Excerpt from HMP 7 Chapter 4
Silver-Halide Recording Materials
Last year, the primary manufacturer of silver-halide holographic recording materials (Agfa) ceased production. Since that time, several other companies around the world have expanded their manufacturing capabilities to meet the various demands of the holography industry.
In this chapter, we have published the most recent technical specifications available from four major manufacturers of silver-halide emulsions. We have also included pertinent commentary from professional holographers who have had a chance to test and review some of the newly available emulsions.
Introduction
Silver-halide recording materials are commonly coated onto films or glass plates for use in traditional cameras, in laboratories, and in factories. However, they were not originally designed for holographic applications.
The important difference between photographic and holographic materials is resolving power, usually expressed in lines per millimeter. Whereas photographic films usually cannot resolve more than 50-100 lines per millimeter, holographic applications require 1,250-2,500 lines per millimeter.
Another difference is sensitivity, which is typically expressed as an ASA number. For example, popular photographic films are rated at ASA 120-400. Exposures are usually measured in hundredths of a second. Silver-halide holographic recording materials are so much less sensitive than standard films that their ASA would only be rated as fractions. Therefore, their sensitivities are usually expressed in micro-joules per square centimeters (or ergs per centimeter squared). Exposure times are typically measured in seconds, or even minutes, depending on the amount of laser light available.
The silver-halide emulsions are coated on either glass plates or film. Glass is preferred for most holographic applications, especially reflection holography, due to its rigidity. The most popular size glass plates are 4" x 5" and 8" x 10", though display holographers often prefer to work with bigger sizes. It is most economical to purchase larger sizes and cut plates to your own specs, if this is feasible.
The same factories that coat glass plates usually offer their emulsions on film, too. A major advantage of film is that it is less expensive than glass, is easier to cut and curve, and is much more suitable for automated reproduction processes — as it is often available on rolls. The main difficulty faced by holographers using low powered CW lasers and film is keeping the film absolutely motionless during the exposure. Although the film can be sandwiched between clear pieces of glass, better results are obtained when the emulsion is left uncovered. Vacuum mounts and various other devices have been designed to accomplish this feat.
Some plates and films are supplied with an annihilation coating on the back, which can be useful when making transmission holograms, as it helps to cut down on unwanted internal reflections. These plates cannot, however, be used to make reflection holograms, so check product codes before ordering.
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