AN ILLUMINATION SYSTEM FOR A COLOR IMAGE PROJECTION

Abstract: An illumination system for an image projection apparatus utilizing a single light valve (62) for modulating light impinging thereon with an image signal (66) which system comprises three projection lamps (82, 84, 88), one for each of the primary colors, which are activated sequentially. Positioned in the light path between two of the lamps (84, 88) and the light valve are occluders (94, 96) which block and unblock the light output from their associated lamp. The lamps which have the occluders are operated such that each lamp may be driven with a series of non-occluded pulses and occluded pulses. The more a desired reduction in light output in one of the colors is required, the non-occluded pulses are reduced and the corresponding occluded pulses are increased. This permits adjustment of the colorimetry of the system without adversely affecting the electrical properties of the lamp.

Description: An iUumination system for a color image projection. This invention relates to an illumination system for a color image projection apparatus system and comprising: a light valve for modulating light impinging thereon in accordance with an image content signal; at least a first, and second light sources for iUuminating said light valve; means for altering the color of the light emitted by said source so that said light valve is iUuminated with first at least a first, and second and color of light. The term image projection apparatus should be understood in a wide sense and comprises a device for displaying, for example, a video image, a graphic image, numerical information or a combination thereof. Most commercially available image projection systems, like video projection systems utilize separate channels for each of the three primary colors. The systems thus require three light valves with separate optical systems which must be accurately converged on the projection screen, which adds to complexity and expense. Recently, projection video systems utilizing only a single light valve have been developed. One such system is a color field sequential system, in which the normal video field, l/50th or l/60th of a second, is broken into three parts, or color subfields of l/50th or l/180th of a second. During the three color sub-fields, the light valve is iUuminated with red, green and blue light sequentially. While the light valve is iUuminated with any given color, the video data corresponding to that color is displayed on the Ught valve. The eye then fuses the three color sub-fields into a single, fuU color field. The eye also fuses successive video fields and frames into fuU motion, full color video. Recently, improved Ught valves particularly suitable for use in projection television systems have become avaUable. One such device is a so-caUed deformable mirror device (sometimes caUed a digital πiirror device or DMD) which is illustrated in U.S. Patent No. 5,079,544 and patents referenced therein, in which the Ught valve consists of an array of tiny movable mirror-like pixels for deflecting a beam of Ught either to the display screen (on) or away from the display optics (off). This device is suitable for use in a field sequential system because its pixels are capable of being switched very rapidly. By controlUng the on/off ratio of the pixels during each color subfield the intensity of the color can be adjusted. In addition to improved Ught valves for use in projection video systems, improved projection lamps are also now available. These projection lamps are highly efficient and have a long Ufe. Furthermore, these lamps are physically quite smaU and have a smaU arc length. Small size and smaU arc length can significantly reduce the size and cost of the optics used to project the Ught onto the Ught valve as weU as onto the projection screen. SmaUer optics can considerably reduce the overaU cost of a video projection system since the optical elements of the system are a very significant portion of the overall cost. Many such lamps are also capable of following an electrical drive signal with good fideUty, i.e. they have a fast rise and fall time and can follow any reasonable waveform, including squarewaves. One such lamp is the PhUips CSL-R100W Ultra High Pressure Projection lamp. However, many otherwise suitable lamps may not have even color distribution across the visible spectrum, i.e. they may be deficient in one or more colors. Furthermore, these lamps have carefuUy designed thermal properties which require operation at a given power level in order to assure optimal power dissipation. Accordingly, such lamps require a consistent power input over time, such as 100 watts. If greater power is input to the lamp, the lamp wiU have a significantly shortened Ufe span but turning down the power input to the lamp wiU cause the lamp to become unstable or go out altogether. The present invention is directed towards providing a three-lamp, single Ught valve projection video system that can take fuU advantage of these improved projection lamps while operating the lamps at optimum parameters. In addition to correcting for any color spectrum deficiencies of the projection lamps used in a projection video system, a suitable video projection system must also provide for color correction of the dichroic filters utilized to convert the white Ught output from the projection lamps to the primary colors. Dichroic filters are manufactured in a batch process and there are sample to sample variations in the colorimetry of these filters. Additionally upon exposure to the intense Ught of projection lamps, the colors of the dichroic filters may fade. Accordingly, any suitable projection system must be able to compensate for batch