Patent Publication Number: US-6698892-B2

Title: Achieving color balance in image projection systems by injecting compensating light in a controlled amount

Description:
RELATED APPLICATION 
     This is a continuation in part of application Ser. No. 09/877,955, filed Jun. 8, 2001. 
    
    
     TECHNICAL FIELD 
     This invention relates to image projection systems and more particularly to a method for improving the brightness and color balance of images produced by such projection systems. 
     BACKGROUND OF THE INVENTION 
     Image projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction. 
     Color image projection systems operate on the principle that color images are produced from the three primary light colors: red (“R”), green (“G”), and blue (“B”). With reference to FIG. 1, a prior art image projection system  100  includes a primary light source  102  positioned at the focus of an ellipsoidal light reflector  104  to produce light rays  105  (not shown) of polychromatic light that propagate along a primary light path  106  through a rotating color wheel assembly  108 . Color wheel assembly  108  includes at least three filter sections, each tinted in a different one of primary colors R, G, and B. Light rays  105  of polychromatic light emitted by primary light source  102  propagate along light path  106  through an optical integrating device, preferably a light tunnel  110  of either a solid or hollow type, to create at its exit end a uniform illumination pattern. (A light tunnel  110  of a solid type is shown in FIG. 1.) Light tunnel  110  works on the principle of multiple reflection to achieve uniform light intensity over a rectangular area with the same dimensional proportions as the final projected image. The illumination pattern is imaged by a lens element system  112 , reflected off a light reflecting surface  114 , and transmitted through a projection lens  116  to form an image. Popular commercially available image projection systems of a type described above include the LP300 series manufactured by InFocus Corporation, of Wilsonville, Oreg., the assignee of this application. 
     There has been significant effort devoted to developing image projection systems that produce bright, high-quality color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projectors in a well-lighted room. 
     To improve the brightness of images they project, image projection systems typically employ a high-intensity discharge (“HID”) arc lamp as primary light source  102 . FIG. 2 shows an exemplary HID arc lamp  120  that includes first and second electrodes  122  and  124  separated by an arc gap  126 , which is preferably between 0.8 and 2.0 mm wide. First and second electrodes  122  and  124  and arc gap  126  are contained within a sealed pressurized chamber  128  that is filled with ionizable gases and solids. A high voltage pulse applied to first electrode  122  by an external voltage source (not shown) causes ionization of the gases and solids contained within chamber  128  such that a steady state reversible reaction occurs, resulting in the formation of plasma. The current flow developed across arc gap  126  is maintained by external lamp driving electronic circuitry, thereby maintaining the plasma generated by the steady state reversible reaction. The plasma emits bright polychromatic light. The components of arc lamp  120  are enshrouded in a glass envelope  130 , and conductive foil plates  132  are attached to electrodes  122  and  124  to dissipate heat and thereby prevent cracking of glass envelope  130 . 
     Thus HID arc lamps produce a point source of intense polychromatic light. Placing the HID arc lamp adjacent to an ellipsoidal reflector allows focusing of the intense polychromatic light with high precision onto a color wheel. HID arc lamps have many favorable attributes, such as high intensity, efficiency, and reliability; but, unfortunately, the polychromatic light emitted by HID arc lamps is not balanced in terms of its emission energy content. Specifically, HID arc lamps provide greater emission energy content at the blue end of the color spectrum than at the red end, causing an emission energy imbalance. There have been several attempted approaches to solving this problem. 
     One attempt to minimize illumination emission energy imbalance entailed increasing the angular extent (physical size) of the color wheel R filter segment relative to the angular extent of the B filter segment and/or increasing the attenuation of the color wheel B filter segment relative to the attenuation of the R filter segment. A second attempt entailed reducing overall brightness levels through color lookup electronics to achieve “headroom” for color adjustments. Unfortunately, these attempts either caused temporal artifacts or decreased image brightness. A third attempt entailed adding a white filter segment to the color wheel to provide a “white peaking” function. The addition of a white filter segment increased image brightness but resulted in a loss of brightness of saturated colors. Unfortunately, these optical components caused a significant amount of light to escape from the primary colors. A fourth attempt entailed simply employing a more powerful arc lamp in the projection system. When implemented in compact portable projectors, this method led to heat, size, weight, cost, and reliability issues. 
