelength, the blue light of this beam passes through dichroic filter  8306 , after which some of its rays glance off low-index gap  8308 . Then it internally reflects on the exit face of prism  8307 , thereby turned downward, after which some of its rays glance off low-index gap  8310 . The blue beam enters large square CPC  8314 , which concentrates it into box  8315 , wherein it is absorbed by the transparent or standard phosphor which box  8315  contains. All along the blue beam&#39;s way, the various low-index gaps keep rays from escaping the beam and ensure the whole beam is directed to enter solid dielectric square CPC  8314 . 
         [0050]      FIG. 6  shows the yellow-emitting operation of remote phosphor system  8300 . The photostimulated transparent phosphor within five-sided reflective box  8315  will emit yellow light, which large CPC  8314  collimates into a beam represented by rays  8318 . They pass upward to be internally reflected by the exit face of prism  8307 . Due to their long wavelengths, the yellow rays reflect off of dichroic filter  8306 , thereafter passing out of the exit face of prism  8307  to form yellow output beam  8319 . Low-index gap  8310  confines glancing rays, as does filter  8306 , so that all yellow rays strike the exit face of prism  8307  instead of leaving the required beam. 
         [0051]      FIG. 7  shows the auxiliary blue operation of remote phosphor system  8300 . Light from auxiliary blue LEDs  8316  is collimated by large CPC  8313  into a beam represented by rays  8320 . The leftmost rays are confined by vertical low-index gap  8310 , but also by dichroic filter  8306 . Because of their glancing angle, vertical rays encountering the filter will be reflected because their high incidence angle has lengthened their effective wavelength and shifted their filter response from transmission to reflection. The right-hand vertical rays internally reflect leftwards off the exterior of prism  8311 , and pass through low-index gap  8312 . The left-hand rays go through low-index gap  8312  and are internally reflected leftwards off air gap  8304 . Because of their short wavelength, all rays can pass through dichroic filter  8306  and pass out of the exit face of prism  8307  to form blue output beam  8321 , coincident with yellow output beam  8319  to thereby form a white beam. Although dichroic filter  8306  is specified for a principal incidence angle of 22°, the full range is 0-40°, so that some of the rays at the edge of this range may encounter reduced efficiency. 
         [0052]    The behaviors of short-pass and long-pass filters at high incidence angles are not commonly considered. In fact for current commercially available designs for incidence angles above 70 degrees, the filter does not behave as a reflector. Typical reflectance values at these incidence angles for commercially short-pass filters are on the order of perhaps 50%. This is not a sufficiently high enough performance for the preferred embodiment of  FIG. 7 . The reason for this is that the thickness of most layers in a thin film is typically around a quarter wavelength, which for a reference wavelength of 500 nm is around 125 nm. At this thickness, evanescent waves can jump across the layers at high incidence angles, via the phenomenon of frustrated total internal reflection. One way to overcome this problem that has been developed by the Inventors is to add one thick layer into the filter. The thickness of this layer should be on the order of two wavelengths. A preferred thickness is approximately 1200 nm. A suitable material for the layer is silicon dioxide. The thick layer must be designed to work in conjunction with the other layers at the angles of incidence required by the short-pass filter (at the lower incidence angles). Examples of such designs using 80 layers are shown in Tables 2 and 3 of the concurrently-filed application (docket no. 047654-0016-P1-US) for the preferred embodiment of  FIG. 8 . Note that the first layer of the coating in Table 3 uses an 1172 nm layer of silicon dioxide. Tables 1 and 4 show respectively 80-layer and 82-layer designs that work nearly as well as those of Tables 2 and 3 but without the thick first layer. 
         [0053]    It is possible to reduce the principal incidence angle to 15°, thereby reducing the range to 0-30°, enabling very high efficiency to be attained. The efficient operation of these preferred embodiments hinges on the design of the dichroic filter. The auxiliary blue output is reduced to the extent that the filter&#39;s blue transmittance is less than 100%, and likewise any loss in transmittance of the photostimulating blue reduces how much yellow light is produced. Then the yellow light itself is reflected by the filter with less than perfect efficiency. In the following preferred embodiments the filter transmits 97.8% for blue and reflects 97.8% for yellow, for an overall yellow-beam score of 95.4%. 
