H. I. B jelkhagen
Recording Materials

The generation of a surface-relief hologram is one way for obtaining transmission
phase holograms already discussed in Chap.S. However, concerning
silver-halide materials the effect is normally limited to low spatial frequencies.
Also sometimes an unwanted surface-relief pattern is created as a result
of certain bleaching processes. Such a pattern is then only considered a
source of noise. We shall therefore devote some time to 'discussing the possibilities
of enhancing and extending the relief pattern to higher spatialfrequencies.
The fact that a surface-relief image can be obtained in a photographic
silver-halide emulsion has been known since the 19th century. The Koppmann
process, for example, uses gelatin relief images for dye printing. In
photography such images have also been used for color reproduction.
The main procedure to obtain a relief structure on photographic materials
is to use either a tanning developer or a tanning bleach, or else a combination
of the two. The gelatin of the emulsion must be soft (very little or
no hardening of the unexposed material). Pyrogallol or pyrocatechol developers
are often used. After development the emulsion is washed in hot water
to remove the gelatin (from unexposed areas) which has not been hardened
during the development. This gives a surface-relief image which is,
however, obtained mainly at low spatial frequencies. A tanning bleach, such
as Kodak R-IO, can also be employed. Concerning the tanning development
of photographic dye-transfer images a good review by Tull has been published
. The following developing agents have a tanning effect on gelatin
according to Tull: pyrogallol, pyrocatechol, amidol, and hydroquinone.
All these tan the gelatin at various degrees when. mixed with suitable alkali.
No sodium sulfite is allowed in the developer. One interesting remark mentions
that the developer promotes a better relief image at a lower temperature
(120 C) than when used at higher temperatures (200 C). Another important
finding was that a supplementary oxidatic:>n step applied before the
hot-water etching takes place, improves the relief structure. The oxidizing
agents tested were potassium ferricyanide, dichromate, and ferric nitrate,
which all enhanced the relief image. After the tanning development the
material is washed, fixed in a nonhardening fixer, washed and then soaked
in the oxidizer for about one minute. After that the material is washed
again and then etched in hot water (800 C). The oxidizing step can be substituted
by an alkali treatment (carbonate). The paper contains a lot of other
useful information that could be important for potential holographic applications
as well. The above-described technique has only been tested for
low-spatial-frequency information recording.
An investigation on relief images on photographic materials (Kodak
Minicard film 6451 and 649-GH) was made by Smith [6.87]. Both tanning
developers and bleaches were tested. Only spatial frequencies between zero
and 200 lines/mm were studied. The main conclusions are:
• The height of the relief image is dependent on the degree of tanning
which is proportional to the amount of silver in the image (proportional
to the optical density).
• The maximum relief is obtained for a certain spatial frequency. This
maximum depends only on the thickness of the emulsion and varies
inversely with the thickness.
• The material processed in a tanning bleach has a higher relief at higher
spatial frequencies than the material processed with a tanning developer.
• The drying method (slow or fast, drying in high or low humidity) does
not affect the height of the relief image.
Later, Smith [6.88] discussed the production of relief images with an
arbitrary profile mainly for low-spatial-frequency applications, such as lenticular
lenses and screens.
Altman [6.89] investigated Kodak high-resolution photographic plates
and the 649-GH film. He established a relation between the height of the
relief image h", and the optical density D. A linear relationship is valid for
densities between 0 and 2.5. In the linear range, the relation is
0= 5.9h", (6.7)
where h", is the height in p.m. This is valid for images developed in the
Kodak HRP developer for 5 minutes at 20° C. This is also an upper limit
for the highest density depending on the recorded line width of the object.

6.86 A.G. Tull: Tanning development and its application to dye transfer images. J.
Photogr. Sci. 11, 1-26 (1963)
6.87 H.M. Smith: Photographic relief images. J. Opt. Soc. Am. 58, 533-539 (1968)
6.88 H.M. Smith: Production of photographic relief images with arbitrary profile. J.
Opt. Soc. Am. 59, 1492-1494 (1969)
6.89 J.H. Altman: Microdensitometry of high resolution plates by measurement of
the relief image. Photogr. Sci. Eng. 10, 156-159 (1966)
6.90 J.H. Altman: Pure relief images on type 649-F plates. App!. Opt. 5, 1689-1690