     What is needed, therefore, is an image projection system that is implemented with an improved technique for achieving increased image brightness and adjustable color balance while minimizing light loss. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is, therefore, to provide an apparatus and a method for improving the brightness and color balance of an image projected by and for minimizing the light loss from an image projection system. 
     Another object of the present invention is to adjust the color temperature of the projected image to a desirable setting without introducing perceptible image artifacts, losing bit depth, or perceptibly reducing image brightness. 
     The present invention achieves improved image brightness and color balance of an image projection system illuminated by a primary source of polychromatic light. The invention entails adding to the image projection system an auxiliary light source from which compensating light co-propagates with the polychromatic light along a primary light path. The compensating light has an emission energy content that minimizes an emission energy imbalance introduced by the primary light source. For example, in the above-mentioned instance of insufficient emission energy content at the red end of the color spectrum, the auxiliary light source provides compensating light whose emission energy content corresponds to red light and thereby minimizes the emission energy imbalance. 
     In a first preferred embodiment, the auxiliary light source is affixed at a location near the entrance end of the optical integrating device of the image projection system such that the compensating light coincides with the primary light path at a location upstream of the place where the first paraxial reflection occurs. Affixing the auxiliary light source at a location near the entrance end of the optical integrating device causes minimal light loss because a minimal amount of polychromatic light is incident near the entrance end of the optical integrating device. For this reason, the auxiliary light source of the first preferred embodiment improves the brightness and/or color balance of the projected image while minimizing the amount of light loss within the image projection system. 
     In a second preferred embodiment, the auxiliary light source is coupled to the light reflector adjacent to the primary light source, thereby allowing the compensating light emitted from the auxiliary light source to be directed through the image projection system with the same efficiency as that of the polychromatic light generated by the primary light source. The light reflector is preferably coated with a color selective transmission coating that transmits the emission energy of the light emitted by the auxiliary light source and reflects all other emission energies. This coating minimizes the loss of light from the primary light source through the area in which the auxiliary light source is affixed. 
     In a third preferred embodiment, the amount of electrical energy flowing into the auxiliary light source, which can be coupled to the image projection system consistent with the teachings of either of the first or second preferred embodiments, can be adjusted such that a desired color temperature may be achieved without introducing perceptible image artifacts, losing bit depth, or reducing perceptible image brightness. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric pictorial view of a prior art color image projection system. 
     FIG. 2 is an enlarged, diagrammatic side elevation view of a prior art HID arc lamp. 
     FIG. 3 a  is a fragmentary oblique view of a first embodiment of an illumination subsystem added to the prior art image projection system of FIG. 1 in which an auxiliary light source is affixed at a location near the entrance end of a solid light tunnel before the location of the first paraxial reflection. 
     FIG. 3 b  is an enlarged fragmentary side elevation view of the first embodiment of the illumination subsystem of FIG. 3 a  implemented with an alternative prism. 
     FIGS. 4 a  and  4   b  are fragmentary side elevation views of different implementations of the first embodiment of the illumination subsystem implemented with alternative optical fibers. 
     FIG. 5 is a fragmentary isometric view of the first embodiment of the illumination subsystem of FIG. 3 a  implemented with a fiber optic bundle attached to an optical integrating device. 
     FIGS. 6 and 7 show for all azimuthal angles on-axis a cone representing the distribution of the intensity of light exiting the light tunnel of, respectively, the prior art image projection system of FIG.  1  and of the image projection system of either of FIG. 3 a  or FIG. 3 b.    
     FIG. 8 is a fragmentary oblique view of a first alternative implementation of the first illumination subsystem embodiment in which the auxiliary light source is offset to a corner of a solid light tunnel. 
     FIG. 9 is a fragmentary oblique view of a second alternative implementation of the first illumination subsystem embodiment in which the auxiliary light source is affixed to an entrance end of a hollow light tunnel. 
     FIGS. 10 a ,  10   b , and  10   c  are fragmentary side elevation views of the illumination subsystem of FIG. 9 implemented with alternative optical integrating devices. 