         [0054]    A novel 80-layer bi-material filter is disclosed in the concurrently-filed application that attains this high score over the 0-30° incidence-angle range. No previous filter of the prior art could do this, but then none was ever needed, at non-normal incidence at least, to both transmit unpolarized blue light and reflect unpolarized yellow light. The 80-layer thickness-specification listed in Table 3 of the concurrently-filed application is operative in the following preferred embodiment. This filter also reflects both blue and yellow light at high incidence angles (above 70 degrees). The filter design employs the aforementioned approach of a thick single thick layer of approximately 1200 nm as the first of 80 layers. In the original optimized design for the short-pass filter for this embodiment, the first layer had a thickness of approximately 94 nm. It was discovered that the performance of the filter in the short-pass mode (for angles 0 to 30 degrees) is insensitive to variations of the thickness of the first layer in the stack. This is not the case for traditional short-pass filters and is made possible by the development of a new algorithm for the design of such devices. (This is the subject of the concurrently filed application.) The performance of the filter at high incidence angles is greatly enhanced by increasing the thickness of the first layer. At 1200 nm thickness, the reflectance at incidence angles above 70 degrees in the direction of the yellow light (from the left as seen in  FIGS. 4 to 7 ) is theoretically perfect, 100%. In the blue light direction, the average reflectance is on the order of 99% over the same range of incidence angles. This is achieved when layer  80  of the filter is oriented so it is the first layer of the stack that is impinged upon by the rays emanating from the phosphor. 
         [0055]      FIG. 8  shows a further remote phosphor system  8400 , comprising photostimulating LEDs  8401 , 10° dielectric square CPCs  8402  (two of four are visible), first right-angle 15° prism  8403  coupled to the square CPCs  8402 , a low-index gap  8404 , a first isosceles triangular prism  8405 , a dichroic filter  8406  with either of the aforementioned 80-layer specifications, a second isosceles triangular prism  8407  (both isosceles prisms  8405  and  8407  have angles of 37.5°, 37.5° and 105°), a low-index gap  8408 , a second right-angle 15° prism  8409 , a large 15° dielectric CPC  8410 , a box  8411  enclosing a transparent or standard phosphor, an air gap  8412  between the diagonal hypotenuse of prism  8405  and an adjacent right-angle 22.5° prism  8413 , a small 15° dielectric CPC  8414  coupled to the prism  8413 , and an auxiliary blue LED array  8415  at the narrow end of the small 15° CPC  8414 . As will become apparent from the following discussion of  FIGS. 9 and 11 , the 22.5° prism  8413  can also be designed as a trapezoidal prism by trimming the knife edge of the right angle triangle prism outside the area of the CPC  8414  (to the right as seen in  FIG. 8 ), because that knife edge part is not used to transmit light. Due to their 15° angles, the wedge prisms  8403 ,  8409 , and  8413  are difficult to manufacture as separate pieces, so in actual practice each would be molded as one piece with its respective plastic CPC  8402 ,  8410 , or  8414 . For optimal performance of dichroic filter  8406 , it is desirable that it be coated on prism  8407 . This position works best for controlling beam expansion of glancing rays in either direction. 
         [0056]    Dichroic filter  8406  has two possible orientations, depending upon whether it is deposited at the top (as seen in  FIG. 8 ) surface of prism  8405  or the bottom surface of prism  8407 . The latter alternative displays somewhat better performance at high incidence angles, and is the preferred embodiment of this filter. 
         [0057]      FIG. 9  shows the photostimulative operation of remote phosphor system  8400 . The system angle α of 37.5° is also shown. The blue emission of LEDs  8401  is collimated by square CPCs  8402  into a beam represented by left-going rays  8416 , which are internally reflected off air gap  8412  to dichroic filter  8406 , with confinement of glancing rays provided by low-index gap  8404  of  FIG. 8 , exemplified by glancing ray  8423 , and by a similar low-index gap (which may be formed by a thick layer as discussed above) designed as a feature of the stack constituting filter  8406 . Due to their short wavelengths, the rays pass through filter  8406 , up into prism  8407 , with confinement of glancing rays by low-index gap  8408 . The rays total internally reflect off the prism&#39;s diagonal face, proceeding horizontally through low-index gap  8408  and across prism  8409  and into CPC  8410 , which concentrates them into box  8411  to be absorbed by the transparent phosphor therein. Box  8411  has highly reflective walls on five of its sides, with only the top available to receive blue light and transmit yellow. Box  8411  can operate with either a transparent phosphor or a standard opaque phosphor material. The opaque phosphor should be highly absorbing of blue light so the blue light only penetrates a short distance. 
