This application is a continuation of application Ser. No. 752,227 filed July 3, 1985, abandoned. 5
BACKGROUND OF THE INVENTION
The present invention relates to optical systems, and, more particularly, to Schlieren imaging systems.
Light valves, or spatial light modulators, have been 10 used in conjunction with Schlieren imaging systems for many years in projector applications where large, bright displays of video information are required. In these projectors, the electronic video information is converted into corresponding phase perturbations 13 across a beam of light by the spatial light modulator. The Schlieren imaging system then converts the phase modulations across the light beam leaving the modulator into light intensity variations at a viewing screen by blocking the unmodulated light and passing a large 20 fraction of the light incident to modulated areas on the light valve. An optical printer based on this approach has also been recently proposed and converts the modulated light to printed form by xerography. Light valve projectors have at least two features that are important 25 in display or printing applications. The light valves themselves are electronically addressable in an areal (display) or linear (printing) manner. This feature makes it possible to present electronically generated data in "real time". Secondly, the light valve is used to gate or 30 control the light from a separate, external source. The properties of the light source can thus be chosen independently to meet system size, power, and cost requirements while achieving the desired display or photoreceptor irradiance level. 35
Since the attainment of the highest possible light level at the final image plane has been a goal of all light valve projector systems, bright, compact light sources such as arc lamps or lasers have traditionally been used together with efficient optical configurations having the highest 40 possible optical throughput. The light modulating characteristics of the light valve and the configuration of the Schlieren stop that is used with it have a critical impact on the attainable optical efficiency. The stop must be tailored to the light valve in order to both efficiently 45 block the background unmodulated light and pass a large fraction of the signal energy that is diffracted from the modulated areas of the light valve.
Several different light valve technologies have been utilized to date, each one incorporating a different type 50 of stop plane discrimination. A brief overview of some of these technologies will be presented together with proposed improvements to the optical system that has been used with previous cantilever beam light valves.
The oldest of the light valve technologies is the elec- 55 trostatically deformable oil film. It has been incorporated into both the Eidophor theatre projector system and the General Electric color television projector ("Color Television Light Valve Projection Systems," IEEE International Convention, Session 26/1, 1-8 60 (1973)). In both systems, a continuous oil film is scanned in raster fashion with an electron beam that is modulated so as to create a spatially periodic distribution of deposited charge within each resolvable pixel area on the oil film. This charge distribution results in the ere- 65 ation of a phase grating within each pixel by virtue of the electrostatic attraction between the oil film surface and the supporting substrate, which is maintained at
constant potential. This attractive force causes the surface of the film to deform by an amount proportional to the quantity of deposited charge. The modulated light valve is illuminated with spatially coherent light from a Xenon arc lamp. Light incident to modulated pixels on the oil film is diffracted by the local phase gratings into a discrete set of regularly spaced orders which are made to fall on a Schlieren stop consisting of a periodic array of alternating clear and opaque bars by part of the optical system. The spacing of the Schlieren stop bars is chosen to match the spacing of the diffracted signal orders at the stop plane so that high optical throughput efficiency is achieved. Light that is incident to unmodulated regions of the light valve is blocked from reaching the projection lens by the opaque bars of the Schlieren stop. Images formed of unmodulated areas on the light valve by the Schlieren imaging system on the projecting screen are therefore dark, while the phase perturbations introduced by the modulated electron beam are converted into bright spots of light at the screen by the Schlieren projector.
The Eidophor and the General Electric light valve projector are commerically available products that have been used for educational, entertainment, military, and NASA applications where a display suitable for a very large audience is required. They are both large, heavy, and expensive and thus unsuitable for high volume, low cost display or printer applications.
Several efforts have been made to improve on the size, cost, and manufacturability of the oil film projectors ("Survey of Developmental Light Valve Systems," IEEE International Convention, Session 26/2, 1-10 (1973)). Many of these efforts have concentrated on replacing the oil film by a thin, reflective membrane that is mounted to the faceplate of a CRT by means of a support grid structure. These light valves are thus also addressed by a raster scanned electron beam. An electrostatic force of attraction is generated between the charge deposited on the glass faceplate by the electron beam and the membrane, which is held at constant voltage. This attractive force causes the membrane to sag into the well formed by the grid structure, thereby forming a miniature spherical mirror at each modulated pixel location. The light diffracted from this type of modulated pixel is concentrated into a relatively narrow cone that is rotationally symmetric about the specularly reflected beam. This type of light valve must thus be used with a Schlieren stop that consists of a single central obscuration positioned and sized so as to block the image of the arc source that is formed by the optical system after specular reflection from unmodulated areas of the light valve. Modulated pixels give rise to a circular patch of light at the Schlieren stop plane that is larger than the central obscuration, but centered on it. The stop efficiency, or fraction of the modulated pixel energy that clears the Schlieren stop, is generally somewhat lower for projectors based on deformable membranes than it is for the oil film projectors discussed above.
Light valve projectors based on deformable membranes have never been turned into commercial products for at least two reasons. The fabrication process is very susceptible to defects that result when even small, micron sized particles are trapped between the membrane and the underlying support grid stucture. The membrane would form a "tent" over these trapped particles whose lateral extent is much larger than the size of the particle itself, and these tents would in turn