BACKGROUND OF THE INVENTION

The invention is related to the field of liquid crystal displays, and more particularly to a projection display using a liquid crystal display with reflective color filters.

Transparent liquid crystal devices (LCDs) incorpora­ting striped or dot-pattern color filters are well known in the art, and are used in commercial products such as small screen LCD television receivers. An exemplary paper describing such a device is "Color LCD for Character and TV Display Addressed by Self-Aligned a-Si:H TFT," by Yasuhiro Nasu et al., SID 86 Digest, pages 289-292. These LCDs use absorptive filters fabricated by depositing and patterning inorganic or organic pigments on a glass substrate. These devices have not been used in projection display systems, in part because the color filters bleach or decompose when exposed to bright light.

Projection display systems using reflective liquid crystal devices have been developed by Hughes Aircraft Company as described in Report NADC-77212-30, "Development of a Color Alphanumeric Liquid Crystal Display," December 1981, prepared for the Naval Air Development Center, by R.G. Hegg and J.E. Gunther. A projection system using transparent liquid crystal matrices is described in the paper entitled "LCD Full-Color Video Projector," by Shinji Morozumi et al., SID Digest, pages 375-378. Both of these projection systems use separate liquid crystal devices for the three primary colors and color selective beam-­splitters. Such an apparatus is expensive and requires considerable space.

Reflective, scattering mode liquid crystal devices using reflective dichroic filters are described in U.S. Patent 4,006,968, assigned to the assignee of the present application. The disclosure of U.S. Patent 4,006,968 is incorporated herein by this reference. These devices are understood to have been used in a directly-viewed mode only, i.e., not in a projected mode.

It would therefore represent an advance in the art to provide a projection display system using liquid crystal devices which do not require a beamsplitter or separate devices for each of the primary colors, and which do not use absorptive color filters in the liquid crystal device.

SUMMARY OF THE INVENTION

A color liquid crystal projection display system is disclosed, wherein a source of white light projects a beam of white projection light onto a single transmissive liquid crystal device. The device comprises a transparent substrate, the liquid crystal material, an electrode structure for applying an electric field to the liquid crystal material so as to define a desired image, a plurality of color filters positioned in correspondence with the electrode structure and two linear polarizers.

In accordance with the invention, the color filters comprise reflective interference filters for transmitting a selected color or colors of light at a particular pixel location and reflecting substantially all other wave­lengths of light. The system further comprises a display screen, and projection optical elements positioned with respect to the liquid crystal device so as to project the image light transmitted through the liquid crystal device onto the screen. The electrode structure is selectively energized so that light of a desired color or colors is transmitted through pixel locations of the liquid crystal device, and light of undesired colors is either reflected from corresponding pixel locations or absorbed in the polarizers. Substantially no light energy is absorbed in the color filters.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will become more apparent from the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings, in which:

• FIG. 1 is a cross-sectional view of a liquid crystal device employing reflective color filters in accordance with the invention.
• FIGS. 2A-2D represent four alternative embodiments of the patterning of the reflective color filters employed in a liquid crystal device in accordance with the inven­tion.
• FIG. 3 is a simplified schematic diagram of a projection system employing a liquid crystal device with reflective color filters in accordance with the invention.
• FIGS. 4A-4C are simplified optical structure and ray diagrams illustrating the absorption of light energy by the conventional LCD device with absorptive color filters (FIG. 4A) and projection systems employing LCD devices with reflective color filters.
• FIG. 5 is a cross-sectional view of an alternate embodiment of a liquid crystal device employing the invention, with a polarizing prism for polarizing the incident projection light.
• FIG. 6 is a simplified illustration of a particular pixel location for the device of FIGS. 1 or 5.
• FIGS. 7A-7D illustrate one fabrication technique for fabricating the patterned reflective color filters em­ployed in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-sectional diagram of a liquid crystal device structure 20 containing color filters. The structure includes linear polarizers 22 and 36 sandwiching the interior elements of the structure. Glass substrates 24 and 34 in turn sandwich the pixel or column electrodes 30, the liquid crystal material 28, the common or row electrodes 26, and the color filters 32.