         [0058]      FIG. 10  is a close-up view of how the remote phosphor system  8400  directs its yellow output. The photostimulated light generated by the transparent phosphor within box  8411  of  FIG. 8  is collimated by dielectric CPC  8410  into a beam represented by rays  8417 , which proceed rightwards through prism  8409  and low-index gap  8408 , with guidance of glancing rays provided by the exterior of prism  8409  and by filter  8406 . The horizontal rays have an incidence angle of 90-α and thus are internally reflected downward by the diagonal face of prism  8407 , onto filter  8406 , at incidence angle 90-2α. The filter reflects them back upwards to the same diagonal face, at incidence angle 90-3α, which is less than the critical angle, so that they are refracted outwards into asymmetric yellow output beam  8418 . 
         [0059]      FIG. 11  is a close-up view of how remote phosphor system  8400  directs its auxiliary blue output beam. Auxiliary blue LED array  8415  of  FIG. 8  emits light that is collimated by CPC  8414  into an output beam represented by rays  8419 , which in passing through prism  8413  are confined on the left by its exterior face. The rays pass through air gap  8412  and through prism  8405  to filter  8406 , which passes them into prism  8407 . They exit therefrom through its diagonal face to form asymmetric auxiliary blue output beam  8420 , which combines with yellow output beam  8418  to produce a white resultant. Varying the output of LED array  8415 , relative to that of photostimulative LEDs  8401 , will alter the color temperature of the white resultant exit beam. 
         [0060]    In system  8400 , the dichroic filter  8406  is designed as a short-pass filter that operates for blue and yellow beams where the central angle of each beam strikes the filter at an incidence angle of 15°. However, it is possible to design system  8400  so that the central incidence angle of the beam (blue or yellow) is less or more by as much as 2°. This is accomplished by adjusting the angles of isosceles prisms  8405  and  8407 , right angle prisms  8403  and  8409 , and prism  8413 . For example, if the central angle is reduced to 14°, then the angles of isosceles prisms  8405  and  8407  must be changed to 38°, 38° and 104° (the small angles are increased by one half a degree) while the 15° right angle prisms  8403  and  8409  become 14° right angle prisms. The angles for the prisms are determined as follows. The angle of incidence of the central ray on the filter is chosen, and that angle becomes the small angle of right triangle prisms  8403  and  8409 . The large angle of isosceles triangles is equal to 90° plus the required angle of incidence on the filter. 
         [0061]    In this example, this results in a 0.5° increase in the slope of the bottom face of prism  8405  proximate to air-gap  8412 . As there is reflection at this interface via TIR, the directional change in degrees is double this or 2*0.5° or 1°. This in turn changes the angle of incidence of the central ray on the filter from 15° to 14°. One must calculate the critical angle for the full beam to determine whether TIR will work. As the incidence angle on the filter is reduced the index of refraction of prisms  8405  and  8407  may have to be increased to insure TIR for all rays. 
         [0062]    The acute angles of right angle prism  8413  (which as mentioned above can be a trapezoidal prism) are also adjusted. In the 15° case the small angle of prism  8413  is 22.5°. In the example of a system with a 14° incidence central angle on the filter, the acute angle is increased by 1° from 22.5° to 23.5°. To determine the small angle of right angle prism  8413 , the change in incidence angle on the filter from 15° is added to 22.5° in the case of reducing the incidence angle on the filter, or is subtracted from 22.5° in the case of increasing the angle of incidence. 
         [0063]    Using the above approach it is possible to vary the central incidence angle on the filter by at least 2°, over a range from 13° to 17°. At the low end there may be some loss in performance unless very high index materials are used for the prisms. 