to batch variation and/or fading of the dichroic filters. FinaUy, a suitable projection video system should also provide for color correction based on user preference, either statically or dynamicaUy. U.S. Patent Application Serial No. 08/141,145 filed October 21, 1993 entitled "Color Correction System for Video Projector", is directed to a method for dynamicaUy color correcting a projection video system utilizing a single projection lamp, a color wheel of dichroic filters and a single Ught valve. The disclosure of U.S. AppUcation Serial No. 08/141,145 is hereby incorporated by reference, as if fuUy set forth herein. The present appUcation is directed to an iUumination system being capable of varying the Ught output of the projection lamps without varying the electrical power input thereto so as to permit the lamps to be driven in accordance with their operating parameters. This system is characterized by occluder means positioned between at least one of said sources and said Ught valve, said occluder means having a first operational state in which it passes Ught from its associated source to said Ught valve and a second operational state in which it prevents the light emitted from its associated source from reaching said Ught valve; Ught source driver means for activating said sources in synchronization with image information input to said Ught valve to sequentially iUuminate said Ught valve with said at least first, and second colors of Ught, said Ught source driver means activating at least one of said sources with electrical power when its associated occluder means is in said first and said second operational state, said sources driver means including means for apportioning the electrical power between a first and a second electrical power appUed to said lamp when said occluder means is in said first operational state and said second operational state respectively so that the Ught output of said lamp source plus associated occluder is adjustable while the electrical input to the source remains constant. This permits adjustment of the color temperature of the system to user preference without adversely affecting the electrical properties of the lamp. As such, the electrical power input to each of the lamps remains within operational parameters but the optical output of a particular color as seen by the Ught valve and thus the viewer is reduced. For better understanding of the invention, reference is made to the detaUed specification to follow, which is to be taken in conjunction with the foUowing drawing figures; Fig. 1 is a schematic diagram of a projection video system using multiple projection lamps and a single Ught valve; Fig. 2 illustrates a schematic diagram of a color projection video system utilizing three projection lamps and a single Ught valve and a means for dynamically adjusting the colorimetry of the system; and Fig. 3 is a timing diagram of the driving and occluded pulses for the three projection lamps. Fig. 1 illustrates schematically an illumination system for projection color video system utilizing three projection lamps to iUuminate a single Ught valve. This illumination arrangement nearly triples the brightness of the projected image over a single lamp, single Ught valve system. Figure 1 illustrates an illumination system 10 for iUuminating a Ught valve 12 by three projection lamps 14, 16 and 18. The projection lamps 14, 16 and 18 are driven from a power source 20 by a sequential switch (commutator) 22 operated by control electronics 23. As is shown in Figure 1, the white Ught emitted by each of the lamps is directed to dichroic mirrors 24, 26. Dichroic mirror 24 reflects green Ught and transmits blue Ught. Thus, the Ught from lamp 14 after passing through dichroic mirror 24 will have color components other than be blue subtracted and the light from lamp 16 after reflecting from dichroic mirror 24 wUl be green as its other components wiU not be reflected from mirror 24. Dichroic mirror 26 reflects red Ught and passes blue and green Ught. Thus, the red components of Ught emitted by lamp 18 wiU be reflected by dichroic inirror 26 to Ught valve 12. The net result of the lamp and filter arrangement is that when lamp 14 is activated by switch 22, Ught valve 12, for example a DMD wiU be iUuminated by blue Ught only, when lamp 16 is activated, DMD 12 will be iUuminated by green Ught only and when lamp 18 is activated, Ught valve 12 will be iUuininated by red Ught only. Integrator optics 28 such as disclosed in US patent 5,098,184 may be disposed between dichroic mirror 26 and Ught valve 12 so as to provide a uniform field of illumination. Light valve 12 modulates the Ught under the control of Ught valve electronics 33 in accordance with the mcoming video information 34. After modulation by Ught valve 12, the Ught passes to projection optics 30 and viewing screen 32. Light valve electronics 33 also provides a sync output signal 38 to the input of electronics 23 which controls switch 22. The color subfields generated will be integrated by the eye into a full color picture. Figure 2 illustrates a multiple lamp, single Ught valve projection video system 60 which permits varying the Ught output of the projection lamps, so as to provide for color control, without varying the electrical power input to the projection lamps so that the optimal lamp operating conditions are maintained. System 60 includes a Ught valve 62 driven by Ught valve electronics 64 which in turn receives an input video signal 66. Light valve 62 modulates Ught impinging thereon in accordance with video signal 66 under the control of the Ught valve electronics 64. Light valve 62 is sequentially illuminated with red, green and blue light. While light valve 62 is illuminated