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Speed.

The speed (sensitivity) is the ability of a photosensitive emulsion to change a given exposure by means of processing into a black or colour density corresponding to the exposure intensity. This property is also known as the “general sensitivity” or “photosensitivity”. It is achieved during production by special ripening of the emulsion (chemical sensitisation). Under the influence of added gold and sulphur compounds, so-called ripening nuclei form on the surface of the silver halide crystal, and determine the general speed of the photographic material (see: sensitisation).

Film materials have different speed ratings (see :DIN, ASA and ISO speed; see : spectral sensitivity).

Speed criterion.

According to DIN 4512, the speed criterion is a value on the density curve which, in black-and-white films for normal exposure conditions, corresponds to a density of 0.1 above minimum density. The resultant value on the x-axis represents the relative speed (see: DIN, ASA and ISO speeds).

The speed criterion can be used in combination with the density curves in the AGFA Technical Data brochures to approximately verify the given film speed. If, for example, we plot the point of the speed criterion (0.1 above minimum density) on the density curve for AGFAPAN APX 25 (see also Technical Data Sheet APX 25) and, if we drop down vertically from this point onto the exposure axis, we obtain the logarithmic value –1.5. If we then leave out the minus sign and the decimal point, we get the DIN speed (15 DIN = ISO 25/15° with the APX 25).

Fig. : Determining the relative speed using the speed criterion.

Status filter.

Status filters are light filters for the densitometric measurement of the integral colour densities of colour materials. They have particularly narrow spectral transmittance for the standardized light source of the densitometer, and are used among other things for plotting colour density and gradation curves.

In contrast to the spectral analytical densities of individual emulsions calculated by the manufacturer, the user can obtain the integral colour densities by carrying out a densitometric measurement of the complete emulsion structure of colour materials (e. g. of control strips for process monitoring). By measuring the three subtractive emulsion dyes yellow, magenta and cyan under blue, green and red optical filters (in accordance with the spectral sensitivity of the emulsion layers), it is possible to find the colour densities of the emulsion dyes.

Status A densitometry (using status AA filters) is intended for the measurement of colour reversal film. Status AA filters have a particularly narrow spectral transmittance in the blue and green spectral ranges.

Status D densitometry is used for the measurement of non-transparent printing materials, and also utilizes Status AA filters.

Status A and D densitometry is used for the measurement of films and printing materials for visual observation. What is measured is therefore the visual optical density.

Status M densitometry (with Status MM filters) is used for densitometric colour density measurements of colour negative films. What is measured here is the printing density, corresponding to the matching of colour negative films to the spectral sensitivity of the subsequent printing material.

Status densitometry thus distinguishes between the evaluation of colour negative and colour reversal film on the one hand and colour printing materials on the other. It produces in colour density data and colour density curves, which correspond to the characteristics of the respective material.

Subtractive colour mixture.

The subtractive colours yellow, magenta and cyan are mixed colours. They are formed when white light passes through yellow, magenta and cyan filters, or when white light is reflected on the opaque surface of yellow, magenta or cyan objects. A yellow filter absorbs the blue spectral components of white light, and only lets the green and red spectral components pass through. The green and red rays of light combine to produce the mixed colour yellow. In the same way, a magenta filter absorbs green and allows only blue and red (see: magenta) through, while a cyan filter absorbs red and allows blue and green light through. This means that colour filters and coloured objects either reflect or allow through the rays of light which correspond to their inherent colour.

Correction filtering by the subtractive method is done with colour filters having the mixed colours of yellow, magenta and cyan. Only with these colours is it possible to produce all the other mixed colours. In the absence of all three filters, we get white, and in the presence of all three filters, we get grey, because the technical dyes in the filters (like the developed emulsion dyes in the colour film) do not completely absorb the light passing through them. If they did absorb the light completely, the result would be black. For this reason, only one or two different filter colours are used for subtractive colour filtering. The combination of three filter colours is not recommended, because the grey resulting from the third filter colour would lengthen the exposure time unnecessarily.

The principle of subtractive colour mixture is also applied in modern-day colour negative films and colour slide films, which have, after exposure and processing, yellow, magenta and cyan dye layers on top of one another in the photosensitive emulsion layers. Only by combining these three subtractive colours (each with different intensity) is it possible to reproduce all the colours of the visible spectral range with three dye layers on top of one another.

Although overlapping of the yellow, magenta and cyan filters (or the corresponding layers of the colour material) does not produce a complete black, because of the previously mentioned incomplete absorption of the light, "black" is nevertheless a relative impression, because even parts of the image which only allow a small amount of light through can appear black compared with other lighter areas. This fact makes it possible to use three-layer colour materials with the colours yellow, magenta and cyan.

Apart from certain printing processes, the principle of subtractive colour mixture is applied in all modern colour films and in most colour printing materials ( see: chromogenic process).

Transfer factor.