         [0064]      FIG. 12  is a perspective view of an embodiment of a light source incorporating remote phosphor system  8400 , showing photostimulating LEDs  8401 , four 10° dielectric square CPCs  8402 , first right-angle 15° prism  8403  coupled thereto, first isosceles triangular prism  8405 , dichroic filter  8406  with one of the two aforementioned 80-layer specifications attached to second isosceles triangular prism  8407  (both at 37.5°), low-index gap  8408 , second right-angle 15° prism  8409 , large 15° dielectric CPC  8410 , box  8411  enclosing a transparent phosphor, air gap  8412  between diagonal face of prism  8405  and adjacent right-angle 22.5° prism  8413 , small 15° dielectric CPC  8414  coupled thereto, and auxiliary blue LED array  8415 . 
         [0065]    Also shown in  FIG. 12  are connecting prism block  8416 , mounting boards  8417  and  8401 b, main holder-block  8418 , and bracket  8419 . Prism block  8416  has a male alignment feature (not shown) that is inserted in a female alignment feature (not shown) in bracket  8419 . Bracket  8419  has threads for insertion of screws (not shown) so that prism block  8416  is mechanically held in place by said screws. The output from square CPC  8414  is designed so none of its rays strike the sides of prism block  8416 , thereby facilitating the use of prism block  8416  as a means of attaching the entire optical assembly to main holder block  8418 . In order to enhance the performance of the system, surfaces proximate to air where rays are exiting or entering, such as the exit surface of prism  8406 , should have a broad band anti-reflective coating. 
         [0066]    Bracket  8419  is used to attach the upper optical assembly comprising components  8402 ,  8403 ,  8405 ,  8406 ,  8409 ,  8410  to the lower optical assembly comprising  8412 ,  8413  and  8414  (which can be molded as one piece). In order to eliminate light leaking at the points where bracket  8419  touches CPC  8410  and prism  8413 , the active surfaces of these optical components proximate to the attachment features of bracket  8419  can be coated with a thick layer of Silicon Dioxide. A suitable thickness for this coating is approximately 1 micron or two wavelengths. Low index layers  8404  and  8408  can be an optical adhesive with an index of refraction of approximately 1.5 or below. A suitable adhesive is OP-21 made by the Dymax Corporation of Torrington, Conn. The recommended thickness of the adhesive layer should be between 8 to 13 microns. This is also the recommended adhesive thickness for attaching prisms  8405  and  8407  together. Prism  8407  has dichroic filter  8406  deposited on its surface so that the adhesive layer is in fact in contact with prism  8405  and dichroic filter  8406 . Alternatively, dichroic filter can be coated onto a thin flat substrate and the filter would be adhesively bonded to both prisms  8405  and  8407 . 
         [0067]    Main holder-block  8418  has mounting feature  8420  and  8421  on two of its faces, to enable attachment of the optical assembly to boards  8417  and  8401   b.  These features are designed so there is a gap of approximately 500 microns between features  8420  and  8421  and their respective boards  8417  and  8401   b.  Once the optical assembly is aligned with one of the boards, the gap is filled with a low-shrinkage adhesive (0.1%) bonding together the board to the optical assembly. Suitable adhesives are available from Dymax Corporation. By using a 0.1% shrinkage adhesive, combined with a precision alignment system, it is possible to align the optics to be within 10 microns in the z-direction of the theoretical ideal position. Additionally, by over-sizing the optics at their base to be approximately 200 microns wider than the size of the LEDs or LED array, very high coupling efficiencies can be achieved, on the order of 97% to 99%. In order to achieve this tolerance requirement of 10 microns in the z-direction and ±100 microns in the x and y directions, a specially designed positioning device is required. A typical system that can be used to achieve the required tolerances requires two separated assemblies. Assembly one is used to hold the board  8417  or  8401   b  (one at a time). This first assembly can come in two configurations, one consisting of two rotary tables mounted on top of each other, with a goniometer on top, and the other consisting of a rotary table with two goniometers mounted on top. Assembly number two is a standard x,y,z positioner. In order to properly align the optics at least two and preferably three cameras are needed, in order that the optic and an LED can be seen from three directions in real time. A suitable lens for these cameras to achieve the aforementioned tolerances is made by Edmunds Optics of New Jersey. The company sells a variety of zoom and fixed lenses that can be attached to CCD cameras using the standard c-mount. The required tolerances can be achieved by using a zoom lens with an optical magnification of 2.5× to 10×, in conjunction with a 19-inch monitor. The CCD or CMOS camera should have a resolution of 640×480 or better. It is advantageous to choose a monochrome or color camera that has a high sensitivity, on the order of 0.1 lux or better. 