with a given color (a color sub- field), the video data corresponding to that color is displayed on the Ught valve by Ught valve electronics 64. The eye fuses the three color sub fields into a single fuU color field and successive video fields into a full motion, full color video. The modulated Ught from the Ught valve is projected by projection optics 68 to a viewing screen 70, which may be of the front or rear projection configuration. A synclironization signal 72 is output from Ught valve electronics 64 to a lamp controUer/driver 74. Lamp controUer/ driver 74 has separate drive (power) outputs 76, 78, 80 to three separate projection lamps 82, 84, 88. Disposed in the output path of projection lamps 82, 84, 88 are dichroic filters 90, 92. Dichroic filter 90 reflects red Ught and passes blue and green Ught. Thus, the red component of the white Ught output of lamp 82 wUl be reflected by dichroic mirror 90 to Ught valve 62. Dichroic filter 92 reflects blue Ught and passes green Ught. Thus, the green component of the white Ught output of projection lamp 84 wUl be passed through of dichroic filter 92 and will impinge on Ught valve 62 after passing through dichroic filter 90 which also passes green Ught. Dichroic filter 92 will also reflect the blue component of projection lamp 88 and iUuminate Ught valve 62 with it after passing through dichroic filter 90. Thus, the net result of the arrangement of projection lamps 82, 84, 88 and dichroic filters 90, 92 is that projection lamp 82 functions as the "red" iUumination lamp, projection lamp 84 functions as the "green" iUumination lamp and projection lamp 88 functions as the "blue" projection lamp. Also disposed in the iUumination path of "green" projection lamp 84 is an occluder (shutter wheel) 94. A second occluder 96, is positioned in front of "blue" lamp 88. Occluders 94, 96 have been illustrated in their simplest form, that is of circular rotating wheels which are approximately 2/3 opaque with Ught transmissive segment 98 in wheel 94 and a Ught transmissive segment 100 in wheel 96. Occluders 94, 96 are driven by phase locked servo motors 102, 103 which are controUed by occluder driver 104 which receives a control input 105 from lamp controUer/driver 74. Lamp controUer/driver 74 includes user inputs 106 R , 106 G and 106 B so that the overall colorimetry of the projected image may be adjusted. AdditionaUy, a color sensor 107 located at the output of Ught valve 62 may also input a signal 108 to lamp controUer/driver 74 to permit automatic adjustment of color temperature. The synchronization and drive arrangement for the three lamps 82, 84, 88 and the two occluders 94, 96 is shown in Figure 3. If extremely precise color control or a greater range of adjustment is needed, a third occluder can be positioned in front of "red" lamp 82, however there is generally no need for occluders in front of aU of the lamps because the relative color balance of the system can be adjusted by changing the light output of two of the three primary colors. As a practical matter, the un-occluded lamp wiU be that of the color for which the lamp is least spectraUy efficient. For the purposes of this discussion, we wiU assume that this is "red" lamp 82. Figure 3 illustrates the electrical power output to the "red", "green" and "blue" lamps through lines 76, 78 and 80. As is indicated on the bottom (red) graph of Figure 3, the small tick marks indicate the color subfields with the video field comprising three color subfields. The lowermost graph of Figure 3 illustrates the power output through line 76 to lamp 82 which forms the red illumination. As is seen a positive going pulse 110 is applied to lamp 82 for one color subfield (in this case red). No power is appUed to lamp 82 for the next two color subfields (i.e. the green and the blue subfields). Thereafter, a negative going pulse 110' is appUed to lamp 82 through line 76. The result of this operation is that lamp 82 is energized for one-third of the video field with an ampUtude A R with pulses both positive 110 and negative 110 going so that the lamp is driven under optimal conditions. In Figure 3 the electrical power input to the lamps is the amplitude of the pulses times the duration of the pulses. As is seen in Figure 3, the ampUtude A R of "red" pulses 110 is greater than that of the other colors as will be described in detail below. The middle timing chart of Figure 3 illustrates the power input to lamp 88 which is the "blue" lamp by the action of dichroic filter 92. Also disposed in the iUumination path of lamp 88 is occluder 96. As is seen, lamp 88 is powered by a series of positive pulses 112 and negative pulses 112' for one-third of the video field (i.e. during the "blue" color subfield). Pulses 112, 112' occur when the Ught transmissive segment 100 of occluder 96 is positioned in front of lamp 88. However, the blue power pulses 112, 112' have an ampUtude A B (iUustrated by the height of pulses 112, 112' in graph B) which is less than that of red pulses 110, 110'. Thus, the total power of non-occluded pulses 112 and 112' is less than the full power requirement of lamp 88. However, as noted above, many sophisticated projection lamps cannot be operated at less than fuU power, averaged over a period of time, without operational difficulties which can lead to premature lamp failure. In order to restore proper electrical power input to lamp 88, it is activated with a series of compensatory pulses 114, 114' which are again both positive and negative going. However, the pulses 114, 114' occur when the opaque portion of occluder wheel 96 is positioned so as to block the Ught output from lamp 88. Thus, there is no Ught output to 7 Ught valve 62 by lamp 88 during pulses 114, 114'. The