This invention relates to frame-sequential color projection displays, and more particularly relates to such displays employing multiple sets of monochromatic light sources such as light emitting diodes (LEDs). Frame-sequential color displays are characterized by having a single electro-optical light modulating device, such as a liquid crystal display (LCD) or a digital micromirror device (DMD), which produces a full color display by sequentially processing primary color sub-frames of a display signal at a rapid frame rate, so that the observer integrates the primary color sub-frames into a full color image. The advantages of frame-sequential color projection displays include lower cost than multi-panel additive color systems, and higher resolution than single-panel spatial filter color systems. However, current systems employ moving parts, e.g., a color wheel, to generate monochromatic illumination for the primary color sub-frames from a wide-band light source, synchronized with corresponding information displayed. If monochromatic light sources are used, synchronization of colored illumination can be achieved by electric switching rather than by mechanical means. Furthermore, illumination power efficiency can be improved since the light sources during the absent color frame time can be turned off. U.S. patent 6,224,216 discloses a frame-sequential color projection display system employing monochromatic light sources. Red, green, and blue light from arrays of light emitting diodes (LEDs) is collected, combined and guided by bundles of optical fibers to a single light integrator and then to a display device to generate frame-sequential color displays. In an alternate embodiment, three separate optical fiber bundles guide the red, green and blue light to three separate display devices. The light modulated by the three display devices is then combined by an X-cube. While the general concept of using monochromatic light sources to illuminate a frame-sequential color projection display without moving parts is known, the use of optical fiber bundles as the means for collecting, combining and relaying the source light results in a bulky arrangement which is difficult to handle and expensive to manufacture. In accordance with the invention, a frame-sequential color projection display system employs a compact light engine having compact light input modules to input light from sets of monochromatic light sources such as LEDs to color combining and light integrating elements. Each light input module includes compact light collection means such as arrays of individual reflectors associated with individual LED chips, and includes a single light coupling element for coupling the light to a color combining element. The color combining element passes the light to a light integrator which integrates the combined light from the input modules. The light integrator may be of any known type such as a lens array, a lenticular array or a lens in a particular configuration, where input and output planes are located approximately at focal planes of the lens separately. This configuration is especially useful for an input source such as an array. The light integrator may also be a homogenizing light guide of the type described in copending U.S. patent application S.N. 10/161,798 (Attorney Docket No. PHUS 020,170), filed June 4, 2002, and assigned to the same assignee as this application. The individual sets of monochromatic light sources are optically arranged and electrically controlled in a way that allows each set of light sources to illuminate the imaging device anytime for any defined duration as needed, enabling them to be switched on time-sequentially and synchronously with the scanning of the device. Thus, a frame- sequential color display is realized without moving parts and at a high power efficiency, and in a compact, easy to manufacture configuration. Moreover, such a frame-sequential color projection display can readily incorporate more than the three primary colors to achieve a wide color gamut. As used herein, the term 'monochromatic' encompasses both true monochromatic sources as well as sources having a relatively small FWHM (Full- Width Half-Maximum), preferably 15 to 40 nm or less. FWHM is defined as the width of the wavelength emission peak at half the height of the maximum intensity of the central wavelength of emission of the source. Moreover, the term 'monochromatic' encompasses two or more sources having the same perceived color, but slightly different peak wavelengths, for example, blue- emitting LEDs having peak wavelengths of 455 nm and 470 nm. LEDs satisfying these conditions are readily commercially available. Moreover, the term 'monochromatic' encompasses LED sources which are combined with one or more light-emitting phosphors in order to enhance or shift the light output of the LED sources. Typically, a phosphor layer in the form of a coating on the outside of the LED is excited by the light output of the LED to emit light in a similar or different wavelength range. In accordance with one aspect of the invention, there is provided a light engine for a frame sequential color projection display system comprising at least two light input modules, the modules having monochromatic light sources of different colors, a color combiner for combining the different colors into a light beam, and a light integrator