The Callier effect is the variation in contrast of images produced by a photographic film with different manners of illumination. It should not be confused with the variation in sharpness which also is due differences partial coherence.
The directed bright-field (see Fig. 1) 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. 2) the illumination of the film is provided through a translucent slab (diffuser), and each point of the film receives light from a wide range of directions.
Figure 2. Diffused-bright-field
The collimation of the illumination plays a fundamental role in contrast of the image impressed on a film.[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 Figure 3 the same silver-based film is reproduced in directed and diffused bright-field setups. 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 4 are reported the images of a dye-based film acquired in directed and diffused bright-field setups; 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 (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]
Figure 5: Callier Q factorversus diffuse density for a silver-based film.
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’.
The correct optical explanation of the Callier effect had to wait until the 1978 papers of Chavel and Loewenthal.[4]
^C. Tuttle. 1926. "The relationship between diffuse and specular density." J. Opt. Soc. Am. 12, 6 (1926), 559–565.
^J. G. Streiffert. 1947. "Callier Q of various motion picture emulsions." J. Soc. Mot. Pict. Engrs. 49, 6 (December 1947), 506–522.
^A. Callier. 1909. "Absorption and scatter of light by photographic negatives." J. Phot. 33 (1909).
^P. Chavel, S. Lowenthal. 1978. "Noise and coherence in optical image processing. I. The Callier effect and its influence on image contrast." JOSA, Vol. 68, Issue 5, pp. 559–568