The transfer factor ( MTF) is an important factor for the sharpness of a photographic emulsion. Its graph is a falling curve. If one or more light/dark edges (e. g. a line grid) are exposed under defined testing conditions onto the film material, then the emulsion layer will, after processing, reproduce the edges with a certain transfer loss depending on the sharpness characteristics of the emulsion. If, instead of a single edge, a whole series of edges is exposed with smaller and smaller gaps between them, then the transfer losses of the photographic material become clearer. The transfer factor curve declines continuously as the distances between the edges become smaller, because the photographic emulsion reproduces the edges less and less sharply. The curve ends when the resolving limit of the material has been reached, a point which is dependent on the specific sharpness characteristics and the granularity of the material (under defined processing conditions). The image sharpness can be visually increased (in this case the transfer factor is greater than 100 %), for example by the edge effect.

Ultra-short-time effect.

With photographic materials, very short exposure times combined with high light intensities result in a loss of speed, flatter contrast and, with colour materials, in colour shifts and cross casts.

Like very long exposure times at low light intensity (see: reciprocity effect), very short exposure times at correspondingly higher light intensity also affect the general speed, the gradation and the relative shapes of the colour density curves of the photo material.

The ultra-short-time effect (a synonym is high intensity reciprocity failure) comes about through the formation of a dispersed latent image (whereby, as the result of an intensive short-time exposure ) , several small, difficult to develop latent image nuclei form on a silver-halide crystal instead of a single large latent image nucleus.

The photographic effect of the exposure is reduced as a result. This is equivalent to a reduction in speed. At exposure times below 0.001 seconds, black-and-white materials can undergo a decline in the relative speed and a flattening of the gradation at the same time. With colour materials, the relative sensitivities of the individual colour-forming emulsion layers can alter (e. g. as with cross cast).

Variable-contrast papers.

With variable contrast papers (e. g. AGFA MULTI-CONTRAST PREMIUM and MULTICONTRAST CLASSIC) the contrast ( gradation) is controlled by exposure with colour-filtered printing light. One single grade of paper can thus be used for the entire range of contrast from extra soft to extra hard. This is made possible by the special sensitisation of the emulsion.

Contrast control with MULTICONTRAST paper .

Fig. : Controlling the contrast grade of MULTICONTRAST papers by a magenta filter (= steep contrast) and yellow filter (= flat contrast).

Unlike black-and-white papers with fixed contrast, which have an emulsion that is sensitive primarily to the blue spectral component of the printing light, the emulsion of variable contrast papers contains a mixture of a non-sensitized emulsion component which is sensitive to blue light, and a green-sensitized emulsion component which is sensitive to blue and green light. The different contrast is achieved by printing with the aid of colour filters. Depending on the blue and green contents of the printing light, either both the emulsion components react or only one of them. If both emulsion components react (to the blue exposure), a steep gradation curve is produced through addition of the density curves of both emulsion components. If, however, only the green-sensitive range reacts (to the green exposure), the characteristic curve will be flat because, in this case, the unsensitized emulsion component, the one which is sensitive to blue light, remains unexposed.

Depending on the blue or green contents of the printing light used for the exposure, (in other words by the use of coloured filters), the paper contrast can be adjusted to virtually anywhere between extra hard and extra soft.

Visual filter (V_λ ).

The spectral density data of black-and-white and colour densities measured densitometrically with the aid of a visual filter (V_λ ) correspond to the visual impression of density perceived by the eye. V_λ represents a given transmission range of the (green) visual filter which, together the spectral characteristics of the light source, results in the desired conformity between the measurement and the visual impression seen by the eye.

Bibliography Berger: Agfacolor, 1st ed.; Verlag W. Girardet, Wuppertal, 1950. Die Sprache der Farben; X-Rite Incorporated, Grandville, MI, U. S. A., 1995.

Folienserie des Fonds der Chemischen Industrie, Textheft 26: Fotografie; Fonds der Chemischen Industrie im Verband der Chemischen Industrie e. V. (editor), Frankfurt am Main, 1999. Mutter: Farbphotographie, Theorie und Praxis; Springer-Verlag, Wien, New York, 1967.

Schmonsees: Sensitometrie in der Schwarzweiß-und Farbphotographie, 1st ed.; Agfa-Gevaert AG, Leverkusen, 1972. Schröder: Technische Fotografie, 1st ed.; Vogel-Verlag, Würzburg, 1981. Solf: Fotografie; Fischer Taschenbuch Verlag, Frankfurt a. M., 1976. Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Vol. A20; VCH Verlagsgesellschaft mbH, Weinheim, 1984, reprinted 1992.

Note Agfa, the Agfa Rhombus, AGFACHROME, AGFACOLOR, AGFAPAN, AGFACOLOR OPTIMA, AGFACOLOR PORTRAIT, AGFACOLOR ULTRA, DIGIPRINT, MULTICONTRAST and SCALA are trademarks of Agfa-Gevaert AG, Leverkusen, Germany. Product improvement is a continuous process, and Agfa-Gevaert therefore reserves the right to make any changes in product specifications without notice.

Appendix 1: Temperature Conversion Table for Celsius and Fahrenheit

The following formulas convert Celsius into Fahrenheit (and vice versa):

°F = (°C × 9/5) + 32                 °C = (°F -32) × 5/9

Appendix 2: ISO/ ASA/ DIN Speed Conversion Table

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