     FIGS. 11 a  and  11   b  are fragmentary oblique views depicting two configurations of a third alternative implementation of the first illumination subsystem embodiment in which multiple auxiliary light sources are affixed to, respectively, corresponding or opposite corners of opposed surfaces of the solid light tunnel. 
     FIG. 12 is a fragmentary oblique view of a fourth alternative implementation of the first illumination subsystem embodiment in which multiple light sources are affixed to the same surface of the solid light tunnel. 
     FIG. 13 a  is an enlarged, diagrammatic side elevation view of a fifth alternative implementation of the first illumination subsystem embodiment in which a pair of flyseye lenslets are implemented as the optical integrating device. 
     FIG. 13 b  is an enlarged view of the illumination subsystem of FIG. 13 a.    
     FIG. 14 is a diagram of a second embodiment of an illumination subsystem in which an auxiliary light source is positioned adjacent to a light reflector and emits compensating light that is coupled with a primary light source. 
     FIG. 15 is a diagram of an alternative implementation of the second embodiment of the illumination subsystem of FIG. 14 in which multiple auxiliary light sources are used. 
     FIG. 16 is a simplified circuit diagram of an electric current control device that sets the amount of light emitted by an auxiliary light source of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Different embodiments of the present invention, described by way of example, position an auxiliary light source at different locations in image projection system  100  of FIG. 1 to compensate for the emission energy imbalance in the emission spectrum of primary light source  102 . Skilled persons will readily appreciate that the present invention can be implemented in other types of image projection systems, such as a three-path projection system. 
     FIG. 3 a  shows a schematic view of a first embodiment of the present invention, in which an auxiliary light source  140  is affixed at a location near an entrance end  142  of an optical integrating device, preferably light tunnel  110 . Auxiliary light source  140  preferably includes a solid state light-emitting device  144 , such as a light-emitting diode (LED), from which compensating light propagates through an optical fiber  146  into an optical coupling device, preferably a prism  148 . Prism  148  directs the compensating light into light tunnel  110  at an appropriate angle to cause the compensating light to coincide with light rays  105  of polychromatic light propagating along primary light path  106 . In the embodiment depicted in FIG. 3 a , optical fiber  146  is affixed to prism  148  on an input prism face  150  that is substantially parallel to a first light tunnel surface  152  to which prism  148  is affixed. Prism  148  is affixed at a location near entrance end  142  of light tunnel  110  upstream of a location  154  where the first paraxial reflection occurs. Providing optical contact between prism  148  and first light tunnel surface  152  before location  154  of the first paraxial reflection minimizes loss of the polychromatic light from light path  106  into prism  148 . 
     Light tunnel  110 , shown in FIG. 3 a , is one exemplary optical integrating device; alternative optical integrating devices are discussed in detail later with reference to certain implementations of this first embodiment. Light tunnels are commonly implemented in image projection systems to create a uniform illumination pattern with the same dimensional proportions as the final desired image. Light tunnels operate on the principle of multiple reflection, wherein transmitted light reflects off all sides of the light tunnel such that light of substantially uniform intensity is emitted from the output end of the light tunnel. Light tunnel  110  is preferably of rectangular shape so that the uniform illumination pattern of light propagating from an exit end  156  of light tunnel  110  of rectangular shape. Light tunnel  110  is also preferably composed of a solid glass rod. Light tunnel  110  is preferably wider than prism  148  so that the total surface area of light tunnel  110  that supports prism  148  is minimized and thus the amount of loss of polychromatic light from primary light path  106  is minimized. An exemplary solid light tunnel is 4.5 mm×6.0 mm×40 mm long. 
     Light-emitting device  144  can be any light source including an LED, a laser, and an arc lamp. An LED is a preferred solid state light-emitting device because it emits virtually monochromatic light and is compact and inexpensive. LEDs that emit light with an emission spectrum corresponding to red light typically emit approximately 30 lumens of red light. This additional red light generally effects a 10 percent increase in red light emission energy content in primary light path  106 . The introduction of red light allows for use of a color wheel with a smaller red segment and larger green and white segments to increase overall light transmission. 