The glass substrate 34 is also coated with color filter materials which are patterned to correspond to the matrix of display picture elements or pixels. Possible patterns for the color filters are illustrated in FIGS. 2A-2D. These patterns include the three color diagonal arrangement of FIG. 2A, the three color vertical strip arrangement of FIG. 2B, and three color horizontal strip arrangement of FIG. 2C, and the two color checkerboard pattern shown in FIG. 2D. Other patterns not shown specifically in FIGS. 2A-2D could also be used. Further, either two (typically red and green) or three (red, green and blue) color filter types may be employed. The number of color filter types and the filter dot or pixel arrange­ment is not critical to this invention.

By selectively energizing the liquid crystal materi­al adjacent to the appropriate color filter dots, symbolic and video information can be shown in color.

The electrodes 30 are coated on one side of the glass substrate 24. The electrodes 26 are coated on the color filters 32. These electrodes are patterned to define a matrix of picture elements, as described, for example, in U.S. Patent 4,006,968. Many techniques for defining and electrically driving the matrix of picture elements in a color liquid crystal device are known in the art. The particular technique employed is a matter of design choice.

In conventional liquid crystal structures, the color filters take the form of patterned organic or inorganic pigments which then absorb the incident light except for the desired narrow color band which is transmitted. This type of color filter arrangement is used in the small LCD televisions on the market today.

In accordance with the invention, the color filters 32 of the liquid crystal structure 20 comprise reflective multilayer dielectric dichroic mirrors. Such mirrors are composed of alternating layers of two materials such that the reflections from the interfaces between the layers interfere upon reflection. Such color-selective mirrors are well known in the art, e.g., The Handbook of Optics, pages 8-58 to 8-63, Chapter 8. Striped reflective di­electric mirrors have been fabricated and incorporated into a reflective liquid crystal device by the assignee of this application, as described in the Final Report NADC-­77212-30, "Development of a Color Alphanumeric Liquid Crystal Display," prepared for the Naval Air Development Center, December 1979, by J.E. Gunther. Such structure is understood to have been employed only for a direct view system.

FIG. 3 shows a simple projection system 100 employ­ing a transmissive liquid crystal device with reflective color filters in accordance with the invention. The system comprises a light source 102, a condensing lens 104 which serves in this exemplary embodiment to collect the light generated by the light source 102 to direct white projection light onto the liquid crystal device 106. In other applications, a reflecting optical element may be alternatively employed instead of the condensing lens to collect the light generated by the light source 102. The device 106 employs the reflective color filters as described above with respect to FIG. 1. The color image light transmitted through the liquid crystal device 106 impinges on the projection lens assembly 108 for projection onto screen 110.

The advantages of the color projection display system 100 can be more fully appreciated with respect to FIGS. 4A and 4B. FIG. 4A illustrates the optical light path through a conventional transmissive liquid crystal device employing absorptive color filters. The incident light impinges on the input polarizer 120, where 50% of the light is absorbed. The remaining light of the desired polarization passes through the polarizer 120 and impinges on the color filter 122 comprising the liquid crystal device. Typically, 70% of the polarized light is absorbed in the filter 122 with only the remaining 30% of the polarized light passing through the liquid crystal layer. The voltages applied to the liquid crystal layer determine the spatial pattern of the light that transits the device to the user. Therefore, much of the light energy is absorbed in the color filter, which for high intensity projection light applications would lead to fading or bleaching of the color filters, resulting in a loss of display contrast or color purity. Thus, the conventional liquid crystal device using absorptive color filters is not suitable for projection display applications.

FIG. 4B illustrates the optical light path through a liquid device employing reflective color filters in accordance with the invention. The incident light im­pinges upon the polarizer 130, where 50% of the incident light is absorbed as in FIG. 4A. The remaining polarized light impinges on the color filters 132, where the un­desired light is reflected, and the desired light colors are transmitted through the filters. Since no light is absorbed by the filters, the liquid crystal device may handle higher intensity input light without degradation. As a result, such advice may be advantageously employed in projection display applications.

The operation of a second embodiment of the inven­tion is shown in FIG. 4C. In this embodiment, the input polarizer of the liquid crystal device is omitted, and a polarizing prism is disposed in front of the device. The light component of the incident light which is polarized in the desired orientation is passed by the prism, while the oppositely polarized component is reflected by the prism. The passed light is treated by the reflective color filter 142 in the same manner as is described with respect to filter 132 in FIG. 4B. Thus, the alternative embodiment results in less light power dissipation in the liquid crystal device as compared to the first embodiment.

