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Illumination & Displays
Holographic screen for image projection
Yuri Denisyuk, Nina Ganzherli, Irina Maurer, Dmitry Chernykh
Research by the late Yuri Denisyuk and colleagues details a new type of
on-axis holographic screen for color image projection systems using a
reference-free volume hologram.
One of the emerging applications of holography is in the fabrication of
screens on which images can be projected. These can be used, for instance, as
displays for driver training. The advantage of this kind of screen is that,
when it reconstructs an image, it produces a viewing zone in a definite,
limited region of space.
Most projection systems are on-axis symmetrical. One of few ways of creating
an on-axis holographic screen was suggested by D. Gabor in 1948 and involves
a thin hologram.1 We initially considered using this type of screen, which
required that a reference source (a diffuser) be placed in the center of the
object being recorded.2 In the resulting centered hologram, the conjugate
image and halo did not distort the reconstructed real image. However, the
bright point of the reference source caused certain distortions to the
projected image. It seemed important, therefore, to develop an on-axis screen
capable of suppressing the zeroth order bright point. (Note that the Leith
off-axis hologram3 has considerable distortion due to the images being
spatially separated and super-imposed on the main images.)
While working on the centered hologram screen we arrived at the idea of using
a reference-free volume hologram. During recording, we wondered if phase
modulation might suppress the zeroth order artifact in the reconstructed
image.4 In order to record such a screen, the diffuser had to be illuminated
using coherent radiation. The rays scattered from different points on the
diffuser to produce a complicated interference pattern which was then
recorded on light-sensitive film. Each point on the diffuser—in this type of
hologram—may be considered as a reference beam for all the other points,
thus creating speckle. At reconstruction, the image of the diffuser acts as a
viewing zone for a two-dimensional image projected onto the screen: the image
reconstructs the hologram of the diffuser, which is projected out into space.
The volume recording medium was made of thick (0.10 to 0.15mm) layers of
self-developing dichromated gelatin, with glycerin added, sensitive in the
blue range.5 The hologram recording process was controlled during exposure.
As the exposure increased, the bright point in the zeroth order of the
viewing zone diffused. Due to this, the zone was uniformly filled with
diffuse light.
Having been recorded in blue, the hologram allows the reconstruction of other
wavelengths, including white light. Thus, the hologram can to be recorded in
the sensitivity range of photographic film, making it useful in color image
projection systems. Figure 1 shows photos of projected images when viewed
through (a) a holographic (speckle) screen and (b) a glass diffuser. The
projected image is brighter in the former.
Figure 1. Shown are projected images from (a) a holographic (speckle) screen
and (b) a glass diffuser. The image on the right is clearly brighter than
that on the left.
Thus, inspired by the thin Gabor hologram, we suggest a new hologram type.
The reference-free volume hologram can compensate for the distortions of the
main reconstructed image typical of a Gabor hologram. This allowed us to
produce an on-axis holographic screen that does not transmit the zeroth order
bright spot. In addition, this screen does not produce a halo or a conjugate
image. The reference-free volume hologram allows recording in one spectral
range and reconstruction in another. This feature simplifies the choice of
recording medium and makes it possible to project a color image through the
screen.
This work was supported by the Russian Foundation for Basic Research (grants
no. 04-02-17593 and NSh-98.2003.2).
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