     Optical fiber  146  can be made of any suitable material, but is preferably plastic or glass. Optical fiber  146  can be any size that is appropriate to the image projection system, but preferably has a diameter of approximately 1 mm because such an optical fiber is inexpensive and more robust than an optical fiber with a smaller diameter. Optical fiber  146  may be of any suitable shape that is appropriate to the image projection system. Optical fiber  146  depicted in FIG. 3 a  is a straight fiber. Alternatively, optical fiber  146  may be curved, as shown in FIG. 4 a.    
     Optical fiber  146  may be directly coupled to light tunnel  110 . Such coupling may be implemented in any suitable, conventional way, but one of the following two arrangements is preferred. In the instance of a hollow light tunnel, an exit end  302  of optical fiber  146  abuts entrance end  142  of light tunnel  110 , as is shown in FIG. 4 a.  In this alternative, optical fiber  146  is preferably attached to a corner of entrance end  142  of light tunnel  110  so that loss of polychromatic light emitted by primary light source  102  and reflected or refracted off optical fiber  146  is minimized. In a second alternative arrangement, exit end  302  of optical fiber  146  abuts a first light tunnel surface  152 , as shown in FIG. 4 b . Both methods allow compensating light propagating from solid state light-emitting device  144  to exit optical fiber  146  and coincide with polychromatic light emitted by primary light source  102 . 
     The benefits of the illumination subsystem of the present invention can also be achieved without the incorporation of optical fiber  146  into auxiliary light source  140 . In an illumination subsystem configured without optical fiber  146 , compensating light propagating from solid state light-emitting device  144  directly enters prism  148 . 
     Alternatively, the benefits of the illumination subsystem of the present invention can be achieved by providing multiple optical fibers  146  in a fiber bundle to direct compensating light emitted by the solid state light-emitting device  144  into an optical integrating device. FIG. 5 shows multiple separate fiber bundles, each formed with multiple fibers. The ends of multiple optical fibers  146  may be embedded in an optical integrating device  306  that is made of an optical material with an index of refraction that corresponds to the index of refraction of the material used to form light tunnel  110 . Optical fibers  146  are embedded at an angle with respect to light path  106  such that the compensating light they emit coincides with polychromatic light from primary light source  102  within light tunnel  110 . Optical assembly  308 , including optical fibers  146  and optical integrating device  306 , may be affixed to any side of light tunnel  110  (alternative attachments shown in phantom lines). One advantage to use of this alternative implementation is that optical assembly  208 , optical fibers  146 , and optical integrating device  306 , can be separately constructed and installed with an optical adhesive, resulting in reduced manufacturing costs. Alternatively, multiple optical assemblies  308  may be attached to light tunnel  110 . 
     Compensating light emitted by solid state light-emitting device  144  and transmitted through optical fiber  146  can be coupled into light tunnel  110  by an optical coupling device. Exemplary optical coupling devices include prisms, glass rods, and mirrors; however the preferred optical coupling device is prism  148 . Prism  148  is preferably attached to optical fiber  146  using an optically transparent adhesive, e.g., a UV-cured adhesive. The attachment of prism  148  to optical fiber  146  is such that the compensating light directed through optical fiber  146  reflects off prism reflection surface  158  with an angle of incidence that allows the compensating light to coincide with the light rays  105  of polychromatic light that propagate along primary light path  106 . For example, FIG. 3 b  shows one exemplary illumination subsystem in which compensating light exiting optical fiber  146  has an approximately 45 degree angle of incidence with respect to a prism reflection surface  158  of a prism  148   a  to allow the compensating light to coincide with light path  106  before the location of first paraxial reflection  154 . As shown in FIG. 3 b , prism  148   a  has an input prism face  150   a  that is angularly inclined relative to light tunnel surface  152  to illustrate an alternative propagation path of light emitted by solid state light-emitting device  144 . Prism  148  need not have an inclined prism face, this implementation is merely exemplary. 
     Prism  148  may be of any size or shape suitable for the image projection system. For example, input prism face  150  depicted in FIG. 3 a  is substantially parallel to first light tunnel surface  152  on which prism  148  is mounted, whereas the input prism face  150   a  depicted in FIG. 3 b  is not parallel to first light tunnel surface  152 . 
