Holographic photochemistry: basic process (no bleaching)

The basics of holographic photochemistry are very similar to those of black and white photography. Photosensitive plates are made of silver halide microcrystals or "grains" (mostly silver bromide with some silver chloride) suspended in a gelatin binder (together, they are called an "emulsion"), which is in turn coated onto a glass or plastic substrate.

During exposure, some of the photons of light are absorbed by the silver halide crystals and produce "electron-hole pairs," of which the photo-electron is quite mobile, and the positively-charged silver ion is relatively stationary. Some of the electrons eventually find their way to the surface and combine with silver ions there to form tiny stable clusters of silver metal atoms, which are the latent image "specks." As the exposure continues, photons strike some part of more and more crystals.

Wet development amplifies the latent image by converting the silver halide grains that have been marked with a silver speck entirely into metallic silver. Silver grains of this small size appear black and opaque. Developers are weak reducing agents - that is, they provide extra electrons during development. Besides their reducing properties, developing solutions may also contain agents that control their alkalinity, dissolve the silver grains slightly or interact with the emulsion gelatin.

After the prescribed development time, the plate is placed in a dilute solution of a weal acid (usually acetic acid), known as "stop bath." Most developers are active only when alkaline, so the stop bath immediately halts the conversion of silver-halide to metallic silver grains, and then rinses most of the chemicals out of the gelatin.

After the stop bath, the plate moves to a solution of a silver halide solvent, usually sodium hypothiosulfate or "hypo." This solution, called the "fixer," dissolves the unexposed and undeveloped grains, leaving behind only the black silver grains. At this point, the plate is no longer sensitive to light. That is, the image is now fixed in place and won't change.

Following fixing, the plate is washed in filtered water. The water bath washes out any soluble silver complexes and residual chemicals. Only metallic silver grains remain in the emulsion. In black and white photography, this silver would form the dark part of a photographic image. In holography, the silver grains make up the fine-scale grating structure that will diffract light.

The final step is drying. When wet, the emulsion swells to five to seven times its original thickness. As the water is removed, the gelatin shrinks dramatically and "catastrophically." That is, the layer attempts to be either fully swollen or fully shrunk, and water moves laterally through the gelatin to allow this to happen. The result is that a drying "edge" or "front" sweeps across the plate as it dries. Any dirt or flaw causes the front to "hang up" and produce an isolated island of wet gelatin, which slowly dries and leaves what is called a "water spot." In holograms, these water spots become especially visible because of the distortion of the gelatin layer than accompanies them. To minimize these effects, a small amount of surfactant (a surface tension reducing agent) is added to a final rinse, so that the water sheets off of the plate smoothly. Much care must be taken to avoid other drying defects during this critical step!