Tabular-grain film
Tabular-grain film is a type of photographic film that includes T-max films from Kodak (with Kodak's T-grain emulsion), Delta films from Ilford Photo and the Fujifilm Neopan films. The silver halide crystals in the film emulsion are flatter and more tabular (hence T-Grain).
Tabular crystals
In panchromatic emulsions, the sensitivity of the silver halide crystal is enhanced by sensitizing dyes that adsorb on the crystal surface. Therefore, sensitivity can be increased by adsorbing more sensitizing dye. This requires to increase the surface area of the crystal, and also improve the dye molecules to form a dense assembly of the dye molecules. Tabular grain emulsion solves the first part of this problem.
Tabular crystals tend to lie along the film's surface when coated and dried. This reduces scattering of light and increases resolution.
Tabular crystals usually have two twinned planes parallel to each other. They are formed at the very beginning of the crystallization. The crystal tends to grow at the edges and not on the main planes, forming very thin crystals of very large surface areas. Tabular crystals probably existed from very early days of silver-gelatin photography. However, it was about 1970 when emulsion engineers could make emulsions that are mainly consisting of tabular crystals. It was also in 1980s when tabular crystals began to be used in production emulsions.
Tabular grain technology brought significant improvements to the image quality of the film, particularly in the improvement of resolution and granularity. However, several more key technologies were implemented into tabular grain products. Many of these concurrent improvements were applied to non-tabular grain products to improve image quality. Therefore, when tabular grain technology is described by uninformed writers, its advantage tends to be overemphasized. For example, excellent reciprocity law is not an inherent property of tabular crystals but rather the result of other techniques introduced at about the same time.
Fixing tabular crystals
Tabular crystals grow along the edges and not on the main planes. Similarly, tabular crystals dissolve mainly along the edges and this causes the crystals more difficult and slow to fix in the fixing stage. Users of tabular grain films are advised to ensure sufficient fixing time, at least twice the clearing time in rapid fixer.
References
- T-Grain at a Kodak Exposure Information page
- Film Development Information by George L. Smyth
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