         [0068]    Main holder-block  8418  has a void feature  8422 , which is used to attach the optical assembly to the x,y,z positioner. The void has two positions to accommodate coupling with a square rod. The two positions are rotated 15 degrees from each other to accommodate the 75 degree angle between board  8401   b  and board  8417 . 
         [0069]    In order to achieve high positioning tolerances, the part of the optics proximate to the LEDs should be designed to have void features to accommodate any wire bond coming up from the LED. It was found that a clearance of 50 to 80 microns around the wirebond in the z-direction is sufficient. A good solution is to have the wire bond of the LED be placed at the corner of the chip and coming out at either 45 degrees or at a 90 degree angle. In the x,y direction the void should accommodate a 100 micron travel in any direction. In addition, it was found that the bottom of each coupling optic should have a void. It was found that the height at the center of this void (at the center part of the optic) should be from 100 to 150 microns above the lowest part (nearest the LED) of the optic. The void then is tapered such that there is nearly zero space at the edge or boundary of the optic. 
         [0070]      FIG. 13  is a perspective view of an alternative arrangement of the same components as in  FIG. 8 , but with a higher parts count. Remote phosphor system  8500  comprises: photostimulating LEDs  8501 ; four 10° dielectric square CPCs  8502 ; first right-angle 15° prism  8503  coupled to triangular prism  8504  (with 37.5° angle  8504   a , 75° angle  8504   b  and 67.5° angle  8504   c ), which in turn is coupled to prism  8503  by low-index gap  8517 . System  8500  further comprises: small isosceles triangular prism  8505  (both angles are 37.5°), which is coupled to prism  8504  by low-index gap  8520 ; dichroic filter  8506  with aforementioned 80-layer specification; large isosceles triangular prism  8507  (angles 37.5°); low-index gap  8508 ; second right-angle 15° prism  8509 , which is attached to large 15° dielectric CPC  8510 ; box  8511  enclosing a transparent or standard phosphor; air gap  8512  proximate to left short face of small isosceles prism  8505  and to upper face of triangular prism  8518  (the mirror image of triangular prism  8504 ) and third right-angle 15° prism  8513  (the mirror image of  8503 ), which is attached to prism  8518  by low-index gap  8519 ; block prism  8514 ; small 15° dielectric CPC  8515  coupled thereto; and auxiliary blue-LED array  8516 . Dichroic filter  8506  ideally should be coated upon the bottom face of large isosceles triangular prism  8506 . Block prism  8514  has inactive faces for means of attaching the optical train onto a mechanical holding fixture (not shown). This system produces the same white output beam as the embodiment of  FIG. 8 . 
         [0071]    In both the designs of  FIG. 8  and  FIG. 13 , the dichroic filter experiences a range of incidence angles, which can be ascertained by ray-tracing.  FIG. 14  shows graph  8610  with abscissa  8611  in degrees of incidence angle of photostimulating blue light  8401  passing through filter  8406  of  FIG. 8 . Ordinate  8612  is in percent relative to the maximum. Solid line  8613  is a graph of the distribution of radiant power passing through the filter, with maximum at 15°, the chief design angle. Dashed line  8614  is a graph of the cumulative distribution of energy, that encircled by a particular angle. It can be seen that the median, or 50%, value, shown by line  8615 , is 22°, showing why a range of incidence angles must be used to evaluate a candidate filter design. 
         [0072]      FIG. 15  shows graph  8620  with abscissa  8621  in degrees of incidence angle of auxiliary blue light  8419  passing through filter  8406  of  FIG. 11 . Ordinate  8622  is in percent. Solid line  8623  is a graph of the distribution of radiant power passing through the filter, with maximum at 12°, due to some extra internal reflections. Dashed line  8624  is a graph of the cumulative distribution of energy, that encircled by a particular angle. It can be seen that the median, or 50%, value is shown by line  8625  to be 18°, but this is similar enough to distribution  8615  of  FIG. 14  as to give the same higher performance. 
         [0073]      FIG. 16  shows graph  8630  with abscissa  8631  in degrees of incidence angle of yellow light  8417  reflecting off filter  8406  of  FIG. 10 . Ordinate  8632  is in percent. Solid line  8633  is a graph of the distribution of radiant power reflected by the filter, with a maximum at 12-15°, and a secondary maximum at 24-27°. Dashed line  8634  is a graph of the cumulative distribution of energy, that encircled by a particular angle. It can be seen that the median, or 50%, value is shown by line  8635  to be 21°. 
