The invention relates to a projection display system.

Projection display systems are applied for projecting image information in both video and monitor systems.

Many different colors are specified as"white"in both video and monitor systems. For example, in cinema work 5400K is specified as"white", but in video work 6550K is specified as"white". Ranges from 3800K to 9300K are used to specify"white"in other applications. Therefore, to enable an end user to accurately judge the color of the finished product which appears on a monitor, it is necessary to adjust the monitor"white" point to the"white"point specified for that particular application.

In CRT monitors, the specified"white"point can be attained by adjusting the relative gains of the red, green and blue video amplifiers so that when all three video signals (R, G and B) are equal, the desired target"white"is displayed. Changing the"white"point in such a system can be effected by changing the relative gain of the red, green and blue amplifiers. This manner of attaining the specified"white"point, however, has several disadvantages when it is incorporated in an LCD display. If the requisite gain is effected in the analog domain, the change not only effects the"white"point, but effects all other colors as well. Accordingly, modified gamma correction curves are then required for each target"white"point. If the gain adjustment is effected in the digital domain, some of the dynamic range must be ignored and not used. Because eight bits is generally considered the minimum number of bits required for effective color display, disregarding portions of the dynamic range will generally lead to serious color artifacts. Even with greater basic resolution, ten or twelve bits, color artifacts can result when interpolating the video signal to a smaller dynamic range.

Therefore, while these methods previously described are suitable for use in color correction for devices such as CRT monitors, these methods are not suitable for adjusting the color of a projection display system.

Using a fixed and variable retarder to correct color temperature, referred to as one dimensional correction, results in changing the color of the light in the illumination path.

This does not permit the"white"point to be fully adjustable, limiting the adjustment to one dimension along the black body line. Accordingly, if the starting or target"white"point is not on the black body line complete adjustment of the"white"point is not possible.

Changing the relative intensity of the red, green and blue color channels, such as through the use of a ColorLink polarizer, to control the amount of light in all three R, G and B channels to reach the target"white"point over-controls the"white"point. Color space is a two-dimensional space, commonly designated x-y color coordinates. The third coordinate in this system is the brightness coordinate. Brightness does not play a role in "white"point correction. Accordingly, only a two dimensional color correction system is necessary to fully control the"white"point of an LCD display.

It is an object of the invention to provide a projection display system having an improved color adjustment.

This object is achieved by the projection display system according to the invention as specified in claim 1.

In the preferred embodiment of this invention, the color of a liquid crystal projection display system may be adjusted without changing the input video signal in either the analog or digital domains. Accordingly, color adjustment is effected optically and is controllable while the projector is operating. In this manner, a fully adjustable"white"point is attained which is not limited to adjustments along the black body line. In the preferred embodiment, the throughput of two different color channels is used to control the"white" point of a projection display thereby effecting two-dimensional color corrections. Partially polarized light produced by an integrator/polarization conversion system combination and two non-pixilated twisted nematic liquid crystal display cells (TN-LCD) or other light modulators are utilized. By adjusting the voltages on the TN-LCD cells, it is possible to adjust the relative amounts of the greatest two of the colors supplied, of the red, green and blue colors, used by the display without changing the light of the color in the least supply.

Because all of the adjustments are done electronically through voltage controls, not mechanically, the"white"point correction system is both simple and reliable. Electronic adjustment can also easily accommodate changing end user"white"point requirements.

Further advantageous embodiments are defined in the dependent claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described herein after. In the drawing:

In the accompanying drawings, like reference numerals indicate corresponding parts throughout.

FIG. 1 is a schematic of the optical path of a reflective color drum projector utilizing a two-channel color polarizer device to control the"white"point in the manner of this invention; and FIG. 2 is a schematic of the optical path of a three-panel transmissive liquid crystal display projector utilizing the color correction method of this invention.

Referring now to the drawings, the following is a description of the preferred embodiments of the invention utilizing the color correction method in a reflective, i. e: single panel reflective and a three-panel transmissive, liquid crystal display (LCD) projector.

Referring first to FIG. 1, there is shown a schematic of the optical path of a single panel reflective color drum projector with two ECB cells for controlling two of the colors in greatest supply such as, for example, red and green, or blue and green, to correct the "white"point without effecting the color in the shortest supply, blue or red, respectively.

Such a light intensity modulator independently controls the polarization of the red, green and blue color bands, and by adjusting the two colors in greatest abundance, would provide satisfactory color control.

This method is preferably implemented by the use of a two color ColorLink color polarizer available from ColorLink Incorporated 2925 55th Street, Boulder, Colorado 80301 and described in U. S. Patent No. 5,751,384, positioned in the illumination path as shown in Fig. 1. For convenience of illustration, the two color polarizer 10 would be red/green, the colors assumed for purposes of illustration to be in greatest supply, with blue being the color in the shortest supply. This integrator/polarization conversion system combination and two non-pixilated twisted nematic LCD cells, provide a system for modulating the light intensity of the two colors in greatest supply, in the example red/green, among the three colors used by the display in order to adjust the"white"point of the projector by electronically controlling the voltage of the color polarizer 10.

