Callier effect

The Callier effect is the variation in contrast of images produced by a photographic film with different manners of illumination.

[1] In case of high scattering fraction, the attenuance provided by the image particles changes considerably with the degree of collimation of the illumination.

In the absence of scattering, the attenuance provided by the emulsion is independent of the collimation of the illumination; a dense point absorbs a big portion of light and a less dense point absorbs a smaller portion, irrespective of the directional characteristics of the incident light.

The ratio between the attenuances provided by a specific point of a photographic film, which were measured in directed (Ddir) and diffused (Ddif) bright-fields, is termed the Callier Q factor: The Callier Q factor is always equal to or greater than unity; its trend versus the diffusely measured density Ddif is depicted in Figure 5 for a typical silver-based film.

[2] These variations (for example with a condenser or a diffuser enlarger) were observed over a long period of time,[3] and they became known as ‘Callier effect’.

Figure 1. Directed bright-field
Figure 2. Diffused-bright-field
Figure 3. Images of the same silver-based film acquired in directed and diffused bright-field setups
Figure 4. Images of the same dye-based film acquired in directed and diffused bright-field setups
Figure 5: Callier Q factorversus diffuse density for a silver-based film.