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Hypersensitization

plates exposure image silver

DAVID MALIN
Anglo-Australian Observatory, RMIT University

This section covers methods of enhancing a silver halide material’s sensitivity before exposure, usually to offset the effects of low-intensity reciprocity failure (LIRF). LIRF reduces the effective speed of an emulsion when exposure times are longer than a few seconds. Hypersensitization is distinguished from latensification, which involves treatment after the image exposure. The term embraces a variety of methods that affect the chain of events involved in the formation of the latent image. Hypersensitizing (hypering) may be part of the manufacturing process or a user-applied pre-exposure procedure. Tough important during most of the 20th century, “hypering” is now mainly of historical interest.

LIRF was first noted by astronomers in the 1890s, who began to explore ways of minimizing it, even though they were not initially aware of the photographic mechanisms involved. Because they often derived photometric data from their plates, astronomers were used to making precise measurements of emulsion sensitivity, so they were able to monitor and document the effects of exposure time on their photographic experiments.

The major suppliers of photographic materials were also keen to improve the speed of their products intended for general use, so there was some collaboration between astronomers and photographic researchers. This was especially true in the United States, where the Kodak Research Laboratories were directed by the English chemist, C. E. Kenneth Mees, who was interested in astronomy. From about 1931 to 1996, Kodak produced a range of spectroscopic plates where the effects of LIRF were intentionally minimized. These emulsions were usually coated on glass and were intended for scientific research involving long exposures, especially astronomy and spectroscopy, and the recording of optical spectra.

Practical, user-applied hypersensitizing techniques evolved over most of the last century and fall mostly into four types of treatments. These broadly involved liquid phase (washing), gas phase (out-gassing and baking and hydrogenation), the use of cold cameras, and pre-flashing.

Washing plates in water, dilute ammonia, triethanolamine, or (more recently) silver nitrate solutions was found to be very effective, especially for red- and infrared-sensitive materials. Later types of fine grain, near-IR-sensitive plates were unusable without such hypersensitizing. However, much skill and persistence was required to obtain consistent and uniform results, especially with large plates, which were often treated at unsocial hours in observatory darkrooms on remote mountain tops.

Some of the earliest gas-phase hypersensitization methods involved exposing the plates to mercury vapor before exposure to light. This was beneficial but was also hazardous and unreliable. More amenable was baking the plates in air in a moderate oven. Used from about 1940, this produced modest speed gains in the then-current coarse-grained emulsions. From about 1970, baking (about 65°C for hours) or prolonged soaking (20°C for weeks) in a flow of nitrogen was used and could achieve a factor of 10 gain in speed for a one-hour exposure.

This was especially important for the new generation of high detective quantum efficiency, fine-grained (but slow) plates Eastman Kodak had developed in the late 1960s. In 1974, researchers at Eastman Kodak announced that plates treated in pure hydrogen after nitrogen treatment were more sensitive at all exposure times than untreated plates, and this was quickly adopted by many observatories, some of whom used non-explosive “forming gas” (a 4 to 8 percent mixture of hydrogen in nitrogen) for reasons of safely. The latest gas-phase processes combine the effects of both drying with nitrogen and reduction sensitization with pure hydrogen to give a sensitivity gain of about 30 times for an hour-long exposure. This also worked very well with fine-grain, high-resolution emulsions on film, typified by Eastman Kodak’s Tech Pan. They are also effective with negative and reversal color film, but are unpredictable and can produce serious changes in color balance.

The liquid-phase of plate washing techniques operates by removing residual soluble bromides or iodides from the emulsion, thereby increasing the silver ion concentration in the vicinity of the photosensitive grain. The gas-phase method, especially nitrogen baking, involves the removal of traces of oxygen and water from the gelatin matrix, which increases the efficiency of the first stages of latent-image formation. Finally, hydrogen is a chemical reducing agent that “seeds” the dry, de-oxygenated silver halide crystal with a few atoms of silver. These are stable, sub-latent image clusters that subsequent photoelectrons from exposure to light can build into a several-atom latent image speck that catalyzes the development of the whole silver halide crystal. Photographic gelatin soaks up ambient moisture rapidly, so in humid climates “hypered” plates were usually exposed at the telescope in an atmosphere of nitrogen.

It had been known since the 1930s that LIRF was less severe during low temperature exposures. Accordingly, many experimenters built film cameras with “cold backs,” metal plates in contact with the film, often cooled with solid carbon dioxide. These were awkward to use because of film embrittlement and condensation, but some good results were obtained with color film. The low temperatures decreased the probability that the initial silver atom would be recombined with a halide ion but did not otherwise seem to affect the formation of the latent image.

Pre-flashing is not strictly a hypersensitizing technique but it was often used in conjunction with Kodak’s spectroscopic emulsions, sometimes together with hypering. It involves a brief, uniform, low-intensity flash of light sufficient to produce a small increase in the unexposed fog level. This was usually done just before a long exposure and gave modest increases in effective speed, but only if the main exposure was filtered or otherwise arranged so that the faint image being recorded was free from sky background or scattered light. The main effect was to change the shape of the toe of the characteristic curve. In photographic terms, pre-flashing lowered contrast and improved the shadow detail without affecting the highlights.

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