     The benefits of the illumination subsystem of the present invention can be achieved without the incorporation of an optical coupling device into auxiliary light source  140 . In an illumination subsystem configured without an optical coupling device, compensating light is injected directly into light tunnel  110  via optical fiber  146  or solid state light-emitting device  144 . 
     Providing optical contact between prism  148  and first light tunnel surface  152  before location  154  of the first reflection minimizes loss of the polychromatic light entering light tunnel  110  through entrance end  142  because little polychromatic light is incident on the side surfaces of light tunnel  110  close to entrance end  142 . This minimal light loss is demonstrated by a comparative relationship of light intensity distributions depicted in FIGS. 6 and 7. FIG. 6 is a schematic diagram showing a cone of light exiting light tunnel  110  of the prior art image projection system  100  of FIG.  1 . The cone of light approximates for all azimuthal angles on-axis the angular light intensity distribution of polychromatic light emitted by primary light source  102  following transmission through light tunnel  110  and upon exit from light tunnel  110  at exit end  156 . In comparison, FIG. 7 is a schematic diagram showing a cone of light exiting light tunnel  110  of the image projection system of either of FIG. 3 a  or FIG. 3 b , in which auxiliary light source  140  introduces compensating light into the image projection system. The cone of light depicted in FIG. 7 approximates the angular light intensity distribution of polychromatic light emitted by primary light source  102  following transmission through light tunnel  110  and upon exit from light tunnel  110  at exit end  156 . A notch  160  at the top of the cone of light shown in FIG. 7 represents an approximately 3 percent loss of polychromatic light due to the affixing of optical coupling prism  148 . Such light loss is minimal in light of the total gain in emission energy content corresponding to red light resulting from the incorporation of auxiliary light source  140  into the image projection system of the present invention. 
     Auxiliary light source  140  of the first embodiment of the present invention may be affixed to the optical integrating device at any location near entrance end  142 . Affixing auxiliary light source  140  at a location near entrance end  142  of the optical integrating device causes minimal light loss because a minimal amount of light is incident on the integrating device near its entrance end. While affixation of auxiliary light source  140  is preferably effected at any location near entrance end  142 , affixation at certain locations offers various benefits, which are discussed below. 
     FIG. 8 depicts a first alternative implementation of the first embodiment of the present invention in which auxiliary light source  140  is affixed to first light tunnel surface  152  and is offset to a corner of light tunnel  110 . This first alternative implementation is especially beneficial because it reduces the loss of polychromatic light propagating through the point of affixation of auxiliary light source  140 . 
     FIG. 9 depicts a second alternative implementation of the first embodiment of the present invention in which auxiliary light source  140  is affixed to a surface of an entrance end  170  of a hollow light tunnel  110   a.  While this alternative implementation may introduce additional thickness to entrance end  170  of light tunnel  110   a  and thereby impact the spacing of color wheel assembly  108 , this implementation allows for the use of a hollow light tunnel instead of a solid light tunnel of the type shown in FIGS. 3 a ,  3   b , and  8 . Hollow light tunnels are less expensive and shorter in length as compared to a solid light tunnel that achieves an equivalent illumination uniformity at the tunnel output end. 
     FIGS. 10 a ,  10   b , and  10   c  depict three alternative optical integrating devices that can be used in the image projection system shown in FIG.  9 . FIG. 10 a  shows use of an injection prism  312  that has a reflectance surface at a 45 degree angle relative to entrance end  142  of light tunnel  110 . Injection prism  312  can be used with a hollow light tunnel of the type shown in FIG. 9 or with a solid light tunnel of the type shown in FIG.  8 . FIG. 10 b  shows use of a beam splitter prism  320  in the image projection system of FIG.  9 . Beam splitter prism  320  includes a dichroic mirror  316   a  with a compensating prism  322  situated at a 45 degree angle to entrance end  142  of light tunnel  110 , thereby forming a beam splitter cube that allows light from primary light source  102  to pass through the beam splitter cube without getting lost. FIG. 10 c  shows use of a dichroic mirror  316   b  inclined at an acute angle to entrance end  142  of light tunnel  110  of the image projection system of FIG.  9 . 
