facilities required for encoding the holograms, but more importantly,

the plotting devices have been the major bottleneck. The typical

procedure for fabricating a computer generated hologram is to have the

digitized interference pattern, which has been calculated and encoded

by computer, drawn to a large scale by a computer-driven plotter. The

drawing is then reduced photographically onto high-resolution film to

the desired final size. Unfortunately, errors are introduced in the

plotting and photo-reduction processes. In addition, optical plotting

devices are limited in spatial resolution and space-bandwidth product,

typically to 106 pixels. The resolution is limited by the number of

discreet points which can be placed on the paper and the accuracy of

the copy process. This printing technique is strictly binary and

requires binary mapping techniques as described in Chapter III.

 To produce continuous-tone holograms, a gray-scale writing device

is needed. With such a device, a pattern can be produced containing

transmittance values ranging continuously from zero to one. One of

the earliest examples is the rotating drum scanner. Such scanners

have been used in the newspaper industry for years to send pictures

electronically over phone lines. In the rotating drum scanner, a

photographic film is wrapped about a drum rotating at a fixed speed.

As the drum rotates, an incoherent light source is focused to a spot

on the film and moves sideways along the length of the film. In this

fashion, the light source scans a helix along the drum, providing a

raster scan on the film. The light is intensity modulated according

to the pattern to be written to the film. These drum writers are

commonly available from a number of companies, along with interfaces

for most common computers.