         [0074]      FIG. 17  shows spectral graph  8710 , with abscissa  8711  for wavelength in nanometers and ordinate  8712  in percentage. Line  8714  graphs the cumulative spectral intensity of the photostimulative blue LEDs  8401  of  FIG. 8 , with the accumulation done with decreasing wavelength. Line  8715  is the cumulative spectral intensity of the yellow phosphor, with the accumulation done with increasing wavelength. Both curves rise up from their crossing point at about 500 nm. Spectral transmission curves  8716  are displayed for incidence angles from 5° to 30°, showing the downward shift in wavelength with increasing incidence angle, as predicted by theory. 
         [0075]      FIG. 18  shows spectral graph  8720 , with abscissa  8721  for wavelength in nanometers and ordinate  8722  in percentage. Line  8723  graphs the photostimulative blue spectrum. Line  8724  graphs the auxiliary blue spectrum, at a  20  nm longer peak wavelength. Line  8725  graphs the phosphor spectrum. Line  8726  graphs the overall filter response, combining the curves  8716  of  FIG. 17  into one, weighted in accordance with the angular distribution of power as shown in  FIGS. 14 ,  15 , and  16 . That is, for the curves  8716  the resultant is the sum of: 3% of the 5° curve, 12% of the 10° curve, 30% of the 15° curve, 30% of the  20 ° curve, 20% of the 25° curve, and 5% of the 30° curve. This equivalent filter curve holds only for this distribution of incidence angles. When the filter curve is weighted by LED curve  8723 , the score is 97%, and by phosphor curve  8725  the score is 98.2%. Their product of 95.2% represents the filter&#39;s excellent combined performance in producing white light. 
         [0076]    This high performance-score of the filter is for the light at low incidence angles. As seen in  FIG. 9  and  FIG. 10 , however, there are some rays striking the filter at high incidence angles, and these rays must be totally reflected in order to remain with the rest of the rays in their respective beams. In  FIG. 9 , the topmost of blue rays  8416  have incidence angles of 80° and more. When these rays encounter the first filter layer, its low refractive index causes total internal reflection. Within prism  8407 , rays at high incidence angle will enter the filter, but its bottom layers turn out to have very high reflectance at all wavelengths. 
         [0077]      FIG. 19  shows a graph of the asymmetric far-field intensity of the white resultant beam. When two of the systems  8400  of  FIG. 8  are adjacently disposed with opposite placement of their blue and yellow sides, they combine into a much more useful beam.  FIG. 20  shows a graph of the high-quality square beam generated in the far field (beyond ten device widths) by the two adjacent remote phosphor systems. Systems  8400  with a combined volume that is relatively compact, about 2″ (5 cm) across can generate sufficient intensity for most downlight applications, given the luminosity of the latest blue LEDs. The square pattern is 60° across at half maximum, suitable for illuminating an 8′ (2.5 meter) long table from a 10′ (3 meter) high ceiling. Such square patterns usually require expensive and bulky projection floodlights, showing the commercial benefits of system  8400  of  FIG. 8 . 
         [0078]    Although specific embodiments have been described, various modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims. For example, the embodiment shown in  FIG. 1  can be designed to operate using a long-pass filter instead of a short-pass filter by exchanging the positions of CPC  8106  with phosphor  8107  and CPC  8104  with auxiliary blue LED  8103 . This alternative approach can be applied to many of the embodiments shown herein, including the embodiments of  FIGS. 4 and 8 , as should now be obvious to one skilled in the art of nonimaging optics or a related field. 
         [0079]    For example, various gaps at which rays with a high angle of incidence are reflected and rays with a low angle of incidence are transmitted are identified in the embodiments as “air gaps” or “low-index gaps.” Any of these gaps may be filled with air, or with a solid, liquid or other material having a lower refractive index than the prisms and/or CPCs bounding the gap. Air, because of its low refractive index, is preferred where a low minimum angle of incidence for reflection is desired. An adhesive material may be preferred where a structural connection between the bounding elements is desired. A solid low-index medium may be preferred to control loss by reflection of low incidence angle rays.