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Speed
The speed is the ability of a photosensitive emulsion to reproduce a
given exposure by means of processing at a density or color density
corresponding to the exposure intensity.
This property is also known as the general sensitivity or
"photosensitivity". It is achieved during production by a
special ripening process of the emulsion (chemical sensitization).
Thus, under the influence of added gold and sulphur compounds,
so-called ripening nuclei form on the surface of the silver halide
crystal, covering the general speed of the photographic material (sensitization).
Film materials are subdivided into different speeds (DIN, ASA and ISO speeds).
Speed criterion
According to DIN 4512, this is a density 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 (DIN, ASA and ISO speeds).
The speed criterion can be used in combination with the density curves
in the Technical Data Sheets to approximately verify the given film
speed: If, for example, we plot the point of speed criterion (0.1 above
minimum density) on the density curve for AGFAPAN APX 25 (see also
Technical Data Sheet AGFAPAN 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).
Density Fig. 23: Determining the relative
speed using the speed criterion (taking AGFAPAN APX 25 as an example).
Status filter
Status filters are light filters for the densitometric measurement of
the integral color densities of color materials. They have particularly
narrow spectral transmission for the standardized light source of the
densitometer and are used among other things for plotting color density
and gradation curves.
In contrast to the spectral analytical
densities of individual emulsions calculated by the
manufacturer, the user can obtain the integral color densities by
carrying out a densitometric measurement of the complete emulsion
structure of color materials (e.g. of control strips for processing
control). 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 obtain the color densities of the emulsion dyes.
A densitometry (using status AA filters) is intended for the
measurement of color reversal film. Status AA filters have a
particularly narrow spectral transmission in the blue and green
spectral range.
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 color density measurements of color negative films. What
is measured here is the printing density, corresponding to the matching
of color negative films to the spectral
sensitivity characteristics "receiving
characteristics" of the subsequent printing material.
Status densitometry thus distinguishes between the evaluation of color
negative and color reversal film on the one hand and color printing
materials on the other. It results in color density data and color density curves, which
correspond to the characteristics of the respective material.
Subtractive color mixture
The subtractive colors yellow, magenta and cyan are mixed colors. They
are formed when white light passes through yellow, magenta and cyan
filters or when white light is reflected on the light-impermeable
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 color of yellow. In the same way, a
magenta filter absorbs green and allows only blue and red (= magenta)
light through, while a cyan filter absorbs red and allows blue and
green light through.
This means that color filters and colored objects either reflect or
allow through those rays of light which correspond to their inherent
color.
Correction filtering by the subtractive method is done using color
filters having the mixed colors of yellow, magenta and cyan. Only with
these colors is it possible to produce all the other mixed colors. 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 color 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 colors are used for subtractive color
filtering. The combination of three filter colors is not recommended
because the grey resulting from the third filter color would extend the
exposure time unnecessarily.
The principle of subtractive color mixture is also applied in
modern-day color negative films and color 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 the three subtractive colors yellow, magenta and cyan (each
with different intensity) is it possible to reproduce all the colors 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 color 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 picture which only allow a small amount of light
through can appear black compared with other lighter areas. This factor
makes it possible to use three-layer color materials with the colors
yellow, magenta and cyan.Apart from certain printing processes, the
principle of subtractive color mixture is applied in all modern color
films and in most color printing materials (chromogenic process).
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