One type of liquid crystal light valve is a projection-type image display apparatus. The light valve comprises an evacuated ceramic envelope that incorporates a pair of opposing transparent windows as part of the envelope wall. One window forms part of a liquid crystal cell. The remainder of the cell is assembled adjacent to the inner surface of that window. Polarized light is typically directed through the envelope windows. The light exiting the liquid crystal cell passes through a polarizing filter or analyzer. Any light passing through the analyzer is transmitted via a projection lens system onto a suitable viewing surface.
The liquid crystal cell includes liquid crystal material captured between two substrates. One substrate is formed of glass and is incorporated into the wall of the envelope to serve as a window as mentioned above. The opposing substrate comprises a thin dielectric material. The cell is preferably of the twisted nematic type, which is constructed so that in the absence of an applied electric field (i.e., with the cell in the "OFF" state) the cell rotates by 90.degree. the polarization direction of the projected light. With the cell in the "OFF" state, no light passes through the analyzer and the viewing surface remains dark. When an electric field is applied to the cell (i.e., when the cell is switched to the "ON" state) the projected light passes through the cell with the polarization direction unchanged. As a result, the light passes through the polarizing analyzer to the viewing surface.
An electron beam-addressed liquid crystal light valve employs an electron beam for modulating the polarization direction of light passing through the liquid crystal cell. Specifically, an electron gun is mounted within the light valve envelope and provides a beam of electrons that strike the side of the liquid crystal cell carrying the thin dielectric substrate, which is called the target substrate. The electron beam direction is deflected by suitable circuitry to raster scan the target substrate. A collector electrode is mounted within the liquid crystal light valve near the target substrate. To produce or "write" an image, the electron beam and collector electrode positioned above and over the target substrate cooperate to develop an electrostatic potential at certain points on the surface of the target substrate that correspond to the desired image. As a result, those points on the liquid crystal cell are switched to the "ON" state, thereby permitting associated portions of the projected light of unchanged polarization direction to pass through the valve and form the image on the viewing surface.
As noted, the target substrate for an electron beam-addressed liquid crystal light valve preferably comprises thin dielectric material. The target substrate must be thin to minimize spreading of the electric field lines produced by the charge deposited on the surface. Spreading of the field lines reduces the resolution of the projected image. A film of five to ten microns in thickness forms a suitable target substrate.
Copending U.S. patent application of Buzak et al., Ser. No. 046,822, entitled Liquid Crystal Light Valve with Electrically Switchable Secondary Electron Collector Electrode, filed concurrently herewith, describes a particular configuration and optimal operation mode for an electron beam-addressed liquid crystal light valve. More particularly, that application describes write and erase modes wherein the collector electrode is configured and arranged to create strong collecting fields to control the redistribution of secondary electrons generated by the electron gun. When in the write mode, the collector electrode is maintained at positive voltage relative to the target substrate as the electron beam bombards the target substrate with electrons of sufficient energy to result in secondary electron emissions from the target substrate surface. The secondary electrons are collected on the positively charged collector electrode. The rate of secondary electron emission is greater than the rate the incident electrons are delivered by the electron gun. Consequently, the beam-addressed area of the target substrate surface is driven positive. This change in potential switches the corresponding region of the liquid crystal cell to the "ON" state
To erase the image (i.e., to switch the previously written region of the liquid crystal cell to the "OFF" state), the collector electrode is switched to a negative potential relative to the written region of the target substrate. An electron beam, either emanating from the same gun as used for writing or from a separate gun, is scanned over the target substrate. The resulting secondary electrons, repelled by the negatively charged collector electrode, are directed to the previously written (positively charged) regions. Accordingly, the positive potential difference at the previously written areas of the target substrate is removed and the corresponding region of the liquid crystal cell is switched to the "OFF" state.
The above-noted copending U.S. patent application presents an operating mode for an electron beam-addressed liquid crystal light valve wherein both a writing electron gun and an erasing electron gun are operated to produce electron beams with energy suitable for generating secondary electrons at a rate greater than the rate the incident electrons are delivered by the beams (the latter rate being the beam current). The ratio of secondary electron emissions to incident electrons is known as the secondary electron emission ratio. Accordingly, for the operating modes just described, the writing and erasing guns are controlled so that the secondary electron emission ratio is always greater than one.
For any given beam energy, the rate of secondary electron emissions from the substrate varies depending upon the material used as the target substrate. Further, it is desirable to bombard the substrate with relatively low incident beam current to minimize the beam spot size and produce a correspondingly higher resolution image.