for integrating the light beam from the color combiner. In addition to having a set of monochromatic light sources, each module also has light collection means for collecting light from the sets of sources, and a light coupling element for coupling the collected light into the color combiner. Preferably, the monochromatic light sources are sets of LED chips, and the light collection means are arrays of reflectors, each reflector associated with one LED chip or one group of more than one closely packaged LED chips. In accordance with a preferred embodiment of the invention, the light engine has three light input modules, one for each of the primary colors red, green and blue, and the color combiner is an X-cube having two internal crossed dichroic filters. The dichroic filters are set to pass light of the source colors into the homogenizing light guide. The light collection means is preferably one or more reflective elements, but could alternately be refractive, e.g., one or more lens elements. The light coupling elements are preferably light guiding elements of the type described in copending U.S. patent application S.N. 10/161,798 (Attorney Docket No. PHUS 020,170), filed June 4, 2002, and assigned to the same assignee as this application. In accordance with another preferred embodiment of the invention, the light engine has more than three light input modules, e.g., five modules, in order to widen the color gamut of the display. In this embodiment, the light input modules are positioned on side and end faces of a color combiner comprising an elongated light guiding element. The color combiner has a series of internal dichroic filters distributed along its length, and positioned to receive light from the light input modules and to direct the light along the length of the color combiner to a homogenizing light guide. In accordance with another aspect of the invention, there is provided a frame- sequential color projection display system comprising a light engine of the invention as described herein; an imaging device for modulating the light from the light engine; relay optics for relaying the light from the light engine to the imaging device; projection optics for projecting the modulated light to a display screen; driving electronics for driving the light engine; and control electronics for inputting a display signal and for providing an image scanning signal to the imaging device and for providing synchronizing signals to the driving electronics. Preferably, the imaging device is a non-emissive micro imaging devices, such as a reflective liquid crystal on silicon (LCOS) or digital light processor (DLP). Such projection display systems are useful in business and consumer applications, for converting digital and analog display signals into full color graphic and still image displays, as well as full motion video and movies. In accordance with yet another aspect of the invention, a frame-sequential color projection display system according to the invention employs a driving method in which the sets of LED chips are driven by pulsed currents having a peak value of at least 5 times the rated maximum DC driving current.

In the drawings: Figure 1 is a schematic diagram depicting a frame-sequential color projection display system of the prior art employing monochromatic light sources;

Figure 2 is a schematic diagram showing one embodiment of a three-color light engine of the invention, suitable for use in a frame-sequential color projection display system of the type shown in Figure 1; Figure 3 is a schematic diagram showing an embodiment of a five-color light engine of the invention, suitable for use in a frame-sequential color projection display system of the type shown in Figure 1; and Figures 4A through 4D are graphs of reflectivity (R) versus wavelength (λ) of dichroic filters employed in the light engine of Figure 3.

FIG.l is a diagram showing a frame-sequential color projection display system K) based on the prior art, employing a light engine 11 for generating light of different colors to produce a color display. Light engine 11 includes light source 12 consisting of multiple sets of monochromatic light sources 1, 2, 3, ... i of different colors, and light collecting and color combination optics 13. Each set of monochromatic light sources may consist of more than one light emitting element of the same color and is driven by driving electronics 17, which turns on one or more sets of light sources for a defined duration at a given time synchronized with the frame synchronization signals 21 from control electronics 18. The light output from the light collection and color combination optics 13 is transmitted via relay optics 14 to imaging device 15. Imaging device 15 modulates the transmitted light by image scanning signals 20 derived from control electronics 18. Control electronics 18 derives the image scanning signals 20 as well as the frame synchronzation signals 21 from an input video signal 19. The light modulated by imaging device 15 is projected by projection optics 16 onto a display screen (not shown). Figure 2 illustrates one embodiment of a three-color light engine 22 of the invention, which is suitable for use in a frame-sequential color projection display system . such as the system JO shown in Figure 1. Light engine 22 includes three light input modules 23, 24, 25, one for each of the