A compact display was designed for white-light-transmission (WLT) holograms (30cm x 40cm). The unit, measuring 40cm x 40cm x 15cm, contains a tungsten halogen bulb and a folded play-back beam, providing an aesthetically pleasing alternative to the usual large space needs of WLT holograms. The inevitable availability, in the near future, of red, green and blue diode lasers will bring the possibility of their use in the play-back of full-color transmission holograms. In this paper I explore the recording and playback requirements as well as several approaches for future display designs.

Keywords: holographic art, holographic display, white-light-transmission holograms, full-color holograms, laser diodes

1. BACKGROUND

Unlike reflection holograms that can hang on the wall, a WLT hologram requires placement at eye level, several feet in front of the wall on which the light fixture is positioned. To accomplish this I usually position the art work on a tripod that stands on the floor, or I suspend the work from the ceiling using monofilament. Both arrangements work successfully for a gallery or a museum, but it takes up a lot of space at home.

The need for space in the display of WLT holograms results from the holographic recording geometry, a two-step process of making a hologram of a hologram. This requires the use of a conjugate reference beam, the beam that is traveling in the exact opposite direction. In the recording step this can be accomplished with a large collimating lens or parabolic mirror. In my work I usually approximate collimation and use a long reference beam in the recording; and in the display I use a long distance between the light source and the hologram.(ref.1)

The best illumination for a WLT hologram is a point source of white light. A close approximation to that is a clear glass light bulb with a single, straight filament, positioned vertically (as viewed from the hologram surface). This arrangement will insure a single, clean and sharp image with good depth of field.

2. NEW DISPLAY OPTION

In 1996 an art collector, commissioned me to create a new piece, stipulating that I do away with the light fixture and incorporate it into the hologram. My first approach was to design a laser- viewable transmission hologram using a very short reference beam and two or possibly three diode lasers for play-back illumination. In this way I would have duplicated the original recording beam, allowing a deep space experience without distortions (ref.2). But at this time, only red diode lasers are available. (In the near future I expect to see affordable red, green and blue diode lasers available for play-back)

Another solution would have been the edge-lit holographic technique that was developed at MIT (ref.3,4). Unfortunately, the size of the requested work (80cm x 30cm) required more research and development, and as an individual artist engineer working alone, I am not in a position to take on such projects at this time.

The next available option was to shorten the play-back beam and to fold it up into the smallest possible package while making sure that the image distortions were acceptable (fig.1). The outside dimensions of the final design ended up measuring 40cm x 40cm x 15cm deep, the hologram being 30cm (height) x 40cm (width) (fig.2). The center axis of the play-back beam makes a 60 degree angle with the axis perpendicular to center of the hologram (the normal).

The distance from the bulb's filament to the center of the hologram is 57.5cm. To accomplish this, I used a 55cm diameter, 225cm focal length parabolic mirror to shape the reference beam for the recording of the hologram into a converging beam (fig.3).

For the light source I use a 12volt, 35watt, tungsten halogen bulb - small compared to the 150 watt halogen bulb that usually illuminates my free standing or hanging art work. The bulb has no reflector or focusing lens. The lamp housing allows only light coming directly from the filament (positioned vertically) to reach the hologram. The rest of the light is blocked and absorbed by the housing.

3. DIODE LASERS TO PLAY BACK HOLOGRAMS?

My original plan, to use laser diodes to view the commissioned artwork, has many untapped possibilities. For one, using laser light for playback will keep both the horizontal and vertical parallax intact, with no astigmatism, and no distortions from the two-step process. By using three colors from three diode lasers, theoretically holography could open up to natural color work. But beyond that, the compact size, low cost, long life time and low energy needs of the diode laser are key features that could make them very useful in holography.

Looking at a laser viewable transmission hologram is always exhilarating. The "solid" experience of virtual space without glasses and the exquisite detail possible, makes this holographic technique stand out. However, up to the present day, the need for expensive lasers has confined this experience to the laboratory. New solid-state laser development could change that. Anticipating the production of red, green and blue, compact solid-state lasers, I ray-traced several options for a color display unit and I outline here some features that would make diode lasers user-friendly.

3.1. RECORDING COLOR IN A TRANSMISSION HOLOGRAM

Colors can be recorded simultaneously in a holographic emulsion that is panchromatic, but to prevent cross-talk between the colors during play-back, each reference beam has to be spatially separated from the other colors by 90 degrees. In play-back with three colors, cross-talk would result in a red, green and blue image for each recorded color, nine miss-aligned colors in all.

