Helmet-mounted Displays: Microlenses improve display efficiency

Novel microlens arrays used in a helmet-mounted display increase light throughput by two orders of magnitude. The subtractive-color helmet-mounted display includes three 1024 × 1024 active-matrix liquid-crystal displays (AMLCDs) that act as spatial filters; one AMLCD for each color creates an image at 460, 610, or 530 nm in this cyan-magenta-yellow display.

Within each AMLCD, each pixel has a clear aperture of about 20.3 × 20.3 µm—the rest of the device area is taken up by electrical bus lines and control capacitors. The AMLCD resolution is about 1000 pixels/in., so the opaque electrical bus lines between pixels act as a diffraction grating, causing light that passes through one pixel on the first AMLCD to spread through several pixels on the second AMLCD and several more on the third AMLCD. The resulting loss of light reduces both the contrast and brightness of the overall output image. Ideally, the light from one array pixel would be focused on the corresponding single pixel on the next array.

To reduce such losses, helmet developers at Hughes Danbury Optical Systems (HDOS; Danbury, CT) sandwich the AMLCDs between microlens arrays that focus light into the active area of each pixel. For every pixel in the active-matrix liquid-crystal display, there is a microlens before and after it in the light path. Diffractive microlens arrays, however, do not work well due to the small size of each lens, the multiple wavelengths at which the lenses must operate, and additional light scattering caused by this type of optical element.

Instead, traditional analog refractive elements are designed and fabricated by a proprietary process at HDOS. The minimum center-to-center spacing of micro lenses made by this method is about 10 µm. According to Enrique Garcia, associate director of research at HDOS, lenslets made this way perform better than diffractive optics. Furthermore, the element-to-element spacing and uniformity of the lenses are better.

With the array of spherical microlenses on a hexagonal grid, about 30% of the illumination reaches the screen location, as opposed to 0.8% of the illumination with no microlenses.

Lenslets

Although the method of making the microlens arrays is proprietary, Garcia says that the process is easily scalable and is compatible with batch-mode fabrication, advantages that stem from the method being based on photolithography and etching. He adds that array sizes are limited by the size of available masks and substrates.

Unlike methods introduced several years ago by other companies, the HDOS process does not depend on a particular glass that expands when exposed to radiation. And, although the company has used fused silica as the substrate, Garcia says the process is compatible with other optical glasses as well as silicon and some III-V materials. Concave lenses are produced and can be converted to positive (focusing) lenses by filling the concavity with another material of appropriate refractive index and adding a cover glass. The process allows the researchers to make spherical and cylindrical lenses (with circular cross sections) says Garcia, although it limits the ability of researchers to create aspheric or acylindrical shapes.

Currently, Hughes Danbury Optical Systems is making the lenses primarily for in-house use. In addition to the color-display application, the company is interested in using the lenses for beam shaping--that is, putting energy into patterns specifically for photolithography applications. Systems with these "beam uniformers" may be able to cheat the wave length resolution limit. Another possible application of the lenses is in optical interconnects.