     The image projection system of the present invention may also include multiple auxiliary light sources. The use of multiple auxiliary light sources allows the user to implement lower output, and therefore less expensive, solid state light-emitting devices while effecting a reduction in emission energy imbalance similar to that effected by using a single high output solid state light source. Alternatively, the use of multiple auxiliary light sources allows the user to effect an increased reduction in emission energy imbalance by introduction of an increased amount of compensating light whose emission energy content reduces an emission energy imbalance. The multiple auxiliary light sources may be affixed to any side of light tunnel  110  or  110   a  but are preferably affixed at a location near entrance end  142  or  170 , before the location of first paraxial reflection  154 . 
     FIGS. 11 a  and  11   b  depict two preferred implementations of a third alternative implementation of the first embodiment of the present invention in which one of multiple auxiliary light sources is affixed to first light tunnel surface  152  and one of multiple auxiliary light sources is affixed to a second light tunnel surface  174  that is opposite first light tunnel surface  152 . FIG. 11 a  shows a configuration in which a first auxiliary light source  176  is affixed to first light tunnel surface  152  and a second auxiliary light source  178  is affixed to second light tunnel surface  174  such that first and second auxiliary light sources  176  and  178 , respectively, are located on opposite corners of light tunnel  110 . FIG. 11 b  shows an alternative configuration in which first and second auxiliary light sources  176  and  178 , respectively, are located on corresponding corners of light tunnel  110 . 
     FIG. 12 depicts a fourth alternative implementation of the first embodiment of the present invention in which the multiple auxiliary light sources  176  and  178  are affixed to either of first light tunnel surface  152  (solid lines) or second light tunnel surface  174  (phantom lines). 
     Skilled persons will appreciate, therefore, that a prism may be placed on the entrance surface, any side surface, or top or bottom surfaces of a solid or hollow light tunnel. 
     FIGS. 13 a  and  13   b  show a fifth alternative implementation of the present invention in which a second exemplary optical integrating device is implemented. This optical integrating device is a pair of flyseye integrator plates each containing an array of lenslets designed to create multiple overlapping images so that any nonuniformity in one lenslet is integrated out at a display device (DMD)  380 . FIG. 13 a  shows an embodiment of the present invention in which light emitted by an HID arc lamp  120  strikes (or is incident on) a first set of flyseye lenslets  350 . Each first lenslet  350  has the same aspect ratio as display device  380 . Light exits first flyseye lenslets  350  and enters a second set of flyseye lenslets  352 , each of which is spatially aligned with corresponding first flyseye lenslets  350 . Second flyseye lenslets  352  image the apertures of first flyseye lenslets  350  onto display device  380 . A condensing lens  116  overlaps the multiple images at lenslets  350  created by corresponding lenslets  352  onto display device  380 . First and second flyseye lenslets,  350  and  352  respectively, may be any size and shape appropriate to the image projection system, but are preferably 4×6 mm rectangular. 
     As shown in FIG. 13 a , auxiliary light source  140  is affixed at a location near first flyseye lenslet  350 . Compensating light emitted by solid state light-emitting device  144  preferably passes through a light collection lens  354 , an integrator tunnel  356 , and an integrator imaging lens  358  before encountering mirror  360 . Mirror  360  may be of metallic or multilayer dielectric type. If mirror  360  is a multilayer dielectric type, it can be designed to reflect the compensating light while still transmitting much of the polychromatic light from the primary light source. Compensating light exiting integrator imaging lens  358  reflects off mirror  360  through one of first flyseye lenslets  350  and through one of second flyseye lenslets  352 , which reflection causes rays of compensating light to coincide with light rays  105  of polychromatic light. This alternative implementation results in first flyseye lenslet  350  being uniformly filled with compensating light such that the resulting image projected by the projection device contains excellent color uniformity. 
     Implementation of the flyseye integrator plates may also involve an auxiliary light source without an integrator tunnel  356 . Thus compensating light emitted by solid state light-emitting device  144  passes through light collection lens  354  and integrator imaging lens  358  before being reflected by mirror  360  through the flyseye optical integrating device. One of first flyseye lenslets  350  is filled with compensating light exiting the auxiliary light source; thus no polychromatic light exiting primary light source  102  enters first flyseye lenslet  350 . One advantage of this alternative implementation is that the auxiliary light device can be easily coupled to the remainder of the image projection system. However, this alternative implementation may result in poor color uniformity in the final image as a consequence of non-uniform filling of first flyseye lenslet  350 . 
