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Grey equivalence.

The ability of a colour material to form grey under certain conditions by combining the three subtractive emulsion dyes is an important precondition for correctly reproducing white and black. The dye densities which (combined with one another) are needed to form a neutral grey shade, free of colour cast, are termed the equivalent neutral densities. The grey equivalence lays down the conditions required for neutral colour rendition. The assessment criterion is the colour perception of the "ideal standard observer" ( CIE system). The grey equivalence thus definess the relationship between the colour densities of a multi-layer colour material in their three main absorption areas, which visually produces grey or is grey for the sensitisation of the subsequent printing material.

For reversal film, we can say that:

Grey equivalence is present when a grey wedge exposed on to the film appears grey visually or when projected, or when the grey wedge is grey for sensitisation of the printing material used in the subsequent printing process ( spectral integral density). For colour negative film, we can say that:

The values adjusted to the grey equivalence for the integrally measured colour densities must act as grey filters from a printing point of view, so that a grey wedge exposed onto the negative film is also grey for sensitizing the subsequent printing material. The grey equivalence thus comes about when certain conditions are fulfilled, under which the colours formed by the emulsion dyes are correctly rendered as far as the human eye is concerned (or have no colour cast for the subsequent printing material). Colour negative materials, like positive materials, are also adjusted to grey equivalence so that they are compatible with comparable products from other manufacturers. The visual grey shown in the AGFA Technical Data Sheets (under the heading "Absorption of the emulsion dyes") in the form of a cumulative curve is a conditional (false) grey, because it does not have the same density at every wavelength, and can therefore only be seen as grey or is grey under certain observation conditions (or under a certain type of printing light).

Inhibitors.

Inhibitors are development-inhibiting reaction products, which form in the emulsion layer of films during development. They optimize the three criteria that play a decisive role in achieving good image quality, namely sharpness, grain and colour rendition. In this way, they make an important contribution to improving the image quality of modern photographic materials. With black-and-white films, inhibitors (e. g. bromide) improve the impression of sharpness. The development-inhibiting bromide, which is formed in the emulsion layer during the developing process, diffuses out of areas of high density, in which a lot of development inhibitor has formed, into areas of lower density (with less development inhibitor). This results in an exaggeration of light/ dark transitions, in other words the density is increased locally in the area that has been most exposed, and reduced in less exposed areas ( edge effect).

Inhibitors have a special significance when it comes to improving the quality of modern colour negative films. The essential criteria for good image quality (grain, sharpness and colour saturation) can be vastly improved by the use of DIR couplers in the emulsion layers. During colour development, the DIR couplers release products which diffuse both horizontally within the emulsion layer and vertically into other emulsion layers.

Improving granularity - Fig. 14: Through the effect of the inhibitor, a smaller ”grain” is formed from the relatively large silver halide crystal (= highspeed), producing a smaller dye cloud.

The granularity can also be improved by specifically inhibiting the full development of silver halide (partial reduction). Although relatively large silver halide crystals (= high speed) are used, the inhibitors nevertheless produce a less grainy impression without having any negative influence on the speed. The sharpness can – like the granularity – also be improved through the inhibitor acting horizontally. The inhibitor diffuses, within one emulsion layer, from areas of high density to areas of lower density. Consequently the inhibiting effect on the development process is reduced in the areas of high density, and intensified in areas of low density. The result is seen as an intensification of light-to-dark transitions (= edge effect), which enhances the visual impression of sharpness. The improvement in colour saturation through vertical inhibition is caused by diffusion of the corresponding inhibitor from areas of high colour density to areas of low colour density in a neighbouring emulsion layer (= interimage effect). As a result, development of the high colour density is inhibited to a lesser extent, whereas development of the lower density areas in the neighbouring layer is inhibited a lot.

Integral density.

(Synonyms: visual grey; mean neutral density.) The integral densities of the developed emulsion dyes are determined by densitometric measurement of a multilayer colour material. Under suitable exposure and processing conditions, it is possible to plot the cumulative curve of the spectral integral density (e.g. in the range of D = 1.0) by the wavelength-dependent addition of the main colour densities and secondary colour densities of all the colour-forming layers of a colour film. In the Technical Data Sheets for AGFACOLOR and AGFACHROME films, the spectral integral density is called visual grey, and elsewhere mean (neutral) density. The spectral integral densities of the various photographic materials have different curve patterns, depending on the particular film type. The spectral integral density of a colour film is a measure of the relative reaction of the film to the transmitted light. With a reversal film, it is a measure of the relative effect of the transmitted light on the eye of the observer (under defined standard lighting and/or projection conditions).

Mean neutral density curve of a colour negative film

Fig. 15: The falling curve of mean neutral density is typical of a colour negative film. The photographic material is geared to the characteristics of the printing material. In the case of colour negative films, the spectral integral density is a measure of the relative effect of the transmitted light on the á spectral sensitisation of the subsequent printing material.

In the case of colour negative films, the spectral integral density is a measure of the relative effect of the transmitted light on the spectral sensitisation of the subsequent printing material. For colour negative films, the typical curve slopes from left to right. The mean neutral density of colour negative films is neutral with regard to the spectral properties of the subsequent printing material. As far as printing materials are concerned, the curve shape is different from a negative film curve. Under defined conditions (type of printing light), materials should reproduce the colour impression of the subject as accurately as possible.

Interimage effect.

The interimage effect involves improving the colour saturation of modern colour-negative films by vertical inhibition: DIR couplers integrated into the emulsion layers react with the oxidation product of the colour developer, and release inhibitors which hinder the formation of undesirable secondary colour densities in the adjacent emulsion layers. They also intensify the main colour density in that colour’s emulsion layer (due to a lower inhibiting effect).

Together with the masking, interimage effects bring about an appreciable improvement in the colour saturation of colour negative films. The differing colour saturation of the AGFACOLOR professional films is based on specially graded interimage effects.

Interimage effects.

Fig. 16: The different colour saturations of the AGFACOLOR professional negative films are based on specially graded interimage effects.

ISO speed.

ISO is the abbreviation for International Standards Organization. The ISO speed standard introduced in 1983 combined the previously used ASA and DIN values. The following table shows the relative figures:

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