Compact image generators that contain illumination sources and electronically controlled spatial light modulators have been invented. Compactness is achieved by folding of the optical paths that link the illumination sources, the spatial light modulators, and the viewing regions into which images are projected. The optical configuration of a device of this type ensures that a large proportion of the light from the illumination source is directed into the viewing region; consequently, the device is unusually energy-efficient for a display device and can, therefore, be operated at a relatively low power (possibly even battery power) for a given display brightness. By virtue of their compactness and low power consumption, these image generators are suitable for head-mounted, helmet-mounted, and eyeglass-mounted displays.

Compactness and Efficient Utilization of Light are achieved by a combination of folding the optical path, polarization, and focusing of light from the modulator and the illumination source.

These image generators can be designed in a number of alternative optical configurations, of which one is depicted in the figure. The precise nature of the illumination source is not critical; the source can consist, for example, of one or more light-emitting diodes, laser diodes, cold-cathode or field-emitter cathodoluminescent sources, or incandescent or fluorescent lamps together with a switchable color filter. The spatial light modulator is of a reflective (as distinguished from transmissive) type that effects modulation by either changing or not changing the polarization of light upon reflection, depending on the electronically controlled ON/OFF status of each pixel. The modulating medium in the spatial light modulator is typically a ferroelectric liquid crystal layer.

Light from the illumination source is directed through an auxiliary polarizer into a polarizing beam-splitting cube. The auxiliary polarizer passes only light that is s-polarized with respect to incidence on the polarizing beam-splitting film in the cube. The film reflects most of this s-polarized light toward the spatial light modulator. The light reflected from the spatial light modulator contains the desired image in the form of pixel-by-pixel variations in the proportions of s-polarized and p-polarized light.

The modulated light goes back into the cube, where it is analyzed by the polarizing beam-splitting film: The s-polarized (unchanged) portion of the modulated light is reflected back toward the illumination source. The p-polarized image-bearing light passes through the film, then out of the cube, then through a quarter-wave plate, until it strikes a concave mirror. After reflection from the concave mirror, this light passes back through the quarter-wave plate.

The double pass through the quarter-wave plate converts the polarization from p to s; consequently, upon striking the polarizing beam-splitting film, this image-bearing light is reflected out of the cube toward a viewing region. The curvature (and thus the focal length) of the mirror is chosen, in conjunction with the other dimensions of the optics, so that (1) to ensure efficient utilization of light, a real image of the illumination source is formed within the viewing region and (2) a magnified virtual image of the pattern of modulated light can be viewed by an eye placed within the viewing region, facing toward the cube.

In some applications, it is desirable to provide for adjustment of the gap between the cube and the spatial light modulator and the gap between the cube and the concave mirror, in order to enable focusing of the viewable image. Optionally, once these adjustments have been completed, the various optical components, including the cube, can be cemented together to produce a rugged assembly that resists misalignment.

This work was done by Mark A. Handschy, Michael R. Meadows, Martin Shenker, and Paul E. Weissman of Displaytech, Inc., for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at  under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Mark Handschy
Displaytech, Inc.
2602 Clover Basin Drive
Longmont, CO 80503

Refer to MSC-22992, volume and number of this NASA Tech Briefs issue, and the page number.