During the past year, monochrome holographic reflectors have brightened displays in pagers and other hand-held devices. The latest development is a white holographic reflector that may soon colorize the displays (see photo on p. 44). The new reflectors were developed by Michael Wenyon, William Molteni, and Philip Ralli at Polaroid Corp. (Cambridge, MA) and demonstrated at the 1997 International Symposium of the Society for Information Display held recently in Boston, MA.1
The early holographic reflectors are volume-reflection holograms. These are monochromatic, usually green. The Polaroid researchers developed the white reflector by mounting a transmission hologram in front of a mirrored surface, such as aluminized plastic film. The transmission hologram operates in white light to produce a white image. The reflector directs incident ambient light into a diffuse white cone of controlled angular extent, normal to the display surface. This produces a gain relative to metallic reflectors.
Prototype holographic achromatic LCD reflectors were fabricated by two different methods. The first method involved optical copying into Polaroid DMP-128 photopolymer, a proprietary photosensitive medium, using a krypton-ion laser exposure to create a volume hologram. In this particular volume hologram—any recording medium that contains information on its surface as well as throughout the body—microscopic interference fringes run throughout the less than 20-µm thickness of the recording layer. Their tilted orientation and spacing both affect the hologram`s final properties.
Without a backing mirror, the photopolymer reflector acts like a traditional transmission volume hologram. The subject of the hologram is a uniform flat object, placed by an imaging lens or a holographic transfer process onto the plane of the final hologram. The illuminating beam first passes inertly through the hologram from the front, because of mismatch between the incoming beam and the Bragg angle of the tilted fringes. When the light reflects from the mirror, it returns to the hologram at the Bragg angle, and the fringes now couple light into the recorded image, a diffuse field that determines the viewing zone of the final hologram.
In the second method of producing holographic reflectors, the researchers mechanically embossed a hologram onto the surface of an aluminized plastic film. The metal layer enhances efficiency and transforms what was originally a transmission hologram into one that can be viewed by reflection. The reproduction is purely mechanical and involves no optical exposure. The holographic "imaging" is the same in both photo polymer and embossed holograms.
Both reflectors demonstrated gain one to three times that of metallic reflectors at typical viewing conditions of 20 overhead point-source illumination and viewing at zero degrees. The researchers recognize that there are trade-offs between the size of the viewing zone and the maximum gain, so they plan to continue to optimize both the size and shape of the reflectors` viewing zones.
According to Wenyon and Ralli, the photopolymer transmission holograms ultimately offer the greatest possible efficiency and hence image brightness, whereas embossed holography has other advantages because it is an existing manufacturing method. Future work will include bringing both processes to the point of qualifying them for LCD-ready quality and manufacturing standards before making relative assessments and determining their optimum areas of application. Commercial products featuring white holographic displays are expected to be on sale by the end of this year.
REFERENCE
1. M. Wenyon, W. Molteni, and P. Ralli, "White holographic reflectors for LCDs," Proc. 1997 International Symposium of Society for Information Display (1997).