Callier effect

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 (D_dir) and diffused (D_dif) bright-fields, is termed the Callier Q factor:

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

カリエQ係数は常に1以上であり、拡散測定された密度D _difに対する傾向は、典型的な銀ベースのフィルムについて図5に示されています。 ^[2]

図 5: 銀ベースのフィルムの Callier Q 係数と拡散密度の関係。

これらの変化（例えば、コンデンサーや拡散拡大装置による）は長期間にわたって観察され、^[3] 「カリエ効果」として知られるようになりました。

カリエ効果の正しい光学的説明は、1978年のシャベルとレーヴェンタールの論文まで待たなければなりませんでした。^[4]

参考文献

1. C. Tuttle. 1926. 「拡散密度と鏡面密度の関係」J. Opt. Soc. Am. 12, 6 (1926), 559–565.
2. JG Streiffert. 1947. 「各種映画用乳剤のCallier Q 」 J. Soc. Mot. Pict. Engrs. 49, 6 (1947年12月), 506–522.
3. A. Callier. 1909. 「写真ネガによる光の吸収と散乱」J. Phot. 33 (1909).
4. P. Chavel, S. Lowenthal. 1978.「光学画像処理におけるノイズとコヒーレンス。I. カリエ効果と画像コントラストへの影響」JOSA、第68巻、第5号、559～568頁