three primary colors red, green and blue, a color combination element 35 and a homogenizing light guide 38. Each light input module includes a set of monochromatic light sources (26, 27, 28) in the form of LED chips, light collection optics in the form of arrays of individual reflectors (29, 30, 31), and a light coupling element (32, 33, 34). The size of the arrays (29, 30, 31) is determined by the system parameters, such as the dimensions of the imaging device 15, the aperture of the projection optics 16, etc. Each reflector is associated with one LED chip. This arrangement has the advantage of facilitating heat dissipation. The development of LED chips with improved heat dissipation will allow LED chips to be packaged more closely together or even one large LED chip matched to the etendue of the optical system, enabling a smaller number of, or even one single light collection means, which has the advantage of decreased assembly complexity. Light coupling elements (32, 33, 34) couple the collected light to a color combination means in the form of X-cube 35, a commercially available product which employs crossed dichroic filters to allows light of selected colors to propagate from its input faces to the defined output face without affecting propagation of light of different colors from the other input faces to the output face. In Figure 2, X-cube 35 has input faces 35a, 35b, 35c and an output face 35d, and two internal dichroic filters 36 and 37, having transmissive surfaces 36a, 37a, and reflective surfaces 36b, 37b, respectively. In operation, red light from color input module 23 (indicated by arrows R) strikes the reflective surface 36b of dichroic filter 36, and is reflected in the direction of arrows G, while green light from color input module 24 (indicated by arrows G) strikes the transmissive sides 36a and 37aof dichroic filters 36 and 37, and is transmitted in the direction of arrows G; and blue light from color input module 25 (indicated by arrows B) is reflected by the reflective side 37b of dichroic filter 37 in the direction of arrows G. The red, green and blue light, in the form of a beam of having the cross-section of the X-cube, propagates in the direction of arrows G through output face 35d of X-cube 35 and enters input face 38a of homogenizing light guide 38, which averages the light intensity across the cross-section of the beam without changing the beam's cross-sectional and angular dimension. The homogenized output beam leaves light engine 22 via output face 38b. Examples of central wavelengths for the R, G and B color LEDs are 625+/-15nm (red), 530+/-15nm (green) and 455+/-15nm (blue). Depending on the arrangement of the color LED modules around the X-cube, different combinations of filters can be used. For the arrangement as shown in Figure 2, filter 36 reflects red and passes green and blue with a cutoff wavelength around 590+/-15nm, and filter 37 reflects blue and passes green and red with a cutoff wavelength around 500+/-15nm. In Figure 2, the light collection optics are depicted as reflection optics. However, it can also be refraction optics, such as lenses or Fresnel lenses, or a combination of reflection and refraction optics. Since an X-cube has only three entrances, it is unable to combine light from more than three different colors. According to another embodiment of the invention, cascaded dichoric filters can be used in a light engine to combine more than three colors. Such an arrangement will allow reproduction of a color gamut wider than what can be reproduced . with traditional red, green and blue primary colors. This objective is readily supported by availability of narrow band monochromatic light sources with colors across the visible spectrum, such as LEDs. As shown in Figure 3, one embodiment of such a light engine 40 consists of five light input modules (41, 42, 43, 44, 45), each having a set of monochromatic light sources (46, 47, 48, 49, 50), an array of reflectors (51, 52, 53, 54, 55), and a light coupling element (56, 57, 58, 59, 60). Input module 41 is located on an entrance face 61a of color combining element 61, while the remaining sets are distributed along opposing side input faces 61b and 61c of color combining element 61. Color combining element 61 contains four internal dichroic filters 62, 63, 64, 65, each having a first transmissive surface (62a, 63a, 64a, 65a) and a second reflective surface (62b, 63b, 64b, 65b). Each filter is positioned to have its reflective surface facing one of the light input modules 41 , 42, 43, 44, 45, and is set at an angle to reflect light from the facing module along a propagation direction P toward a homogenizing light guide 66. The filters are designed to have transmission and reflection characteristics so that the first dichroic filter 62 transmits light from the module 41, and also reflects light from the second module 42; the second dichroic filter 63 reflects light from the third module 43, and also transmits light from the first and second modules 41 and 42; and so on. Thus, each dichroic filter is reflective for the adjacent color input module, but transmissive for all of the previous colors. The output beam from color combiner 61 passes from output face 61b into homogenizing light guide 66 via input face 66a, where it is homogenized before it exits the light engine 40 via output face 66b. The dichroic filters may