     As shown in FIG. 13 b , the auxiliary light source may lack an integrator imaging lens  358 . In such a image projection system, compensating light emitted by solid state light-emitting device  144  passes through light collection lens  354  and integrator tunnel  356  before being reflected off mirror  360 , located near exit end  362  of integrator tunnel  356 . The compensating light reflects off a 45-degree angled exit end  362  of integrator tunnel  356  by total internal reflection or a mirror coating and is directed through first flyseye lenslet  350 . 
     In a second preferred embodiment, the auxiliary light source is coupled to a light reflector adjacent to the primary light source and thereby allows the compensating light to propagate through the image projection system with the same efficiency as that of the light generated by the primary light source. 
     FIG. 14 shows a schematic diagram of a second embodiment of the present invention in which auxiliary light source  140  is positioned adjacent to an outer surface  186  of light reflector  104  and is coupled to primary light source  102 , which is preferably an HID arc lamp  120 . Auxiliary light source  140  emits a compensating light beam that is focused by an optical focusing element  180 , and propagates through a compensating light entrance zone  184  on light reflector  104  to pass through arc gap  126  of arc lamp  120  and strike an inner surface  188  of light reflector  104 . 
     To enable propagation of the compensating light beam through light reflector  104 , inner surface  188  of light reflector  104  at compensating light entrance zone  184  carries no coating, a low reflection coating, or preferably a wavelength selective transmission coating, the last of which transmits light of wavelengths equal to the compensating light wavelength while reflecting visible light of wavelengths not equal to the compensating light wavelength. This compensating light entrance zone coating is generally a different coating material from that applied to the remainder of inner surface  188  of light reflector  104 . Inner surface  188  typically carries a metallic or dielectric coating to achieve maximum reflectance over the operating range of wavelengths of image projection system  100 . Light reflector  104  is preferably coated with a spectrally selective transmission coating, which transmits compensating light through the image projection system with the same efficiency as polychromatic light generated by HID arc lamp  120  and reflects light transmitted by other light sources. Polychromatic light emitted by HID arc lamp  120  may be lost through compensating light entrance zone  184  of light reflector  104 . The wavelength selective transmission coating minimizes the loss of polychromatic light emitted by HID arc lamp  120  which light would otherwise pass through an uncoated compensating light entrance zone  184 . 
     Light reflector  104  is preferably made of a material such as glass that transmits light so that the compensating light beam can pass through the light reflector wall on its way to arc gap  126 . Depending on the design goals and the details of downstream optical parts for the image projection system, light reflector  104  may have an ellipsoidal, a paraboloidal, a general aspheric, or a faceted form. Because it provides illumination beam collection and focusing, light reflector  104  preferably includes a cold mirror. Since outer surface  186  of light reflector  104  is effectively an additional lens surface that refracts the incoming compensating light, outer surface  186  preferably is smooth and well controlled. Other specifications such as size, length, focal length, and thermal characteristics are determined by the design goals of the image projection system. 
     As stated above with respect to the first embodiment of the present invention, the solid state light emitting device contained within auxiliary light source  140  can be any solid state light source including an LED, a laser, or an arc lamp. LEDs are preferred because they emit virtually monochromatic light and are compact and inexpensive. LEDs that emit light with an emission spectrum corresponding to red light typically emit approximately 30 lumens of red light. This additional amount of red light generally effects a 10 percent increase in red light emission energy content in the primary light path. 
     Compensating light can alternatively be delivered using fiber optics to transfer the compensating light from auxiliary light source  140  to optical focusing element  180 , which collects and focuses the compensating light propagating through compensating light entrance zone  184  of light reflector  104  and into arc gap  126 . 