In the embodiment illustrated in Fig. 1, there is shown a lamp and reflector 2, and a first optical integrator plate 3 in optical alignment with a second optical integrator plate 4 to provide a well defined uniform light distribution on a light polarization conversion system, (PCS) 5, positioned in the optical path to a first relay lens 8. The two color channel, red/green, ColorLink shutter or filter 10 is positioned adjacent a first polarizing beam splitter (PBS) 15 to control the red/green light passed to the PBS by the ColorLink shutter 10. The

blue light from the ColorLink polarizer 10 passes straight through the PBS 15 and a quarter wave plate 12 and cylindrical lens 13 to a color drum 14 which reflects the blue light back through the lens 13 and the quarter wave plate 12. Because the two passes through the quarter-wave plate 12 has rotated the polarization of the light by 90°, the light now reflects at the first PBS 15 through a second relay lens 16.

When both the green and red channels of the ColorLink device are deactivated, the ColorLink device does not change the polarization of these two colors and they follow the same path as the blue light. In this case, all three colors reflect at the second PBS 25 onto the reflective LCD 30 where the colors are modulated separately. These modulated colors are then transmitted through the second PBS 25 and the projection lens 35 to make a color image on the projection screen (not shown). Since we have assumed for this example that there is too much green and red relative to the amount of blue in this projector, the image would be perceived by the viewer as"yellowish"compared to the target"white" point color.

When one channel, for example green, of the ColorLink polarizer 10 is partially activated, some of the green light has its polarization rotated 90°and some of the light is left with the original polarization. Light where the polarization was rotated is reflected at PBS 15 and is lost to the system. Green light where the polarization is not rotated, transmits through PBS 15 and eventually reaches the LCD 30 and the projection screen as described above. Since some green light was lost at the PBS 15 there is now less green light in the image.

Similarly, when the red channel in the ColorLink device is partially activated, less red light reaches the LCD 30 and the projection screen, reducing the amount of red light in the image. By controlling the degree of activation of the green and red channels of the ColorLink device, it is possible to achieve a wide variety of target"white"points at the projection screen.

In incorporating the method of this invention in a three-panel projector, for example a three-panel, single polarizing beam splitter reflective LCD projector, the projection system illustrated in Fig. 1 would be modified by substituting three-panel optics for the drum optics illustrated.

In three-panel transmissive projectors and three-panel three polarizing beam splitter reflective LCD projectors, however, the method of this invention may be incorporated in the manner illustrated in Fig. 2. In such a three-panel transmissive LCD projector, the amount of light is controlled in two of the three color paths. As previously described, the

amount of light is controlled in two of the color paths, those paths which have the greatest color supply which will depend on the particular lamp used, the dichroic filters applied and other optics in the system as known to those skilled in the art. Control of the two lights in the red/green color channels, which are for purposes of illustration in the greatest supply, may be effected by the use of a non-pixilated twisted nematic LCD positioned in the optical path.

As illustrated in Fig. 2, light from lamp and reflector 2 is passed through a pair of integrator plates 3 and 4, and the polarization conversion system 5, in the manner of the embodiment illustrated in Fig. 1. The light passing through the PCS 5 passes through a first dichroic mirror 41 which reflects the blue and green light to a second dichroic mirror 51.

Red light passes through the first dichroic mirror 41 and is reflected by a mirror 43 through a red light color channel control 45, such as a non-pixilated twisted nematic liquid crystal display (TN-LCD) cell, an LCD with a polarizer attached 46, to a color-combining x-cube 50 positioned in the optical path to pass the controlled red light through a projection lens 60 to a projection screen, not shown.

The blue and green light is reflected by the first dichroic mirror 41 to the second dichroic mirror 51. The second dichroic mirror 51 reflects the green light to a green color channel control 55, such as a non-pixilated TN-LCD cell, an LCD with a polarizer attached 56, which passes the controlled green light to the x-cube 50 and projection lens 60 to the projection screen. The blue light is transmitted through the second dichroic mirror 51 and reflected by a pair of mirrors 57 and 58 through a blue LCD 59 into the color-combining x- cube 50 and projection lens 60 to the projection screen.

Through this embodiment, the amount of light in greatest supply, red and green, is controlled in their two respective color channels by the polarization controllers 45 and 55, respectively to control the"white"point of the projector.

In operation, fully adjustable"white"point control may be effected electronically through voltage controls without any mechanical components. Positioning polarization controllers for two color bands, either 10 or the pair 45 and 55, in the optical path to a projector, 35 and 60, permits the selective control of the two colors which are in the greatest abundance in the system. The controlled reduction of these abundant colors can be matched to the light intensity of the color in least supply to provide a fully adjustable"white" point correction system.

Other aspects and features of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims.