# Callier effect

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The '''Callier effect''' is the variation in [contrast](/source/Contrast_(vision)) of images produced by a [photographic film](/source/photographic_film) with different manners of [illumination](/source/Light).  It should not be confused with the variation in [sharpness](/source/Sharpness_(visual)) which also is due differences partial [coherence](/source/Coherence_(physics)).   

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

{{Anchor|Figure 1}}thumb|Figure 1. Directed bright-field
On the other hand, in a diffused [bright-field](/source/Bright-field_microscopy) setup (see [Fig. 2](/source/Callier_Effect)) the illumination of the film is provided through a translucent slab ([diffuser](/source/Diffuser_(optics))), and each point of the film receives light from a wide range of directions.

{{Anchor|Figure 2}}thumb|Figure 2. Diffused-bright-field
The [collimation](/source/collimation) of the illumination plays a fundamental role in [contrast](/source/Contrast_(vision)) of the image impressed on a [film](/source/photographic_film).<ref>C. Tuttle. 1926. "The relationship between diffuse and specular density." ''J. Opt. Soc. Am.'' 12, 6 (1926), 559–565.</ref> 

In case of high scattering fraction, the [attenuance](/source/Absorbance) provided by the image particles changes considerably with the degree of [collimation](/source/collimation) of the illumination. In [Figure 3](/source/Callier_Effect) 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.

{{Anchor|Figure 3}}thumb|Figure 3. Images of the same silver-based film acquired in directed and diffused bright-field setups
In the absence of [scattering](/source/Light_scattering), the [attenuance](/source/Absorbance) provided by the [emulsion](/source/Photographic_emulsion) is independent of the [collimation](/source/collimation) of the illumination; a dense point [absorbs](/source/Absorption_(electromagnetic_radiation)) a big portion of light and a less dense point [absorbs](/source/Absorption_(electromagnetic_radiation)) a smaller portion, irrespective of the directional characteristics of the incident light. In [Figure 4](/source/Callier_effect) 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.

{{Anchor|Figure 4}}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 (''D''<sub>dir</sub>) and diffused (''D''<sub>dif</sub>) bright-fields, is termed the Callier ''Q'' factor:

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

The Callier ''Q'' factor is always equal to or greater than unity; its trend versus the diffusely measured density ''D''<sub>dif</sub> is depicted in [Figure 5](/source/Callier_Effect) 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}}thumb|Figure 5: Callier Q factorversus diffuse density for a silver-based film.
These variations (for example with a condenser or a diffuser [enlarger](/source/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’'''.

The correct optical explanation of the Callier effect had to wait until the 1978 papers of Chavel and Loewenthal.<ref>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</ref>

==References==
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Category:Photographic techniques

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Adapted from the Wikipedia article [Callier effect](https://en.wikipedia.org/wiki/Callier_effect) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Callier_effect?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.