     FIG. 15 depicts an alternative implementation of the second embodiment of the illumination subsystem of the present invention in which multiple auxiliary light sources  140  (two shown) are placed around the perimeter of light reflector  104  to more uniformly distribute compensating light within the illumination beam striking color wheel assembly  108  (not shown) and thus increase the uniformity with which the compensating light is distributed in the final projected image. Each auxiliary light source  140  is focused through arc gap  126  and is aligned so that each compensating light beam, having propagated through arc gap  126 , does not impinge upon any other compensating light beam entering through compensating light entrance zone  184 . The quantity, beam size, location, and orientation of the auxiliary light sources  140  are determined by the specific performance goals of the image projection system. 
     Thus, the auxiliary light source emitting compensating light having an emission energy content that minimizes an emission energy imbalance may be affixed to the image projection system of the present invention in any of the above-mentioned locations in order to effect improved image brightness and color balance. The addition of the auxiliary light source also allows the user to control the color temperature of the projected image without perceptibly decreasing image brightness or bit depth. 
     As shown in FIG. 1, prior art image projection systems have a primary light source  102  operating with a constant flow of electric current that creates an image with a color temperature of 8000-10,000 Kelvin. Because the desired color temperature of a projected image is approximately 6500 Kelvin, color temperature adjustment may be necessary. In the prior art image project systems, color temperature adjustment entailed discarding spectrally selective light, typically light in the green and blue wavelength ranges, by implementing software that decreases the transmissivity of light corresponding to these colors. However, discarding light to achieve the desired color temperature causes a decrease in light exiting the image projection system, resulting in a consequent decrease in image brightness. Further, the spectrally selective disposition of light creates undesirable shading artifacts in the projected images. 
     For example, an exemplary LP 335 image projection system manufactured by InFocus Corporation, of Wilsonville, Oreg., the assignee of this application, contains a 45 degree white segment and has a color temperature of 7100 Kelvin. To adjust the color temperature to the desired 6500 Kelvin, the green and blue light transmissions are reduced by 40% each. This reduction results in a 27% decrease in image brightness and a green and blue bit depth loss of 40% each, creating shading artifacts in the projected image. 
     In contrast, the image projection system of the present invention allows the user to achieve the desired color temperature without decreasing image brightness or creating shading artifacts. Specifically, the image projection system of the present invention allows the user to control the electrical energy flowing through the auxiliary light source and thereby effect color adjustment without discarding spectrally selective light. 
     FIG. 16 is a simplified circuit diagram of an electric current control device that sets the amount of current flowing through light-emitting diode  144  and thereby sets the amount of red light it emits. With reference to FIG. 16, a controller (not shown) that synchronizes the operation of color wheel assembly  108  and positions of light reflecting surface  114  applies at particular times a voltage to an inverting input of an operational amplifier  402 . The output of amplifier  402  is connected to the base terminal of a PNP transistor  404  to control the amount of current flowing through it in response to the voltage applied by the controller. The emitter and collector terminals of transistor  404  are connected to, respectively, a bias voltage supply and the anode of light-emitting diode  144  to cause current flow through it. The cathode of light-emitting diode  144  is connected to a sampling resistor  406 , which is connected to ground. The value of sampling resistor  406  develops a voltage that is applied to the noninverting input of amplifier  402 . 
     Whenever there is a zero steady-state potential difference across the inverting and noninverting inputs of amplifier  402 , the current flowing through light-emitting diode  144  remains fixed at a value corresponding to the voltage level set by the controller. Thus, operational amplifier  402  and transistor  404  function as a voltage-to-current converter that sets the amount of current flowing through light-emitting diode  144 . In operation, the controller applies a specified voltage to the inverting input of amplifier  402  during the time when the R filter segment of rotating color wheel assembly  108  intercepts polychromatic light rays  105  propagating from primary light source  102  to enhance the amount of red light transmission. 
     Thus the amount of compensating light emitted by auxiliary light source  140  can be adjusted to achieve the desired color temperature. Referring to the above-mentioned exemplary LP 335 image projection system, a user of the present invention can achieve the desired color temperature of 6500 Kelvin by increasing the amount of electrical energy flowing through auxiliary light source  140  so that it emits 45% of the total red light. Because compensating light is added to the image projection system, as compared to being discarded, as occurs in prior art image projection systems, the image brightness increases by 4% and image artifacts are not introduced. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.