The second embodiment is shown in further detail in FIG. 5. Here, a polarizing prism 150 is assembled with a liquid crystal structure 20′ which in this embodiment is identical to structure 20 of FIG. 1, except that the linear polarizer 36 of structure 20 has been replaced with the prism 150. The prism 150 can be assembled to the glass substrate 34′ by index-matching glue. Thus, the structure 20′ comprises reflective color filters 32′, common or row electrodes 26′, the liquid crystal material 28′, the pixel or column electrodes 30′, the glass sub­strate 24′ and linear polarizer 22′.

The operation of the projection display system of FIG. 3 may be understood with reference to FIG. 6, which shows an exemplary pixel element 40 of the patterned color filters 32, comprising three adjacent red (R), green (G), and blue (B) filters, and with reference to FIG. 1 which shows the liquid crystal device. The color filters are shown much larger than the actual size. The dimension of the three respective color filters at each pixel location are in reality smaller than the resolving power of the human eye. The projection light beam is of white light, and is incident on the liquid crystal structure 20, initially impinging on the linear polarizer 36. The incident light is polarized by element 36, so that substantially only light energy of the desired polarization orientation is passed through the polarizer 36, and is incident on the color filters 32, the transparent electrode structure 26, and the liquid crystal material 28. The color filters 32 selectively reflect incident light so that only the desired light color will be transmitted through a particular filter. The red filter transmits the red light while substantially reflecting light of other visible wavelengths. Similarly the blue filter transmits the blue light while reflecting other wavelengths, and the green filter transmits the green light while reflecting other wavelengths.

As will be appreciated by those skilled in the art, depending on the particular design, the combination of the liquid crystal material and linear polarizer is either substantially transparent or opaque to the incident polarized light when no potential is applied to the material. Assume that the liquid crystal material 28 and polarizer combination is of the type which is opaque when no potential is applied, and that the pixel location 40 is to appear red. Then only the electrode adjacent the red color filter is energized, so that substantially no light is transmitted by the liquid crystal material adjacent the blue and green filters, i.e., so that no blue or green light is passed through the liquid crystal material 28 for the pixel location 40. Only red light is permitted to transmit through the pixel location so that the viewer perceives a red spot of light at this pixel location. This operation is repeated for the hundreds or thousands of other pixel locations of the liquid crystal device, to form a full color image defined by the electrode address­ing circuitry. The image may be varied dynamically and periodically refreshed by the electrode addressing cir­cuitry, as is well known in the art.

The color filters 32 could be placed "downstream" of the liquid crystal light, so that the incident projection light first passes through the liquid crystal material, instead of the arrangement shown in FIG. 1 without signif­icant impact on this invention.

FIGS. 7A-7D illustrate one technique by which the patterned reflective color filters 20 may be fabricated on the substrate 34. This is a "lift-off" photolithographic process as is commonly used in LSI semiconductor device fabrication. Other additive or subtractive processes can also be used to fabricated the patterned filter.

In FIG. 7A, a patterned photoresist layer 202 has been formed on the substrate 34, with opening pattern 203 which corresponds to the position of particular color filter locations, e.g., the red filters. Next, the thin film layers 204 comprising the red filters are deposited on the structure shown in FIG. 7A, with the resulting structure shown in FIG. 7B. The layers are formed not only on the substrate through the openings 203, but also on the photoresist 202. In the next step, the photoresist layer 202 is removed, with the layers 204 deposited on top of the photoresist 202 being lifted-off as the photoresist is removed by a dissolving agent. The resulting struc­ture, shown in FIG. 7C, has only the properly located red filters 204 deposited on the substrate 34.

The process is then repeated to form the green and blue filters, e.g., filters 208 and 210. Lastly, a light absorbing material 212 is commonly deposited in the interstices of the filters to enhance the contrast. The resulting patterned filter is shown in FIG. 7D.

The invention has utility for such applications as color VIDs or head-up displays for automotive applica­tions.

It is understood that the above-described embodi­ments are merely illustrative of the possible specific embodiments which may incorporate principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope of the invention.