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The '''Callier effect''' is the variation in [[Sharpness (visual)|sharpness]] and [[Contrast (vision)|contrast]] of images produced by a [[photographic film]] with different manners of [[Light|illumination]].

The directed [[Bright-field microscopy|bright-field]] (see [[Callier Effect#Figure 1|Fig. 1]]) has extremely strong directional characteristics by means of a point source and an optical system ([[Condenser (optics)|condenser]]); in this case, each point of the photographic film receives light from only one direction.

{{Anchor|Figure 1}}[[File:Condensed-bright-field-setup.pdf|thumb|Figure 1. Directed bright-field]]
On the other hand, in a diffused [[Bright-field microscopy|bright-field]] setup (see [[Callier Effect#Figure 2|Fig. 2]]) the illumination of the film is provided through a translucent slab ([[Diffuser (optics)|diffuser]]), and each point of the film receives light from all the directions.

{{Anchor|Figure 2}}[[File:Diffused-bright-field.pdf|thumb|Figure 2. Diffused-bright-field]]
The [[collimation]] of the illumination plays a fundamental role in [[Sharpness (visual)|sharpness]] and [[Contrast (vision)|contrast]] of the image impressed on a [[photographic film|film]].<ref>C. Tuttle. 1926. The relationship between diffuse and specular density. J. Opt. Soc. Am. 12, 6 (1926), 559–565.</ref> The light [[Light scattering by particles|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 [[Absorbance|attenuance]] provided by the image particles changes considerably with the degree of [[collimation]] of the illumination. In [[Callier Effect#Figure 3|Figure 3]] 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.

{{Anchor|Figure 3}}[[File:DiffVScond.pdf|thumb|Figure 3. Images of the same silver-based film acquired in directed and diffused bright-field setups]]
In the absence of [[Light scattering|scattering]], the [[Absorbance|attenuance]] provided by the [[Photographic emulsion|emulsion]] is independent of the [[collimation]] of the illumination; a dense point [[Absorption (electromagnetic radiation)|absorbs]] a big portion of light and a less dense point [[Absorption (electromagnetic radiation)|absorbs]] a smaller portion, irrespective of the directional characteristics of the incident light. In [[Callier Effect#Figure 4|Figure 4]] 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.

{{Anchor|Figure 4}}[[File:DiffVScond-color-eps-converted-to.pdf|thumb|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:

:<math>Q=\frac{{D_{dir}}}{{D_{{dif}}}}</math>

The Callier ''Q'' factor is always equal to or greater than unity; its trend versus the diffusely measured density ''Ddif'' is depicted in [[Callier Effect#Figure 5|Figure 5]] for a typical silver-based film.<ref>J. G. Streiffert. 1947. Callier Q of various motion picture emulsions. J. Soc. Mot. Pict. Engrs. 49, 6 (December 1947), 506–522.</ref>
{{Anchor|Figure 5}}[[File:Qfactor-CC.jpg|thumb|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,<ref>A. Callier. 1909. Absorption and scatter of light by photographic negatives. J. Phot. 33 (1909).</ref> and they became known as '''‘Callier effect’'''.

==References==
{{Reflist}}

[[Category:Photography]]

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62.220.2.102: "in during" -> during

en>Monkbot: Fix CS1 deprecated date parameter errors