duration and amplitude of the pulses 114 and 114' are adjusted so as to restore the total electrical power input to lamp 88 to the desired amount so that its operational characteristics wiU not be affected. The ampUtude A B of non-occluded pulses 112', 112 is less than that of "red" pulses 110, 110'. However, the total electrical power input to lamp 88 is the sum of non-occluded pulses 112, 112', and occluded pulses 114, 114'. The result of this operation is that the optical output of lamp 88 to Ught valve 62 is reduced but its electrical input remains at the optimal level so that its operational characteristics are not affected. Similarly, "green" lamp 84 is driven with a series of non-occluded pulses 116, 116' and a series of occluded pulses 118, 118'. It is seen that the ampUtude A G of non-occluded pulses 116 are the lowest which means that the non-occluded electrical input to the "green" lamp 84 is the lowest, which would be the case where the lamp is spectraUy efficient in green. Accordingly, the compensatory occluded pulses 118, 118' are the largest so that the total power input to lamp 84 remains at the optimum level. In summary, as the drive arrangement in Figure 3 Ulustrates, aU of the lamps see exactly the same input electrical power so that their operating characteristics are optimum. In operation, if the user deems the picture on screen 70 to be "too green", the user would operate control 106 G which causes lamp controUer 74 to alter the relationship of the driving pulses on Une 78 to lamp 84. If a reduction in green is desired, non-occluded pulses 116, 116' are reduced in ampUtude. However, in order to maintain proper electrical power input to lamp 84, occluded pulses 118, 118' are increased in ampUtude so that the total electrical power to lamp 84 remains the same. Since, however, the non-occluded pulses have been reduced in ampUtude, the total Ught output of lamp 84 is reduced and thus the overabundance of green is compensated for. A sirmlar operation will occur with respect to blue lamp 88. If the picture projected on screen 70 is too blue, non-occluded pulses 112, 112' to lamp 88 are reduced and occluded pulses 114, 114' would be increased by operation of control 106 B . The question arises as to how to compensate for a picture that is "too red" since "red" lamp 82 has no occluding device positioned in front of it and, as noted above, its power input cannot be turned down without possible malfunction. The answer is that both blue and green power is increased by controls 106 G , 106 B so that the relative amount of red decreases. The automatic control of color sensor 107 would also cause lamp controUer/driver 74 to operate in a sirmlar manner to adjust the color balance to a preset point. Occluder driver 104 drives motors 102, 103 so that the Ught transmissive portions of occluders 94, 96 are positioned in front of their respective lamps 84, 88 during the time that the non-occluded pulses occur. Occluder driver 104 receives a control input from lamp controUer/driver 74 which in turn is synchronized to Ught valve 62 by Ught valve electronics 64 so that occluders 94, 96 are synchronized to the mcon ing video and iUumination signals. As a practical matter the requirement that the Ught valve be loaded with video data constrains the start and stop points of the non-occluded pulses to defined non- arbitrary locations. However the occluded pulses 114, 114', 118 and 118' may occur at any time during the other two-thirds of the video field, the timing between occluders 94, 96 and the occluded pulses is thus not particularly critical. It is merely necessary that the occluded pulses occur during the period when the output of the respective lamps are occluded. Further, the waveform of the occluded pulses is not critical and may be of any form sufficient to drive the lamps under optimum operating conditions. The waveforms of the occluded pulses may also be utilized to facilitate re-ignition of the lamps by the non-occluded pulses. The devices used to occlude the Ught output from the projection lamps need not be motor driven shutter wheels as illustrated in Figure 2. The occluders may be any form of controUable shutter suitable for occluding the output of projection lamps. Such suitable occluders can be mechanical shutters operated electrica y or shutters in the form of electricaUy operated dispersive Uquid crystal devices. The only requirement is that the shutter be capable of a synchronized operation with the iUumination of the lamps. Mechanical variable density occluders could also be used to provide the function of the occluder wheels, however this would preclude dynamic color adjustment. Lamp controUer/driver 74 may be implemented in a number of ways. Similar to the lamp driver in appUcation Serial No. 141,145 referred to previously; controller driver 74 may consist of a voltage output square wave generator coupled to a current amptifier whose three outputs follow the voltage inputs. Many commerciaUy avaUable power suppUes may also be used, the only requirement is that the controUer driver be capable of proportioning the power output between the non-occluded and occluded pulses so that the total power suppUed to each lamp remains constant. The three separate color controls 106 R , 106 B , and 106 G may also be replaced with a single "tint" control. The above-described embodiments are merely illustrative of the principles of the present invention. Numerous modifications and variations thereof wiU be readUy apparent to those skilled in the art without departing from the spirit and scope of the present invention. For example, instead of three lamps, the iUumination system may comprises two or four lamps and one and three occluders respectively.

