Submitted by Al …
(Adapted from The Encyclopedia of Photography ©1963)
The Callier Effect is a name applied to the difference in contrast of a photographic image when projected (or viewed) by diffused light as compared to the contrast of the same image when projected by means of a concentrated or parallel light beam. The effect is named after the Belgian physicist Andre Callier, who first investigated it in 1909.
The effect is due to the scattering of light by the silver grains in the emulsion. If a negative is viewed by parallel illumination (a ‘collimated’ beam), the light passing through the clear parts will continue on its way without loss. However, in the denser parts of the negative some of the light will be absorbed by the silver image itself; a further quantity will be scattered in all directions by the silver grains of the emulsion and will be lost. Thus, since the clear parts scatter the least and the dense parts scatter the most, the contrast of the negative will appear higher in the collimated beam of light than when viewed by diffused light.
If the density of a part of a given negative is measured in both parallel and diffused light, the measurement will be higher in the first case than in the second. The ratio between the two is called the Callier Quotient, and it depends mostly on the grain size of the image. Thus, very fine-grained negatives show a lower Callier Quotient than coarse grained ones.
From a practical standpoint, the Callier effect concerns the photographer to the extent that a negative will produce a more contrasty print when enlarged than when contact printed. The exact increase in contrast depends on the Callier Quotient of the negative itself and also on the type of enlarger used.
A condenser enlarger using a clear lamp with a small filament and clear condensers will produce the greatest increase in contrast.
A condenser enlarger using an opal bulb or frosted condensers or both, will produce some increase in contrast too, but not as much.
Enlargers using diffused light sources such as integrating-sphere heads, reflected-light heads with mercury-vapour lamps, or simple enlargers using an opal glass diffusing plate produce the least increase in contrast.
Even here though, some increase in contrast over contact printing is found because only a narrow beam of light can reach the lens.
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