Callier effect

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

The directed bright-field (see fig.) has extremely strong directional characteristics by means of a point source and an optical system (condenser); in this case, each point of the photographic film receives light from only one direction.

Figure 1. Directed bright-field

On the other hand, in a diffused bright-field setup (see fig.) the illumination of the film is provided through a translucent slab (diffuser), and each point of the film receives light from all the directions.

Figure 2. Diffused-bright-field

The collimation of the illumination plays a fundamental role in sharpness and contrast of the image impressed on a film. The light scattered by the image particles has specific directional characteristics, which are revealed by the directional characteristics of the illumination.

In case of high scattering fraction, the attenuance provided by the image particles changes considerably with the degree of collimation of the illumination. In figure the same silver-based film is reproduced in directed and diffused bright-field setups. The image on the left is much ‘crisper’ and the film grain is emphasized together with dust and scratches; the image on the right appears ‘softer’ and the details are smoothed. The global contrast also changes: the contrast on the left is much stronger than that on the right.

Figure 3. Images of the same silver-based film acquired in directed and diffused bright-field setups

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. In figure are reported the images of a dye-based film acquired in directed and diffused bright-field setups; with regard to dust and other imperfections that entail scattering, the image on the left is sharper, but the global contrast of the two images is about the same.

Figure 4. Images of the same dye-based film acquired in directed and diffused bright-field setups

The ratio between the attenuances provided by a specific point of a photographic film, which were measured in directed (D_dir) and diffused (D_dif bright-fields, is termed the Callier Q factor:

$Q={\frac {D_{dir}}{D_{dif}}}$

The Callier Q factor is always equal to or greater than unity; its trend versus the diffusely measured density D_dif is depicted in Figure 5 for a typical silver-based film.

Figure 5: Callier Q factorversus diffuse density for a silver-based film.

On the local scale of the small image details, the direct effect of the Q factor is the variation in sharpness of the images produced illuminating with different light collimation; on the global scale of the entire image, the equivalent effect is the variation in contrast. These variations (for example with a condenser or a diffuser enlarger) were observed over a long period of time, and they became known as ‘Callier effect’.