be of any known type, such as dielectric thin film stacks or holographic filters. The filters may be notch (band) filters, which transmit and/or reflect in a defined band of wavelengths. Alternatively, edge filters, which transmit and/or reflect from a certain wavelength and beyond, can also be used if the sets of monochromatic light sources are arranged consecutively according to their central wavelength. In this case it is preferred that the first input module 41 have a light output with the longest wavelength and the remaining modules 42, 43, 44, 45 have outputs with progressively decreasing wavelengths. In such an arrangement, the reflection bands of the dichroic band filters 62, 63, 64, 65, expressed as reflectivity versus wavelength, are indicated graphically in Figures 4A, 4B, 4C, 4D, respectively. In the alternative, edge filters may be used, having the same progression of decreasing wavelengths as the band filters. Typical colors and central wavelengths for such a five color system are; blue (455+/-15nm), cyan (495+/-15nm), green (530+/-15nm), yellow (590+/-15nm) and red (625+/-15nm). Each color has a bandwidth (FWHM) of about 20nm. The filters can be made to match these central wavelength and bandwidth values. Typical cut-off wavelengths for cut-off filters lie midway between adjacent wavelengths. For example, the filter reflecting cyan (495nm) and transmitting blue (455nm) has a cutoff wavelength around 475+/-5nm. Since in a frame-sequential color display, one particular color is displayed only for a fraction of an image frame time (nominally, less than 1/n of the image frame time, where n is number of primary colors) it is possible to pulse drive the LEDs at significantly higher current than the rated maximum DC driving current. This is advantageous because it results in a significantly higher instantaneous light output. For example, we have demonstrated that a Luxeon green LED can be driven with a current at 7 times the rated maximum DC current at a 1/5 duty ratio to deliver instantaneous outputs 4 times that obtained at the rated max. DC current. Because heating of the chips is mainly determined by the RMS value of the driving power, it is permissible to drive LEDs at a significantly higher current (5-7 times max. DC) for a shorter period of time to achieve a higher instantaneous output without damaging the LED or shortening its lifetime. Thus, it is preferred to have all of the LEDs driven with high pulsed currents. Another way to increase brightness is to employ multiple LED sources with slightly different colors. As disclosed in published patent application WO 01/43113, by Pashley and Marshall and assigned to the present assignee, the separate colors may be combined into a single beam having increased lumen output with little or no increase in etendue. For example, commercially available blue LEDs with peak wavelengths of 455nm and 470nm may be combined for such an effect in the blue channel, green LEDs with peak wavelengths of 525nm and 560nm may be combined for such an effect in the green channel and red LEDs with peak wavelengths of 617nm and 625nm may be combined for such an effect in the red channel. On the other hand, commercial LED products have a certain distribution of peak wavelength for a nominal color, which is typically on the order of 30nm. LED manufacturers can sort them into different bins based on peak wavelength distribution. If needed, one can order LEDs based upon a desired difference in wavelength and use them to form a brighter beam. This technique of employing multiple LED sources with slightly different peak wavelengths may be used on all of the colors to achieve an overall brightness increase, or on some of the colors, such as to achieve color balancing. For example, if the green LED has limited brightness, then two green LEDs having different spectra may be combined with an optical filter to produce a higher lumen output effective GREEN source (and hence overall white balanced lumens); i.e. the two spectra are separated in wavelength about the nominal system green primary and a holographic or dichroic filter is used to optically combine the beams. Alternatively, the overall lumen output of the red and blue LEDs may be reduced to obtain the correct white point. Since the output intensity of the color LEDs may not match the intensity needed for a desired color temperature, it is sometimes necessary to drive one color set longer than another. Thus, that particular color set may be displayed longer than 1/n of the image frame time. The driving pulses can be adjusted to achieve a variety of results which would be difficult to achieve in a mechanical system, such as a color wheel system. For example, the color temperature of the display can be readily adjusted by changing the pulse widths or pulse heights of the driving current to one or more of the colors. For example, to achieve a higher colour temperature, the pulse width of the driving current to the red LED set can be shortened; to achieve a lower colour temperature, the pulse width of the driving current to the blue LED set can be shortened. The invention has necessarily been described in terms of a limited number of embodiments. However, other embodiments and variations of embodiments will be apparent to those skilled in the art, and these are intended to be encompassed within the scope of the appended claims.