Claims: Claims: 1. An iUumination system for a color image projection apparatus system and comprising: a Ught valve for modulating Ught impinging thereon in accordance with an image content signal; at least a first, and second Ught sources for iUuminating said Ught valve; means for altering the color of the Ught emitted by said source so that said Ught valve is illuminated with first at least a first, and second and color of Ught; occluder means positioned between at least one of said sources and said Ught valve, said occluder means having a first operational state in which it passes Ught from its associated source to said Ught valve and a second operational state in which it prevents the Ught emitted from its associated source from reaching said Ught valve; Ught source driver means for activating said sources in synchronization with image information input to said Ught valve to sequentiaUy illuminate said Ught valve with said at least first, and second colors of Ught, said Ught source driver means activating at least one of said sources with electrical power when its associated occluder means is in said first and said second operational state, said sources driver means including means for apportiorύng the electrical power between a first and a second electrical power applied to said lamp when said occluder means is in said first operational state and said second operational state respectively so that the Ught output of said lamp source plus associated occluder is adjustable while the electrical input to the source remains constant. 2. The iUumination system as claimed in claim 1 wherein said means for altering the color of the Ught emitted by said first, second and third sources comprise dichroic filters positioned between said sources and said Ught valve. 3. The Ulumination system as claimed in claim 1 or 2 wherein said occluder means comprise a rotating wheel located between at least one of said first, second and third sources and said means for altering the color of the Ught, said occluder means having a Ught transmissive portion providing said first operational state and an opaque portion providing said second operational state. 4. The Ulumination system as claimed in claim 1, 2 or 3 wherein said occluder means are positioned in front of two of said first, second and third Ught sources. 5. The Ulumination system as claimed in claim 3 wherein said rotating wheel is driven by a servo motor under the control of occluder driver means which receives a control input to synchronize the position of the rotating wheel in accordance with the source driver means. 6. The illumination system as claimed in claim 1, 2, 3, 4, or 5 further including color sensing means for sensing the color of the Ught modulated by the Ught valve. 7. The illumination system as claimed in claim 1, 2, 3, 4, 5 or 6 wherein said source driver means activate said source with a series of pulses of predetermined amplitude. 8. An image projection system having an illumination system a Ught valve for modulating Ught impinging thereon, and a projection lens system, wherein the iUumination system is a system as claimed in claim 1, 2, 3, 4, 5, 6 or 7, and wherein said Ught source driver means operate said sources by a series of first and second pulses of electrical power, said first series of pulses occurring when said occluder means is in said first operational state and said second series of pulses occurring when said occluder means is in said second operational state; and means for adjusting the proportionality between the electrical power of the first series of pulses and the electrical power of said second series of pulses. 9. The projection system as claimed in claim 8 wherein said Ught valve comprises an array of reflective pixels moveable between an on and off state. 10. The projection system as claimed in claim 13 further including means for adjusting the proportionaUty between said first and second series of pulses based upon a signal output by said color sensing means. 11. The projection system as claimed in claim 8,9 or 10 further including user input means for varying the proportionaUty between said first and second pulses to said lamps.

Inventors: STANTON DOUGLAS, STANTON, DOUGLAS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H04N9
IPC codes: G03B21, G09G3, H04N9