Method and apparatus for the reduction of LCD flicker in micropol based projection systems

The object of the present invention is to eliminate the flicker of an LCD/Micropol assembly by reducing the excess heat caused by the attachment of the Micropol to the LCD. The present invention solves the problem of LCD flicker caused by excess heating of the LCD/Micropol assembly by reducing the amount of heat transferred to the LCD from the projection lamp. This solution is realized through the use of a special wide-band IR filter that prevents IR radiation from reaching the LCD/Micropol assembly. The IR filter used in the preferred embodiment is based on cholesterol liquid crystal reflectors that are made to reflect IR radiation back into the projection lamp. The excess heat is removed by the projection cooling system and is not transmitted to the LCD display.

DETAILED DESCRIPTION OF THE INVENTION The present invention solves the problem of LCD flicker caused by excessive heating of the LCD/Micropol assembly by reducing the amount of heat transferred to the LCD from the projection lamp. This solution is realized through the use of a special wide-band IR filter that prevents IR radiation from reaching the LCD/Micropol assembly. The IR filter used in the preferred embodiment is based on cholesteric liquid crystal reflectors that are made to reflect IR radiation back into the projection lamp. The excess heat is removed by the projection cooling system and is not transmitted to the LCD display. FIG. 1 illustrates the heat flow problem in a typical LCD projection system 10 . In addition to the production of visible light, the high power lamp used to illuminate the projected image also produces a great amount of infrared radiation 12 that can be transmitted to the LCD light valve and converted to heat. Heating of the LCD produces many undesirable effects including image flicker, not to mention severe structural damage. Typical projection systems reduce the heat transmitted to the LCD by positioning the input polarizer away from the panel. This configuration ensures that heat absorbed by the polarizer is not directly conducted to the LCD cell. In addition, the polarizer is frequently laminated to an IR filter 14 that reduces the amount of heat absorbed by the polarizer. The filter, which is typically made using a glass substrate, provides a convenient carrier for the polarizer. Finally, a forced-air cooling system 16 is typically used to remove heat from both the polarizer and the LCD display 18 by convection. The fan system must provide sufficient air circulation for heat removal in order to keep the LCD panel within a specified operating temperature range. FIG. 2 illustrates how a Micropol affects the heat removal system in a 3 D stereoscopic projector 20 . In this case, the IR radiation 22 received by the LCD display from the IR Filter/Polarizer 24 is the same as in the non-micropol case. However since the micropol 30 is mounted to the surface of the LCD 28 , heat cannot be directly removed from the LCD surface by convection. The micropol 30 acts as an insulator and causes the LCD temperature to rise significantly resulting in poor image display performance. FIG. 3 illustrates the same system 40 with the addition of a CLC IR reflecting filter 52 placed prior to the original IR filter 44 . In this case, lowering the amount of IR radiation transmitted by the projection lamp reduces heating of the LCD element. The reduction is accomplished through the use of a CLC IR filter that reflects a portion of the IR radiation back in to the projection lamp. CLC material typically reflects either right or left handed circularly polarized light. The filter used in this case is constructed from two filters, one to reflect left handed light and one to reflect right handed light. The CLC reflecting filters are designed in such a way as to reflect radiation in the infrared wavelength range. In addition, using a CLC based filter can improve the overall cooling system performance by eliminating IR radiation in a broader range of wavelengths than typical IR filters used in projections systems. Using a CLC reflector has another advantage in that since IR radiation is reflected rather than absorbed, the filter will not act as a source of stored heat. FIG. 4 illustrates an alternative to the drawing of FIG. 3 in which the original projector IR filter has been completely replaced by a CLC reflecting IR filter 62 . In this case the LCD polarizer 58 has been laminated directly to the CLC filter. The heat reduction properties are the same as in the previous case. Alternatively the CLC filter can be constructed in such a way that all wavelengths of right (or left) handed circularly polarized light is reflected back into the lamp, and infrared wavelengths of left (or right) handed circularly polarized light is also reflected back into lamp. This configuration allows only left (or right) handed circularly polarized light in the visible spectrum to pass through the filter. A ¼-wave plate attached to the CLC filter (instead of a linear polarizer), then converts the left (or right) handed circularly polarized light in to linear polarized light required by the LCD display. In this case a minimally-absorptive linearly polarizing filter combined with an IR reflective filter is produced. FIG. 5 illustrates the narrow-band filter response of a typical IR filter used for projectors. In this figure IR radiation is reduced in the narrow range of about 750 nm to 950 nm. IR filters with this narrow-band response may be sufficient for many projector applications. However for the special case in which a micropol is mounted to the LCD, a wider band filter will reduce the heat transferred to the LCD. FIG. 6 illustrates the spectral response of a CLC IR reflector made using both left and right-handed CLC material. The plot shows the much wider IR blocking characteristic of the CLC filter. Eliminating a wider range of IR radiation helps to reduce the radiant heat transmitted to the LCD that results in an elimination of the flicker problem associated with micropol based projectors. The method of accomplishing this flicker improvement can be done two different ways. A projector having an existing micro polarization device may be modified by attaching the CLC filter to the lens side of the micro polarization device. The second method is installing a combination micropolarizer-CLC filter into the LCD projector with the CLC filter mounted on the lens side of the combination device. The present invention has been described with reference to the above illustrative embodiments. It us understood, however, modifications to the illustrative embodiments will readily occur to persons with ordinary skill in the art. All of such modifications and variations