The play-back geometry has to duplicate the recording geometry. Accommodating three or four separate reference beams makes this recording set-up truly three-dimensional (fig.6), as opposed to the usual two dimensional structure of a holographic set-up with one reference beam. For the illumination of the subject matter, the beams need to be combined and aligned with dichroic mirrors before reaching the object.

Unlike a photograph, a hologram cannot record fluorescent colors, and it can be difficult to record colors of high saturation. For instance, when the object's reflected narrow band of wavelengths falls outside the region of wavelengths of the illuminating lasers, those colors are not recorded (ref.5).

3.2. LASER PLAY-BACK

Figure 4, drawn to scale, shows the display for a laser viewable hologram, measuring 60cm x 60cm. To keep the undiffracted (zero order) light out of the viewing area, I positioned two reference beams off-center, in the top two corners. The third reference beam is centered and comes from below.

To illuminate the hologram evenly, I folded and directed each beam with a front surface mirror. The lasers with expanding optics are placed at the focii of the separate color holograms (marked by dots in the drawing, rd, gr and bl). Those focii are recorded in the hologram as the locations in space where the reference beams originated (usually a pinhole in front of a lens). The alignment is done by positioning the mirrors, the laser, or both. A fiber optic, delivering laser light and fitted with an expanding lens, could be very effective

A simpler version, one without mirrors, requires fast expanding optics. In a four color version (fig.5) the distance from each laser to the center of the of the hologram is 75 cm as compared to 142 cm in the mirrored version (fig.4). And because of the extreme off-axis nature of the recording and play-back, the intensity distribution of the light over the hologram surface becomes more uneven. Positioning of the beams for the recording will be a challenge. Hopefully fiber-optics will make this more manageable. Figure 6. helps to visualize the position of four reference beams on an optical bench, and the object space that is available in the hologram.

3.3. SUGGESTIONS FOR A USER-FRIENDLY PLAY-BACK LASER

Ideally, the laser (diode) housing incorporates the adjustable expanding lens that illuminates the hologram evenly at the intended distance. At the same time, this lens should minimize astigmatism of the lazing source itself. Diode lasers usually have a linear aperture emitting light. Astigmatism could be a problem when the recording and play-back lasers are different. Using the same lasers to record and playback the hologram insures the most accurate spatial image reproduction (no size change from wavelength difference). This leaves the optical alignment and the shrinkage of the emulsion as the two remaining sources of distortion in the system.

Smearing (blurring) the laser speckle (while maintaining spatial coherence) would improve the visual appearance of the holographic image and erase the imperfections of the expanding optics at the same time. This makes spatial filtering, to "clean" the beam, unnecessary. It could be accomplished opto-electronically or electrically by shifting (moving) the phase of the emitted light randomly.

In the best of scenarios, a fiber optic, coupled on one side to the laser diode and at the other end fitted with a build-in expanding lens to illuminate the hologram, would make the display lightweight, easy to adjust, and aesthetically exciting.

Depending on the efficiency of the hologram and the ambient light, a 10 mW laser, at 635nm, is adequate to play back a hologram measuring 20cm x 25cm. When fiber optics are guiding the light, the power requirements might double due to coupling losses.

CONCLUSION

The white light display works very well and has given me new freedom to exhibit and sell my work. The prospect for a successful color display depends a lot on new photonic tools that become available in the future. I've been working for more than 22 years in holography and, in that time, the tools have not changed that much. It has only been in the last couple of years that affordable, solid-state lasers have become available. I have no doubt that these new products are the beginning of an exciting new chapter in photonic research and development, which will lead to new, affordable and user-friendly tools for holography.

5. REFERENCES

1. Rudie Berkhout, "Using HOE's in the holographic image making process", Proceedings of the SPIE, Vol. 2652, 1996, pp. 204-212

2. E.N. Leith and J. Upatnieks, "Wavefront reconstruction with diffused illumination and three- dimensional objects", J. Opt. Soc. Am., 54, pp. 1295-1301, 1964

3. S.A. Benton, S.M. Birner, and A. Shirakura, "Edge-lit rainbow holograms", Proceedings of the SPIE, Vol.1461, 1991, pp. 149-157

4. J. Upatnieks, "Edge-illuminated holograms", Applied Optics, Vol. 31, 1992, pp. 1048-52

5. Graham Saxby, Practical Holography, second edition, Chapter 17, pp. 273-283, Prentice Hall, New York, 1994

ã 1998 Rudie Berkhout. All Rights Reserved.

email: rudieberkhout@mindspring.com

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