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D
Densitometer
Photosensitive, photoelectric metering unit for measuring the density
(in the form of logarithmic densities) of black-and-white materials or
the color densities of color materials. Densitometers are used in
printing and repro work and for photographic process control.
The key components of a densitometer are the light source, measuring
filters (in high-quality densitomers, use is made of status filters), a circular
measuring diaphragm, a multiplier for converting the reflected or
transmitted test light flux into electric signals and an analog or
digital data display.
A distinction is made between two different types of densitometers:
Reflection densitometers measure the proportion of incident light
reflected by the surface of the material, e.g. by a photographic paper
or by a color printed on paper.
Transmission densitometers measure the proportion of light from the
densitometer light source which passes through transparent material,
e.g. a photographic film.
For an explanation of the measuring filters used in densitometers see status filter, visual filter.
Density curve
The density curve, also known as the characteristic curve, is the most
important element in sensitometry.
The density curve is the graphic description of the optical density D
of a developed photographic material as a function of the exposure H.
In black-and-white photography, the density curve is also called the
characteristic or H and D curve, whereas in color photography, we also
speak of color density curve(s).
Color photographic materials always have three color density curves in
line with the sensitization of their different emulsion layers.
The density curve of a photographic material is obtained by plotting,
in a system of coordinates, the optical density D measured with a
densitometer (on the y-axis) against the logarithm of the exposure H
= l x t (on the x-axis).
Fig. 7: Schematic representation of the
density curve of a black-and-white photo material.
At the beginning of the density curve we have the minimum density
(lowest exposure, not producing any image), then comes the threshold
(beginning of the exposure which does generate an image). This is
followed by a virtually straight-line section (area of normal
exposure), then comes the shoulder (over-exposure) and the maximum
density. In the case of certain special photo materials (not shown in
Fig. 7) there is then the area of solarization, in which a further
increase in the light intensity lowers the density again.
Fig. 8: The color density curves of a color
reversal film (AGFACHROME PROFESSIONAL) progress in the opposite
direction than those of a negative film (compare with the progress
shown in fig. 7)
The lux-second is used as the unit for measuring the exposure lg H. In
the diagrams shown in the Technical Data Sheets, 1 lux-second
corresponds to the value 0 on the scale of the exposure axis. The
positive values +1.0, +2.0 ... represent 10 and 100 lux-seconds
respectively. The negative values on the exposure axis, -1.0, -2.0 and
-3.0 represent fractions of a lux-second, i.e. 0.1 lxs, 0.01 lxs and
0.001 lxs respectively.
Unlike the density curves of negative materials, those of reversal
materials do not go from left to right, but upwards from right to left.
The area of low (i.e subliminal) density in negative materials
represents, conversely, a high density with the reversal materials,
which is also called maximum density.
DIN, ASA and ISO speeds
Film material for taking photographs is divided into various speed
categories. A distinction is made between slow, medium, fast and
ultra-fast films. Different systems are used for stating the film
speed: the arithmetic ASA system and the logarithmic DIN system.
Doubling or halving the photographic speed is equivalent to doubling or
halving the ASA number in the ASA system or raising or lowering the DIN
number by three units in the DIN system. Both speed systems are
combined in the international ISO standard:
25 ASA
100 ASA
400 ASA
15 DIN
21 DIN
27 DIN
ISO 25/15°
ISO 100/21°
ISO 400/27°
There is also an international standard for stating the speed of
black-and-white photo papers (ISO 6846).
This standard is an arithmetic speed value, in other words doubling or
halving the figure means doubling or halving the speed. To distinguish
it from the ISO standard used for films, the speed designations for
black-and-white photo papers have a letter P in front of the figure,
for example ISO P 400.
DIN speed
The absolute measure of the speed of black-and-white negative films. In
line with DIN 4512/1, the DIN speed is determined for a medium contrast
at a medium luminance ratio of the subject (see also speed as per DIN, ASA and ISO).
DIR couplers
The DIR couplers used in modern color negative films produce much
improved color saturation, a finer grain and better sharpness compared
with conventional films.
The first color film with built-in color couplers was launched in 1936
(AGFACOLOR NEU). The next technical breakthrough came in the seventies
with the introduction of the DIR couplers. DIR stands for
"Development Inhibitor Releasing". These couplers release
development-inhibiting compounds (inhibitors), more in areas
of higher density than in areas of lower density (in proportion to the
exposure and development activity). The inhibitors diffuse in all
directions in the film layers, are adsorbed by developing silver halide
crystals and inhibit their further development. The coupler can be
either colorless or colored.
The DIR couplers of the first generation improved one property in
particular: by preventing complete development of adjacent silver
halides (partial reduction of the silver halide), a finer grain was
produced than in conventional processes without DIR couplers, even
though relatively large silver halide crystals were used.
DIR couplers of the latest generation also improve in particular the
sharpness and color rendition. The inhibitors diffuse within the
emulsion layer (= horizontal inhibition) from points of high density
(where development is inhibited to a lesser extent) to points of low
density (where the development inhibition is greater). The higher
microcontrasts obtained in this way increase the impression of
sharpness (edge effect; transfer factor).
The effects of modern DIR couplers on the color rendition are even more
pronounced; the inhibitors diffuse from layers of high density into
neighbouring layers of lower density (= vertical inhibition, also known
as interimage effect). As
a result, the color density of the dominant color layer is intensified,
whereas in the adjacent layer of low density, dye formation is
suppressed. In this way, undesirable secondary densities (impurities of
the main color density) can be compensated (e.g. the yellow and cyan
secondary densities of the magenta dye). The net result is greater
color purity.
DX code
All the major film manufacturers now make use of the DX coding system.
It is an electronically readable information system for 35 mm films.
The DX system comprises the following components:
Checkered code on the film cartridge:
This electronically readable code enables DX cameras to automatically
control film speed and to identify exposure latitude and number of
exposures. The checkered code consists of two rows of electronically
conductive (silver colored) and non-conductive (black) rectangles.
Fig. 9: DX coding on the film cartridge.
Bar code and numerical code on the film cartridge:
The electronically readable cartridge bar code provides processing
information for automatic identification of the manufacturer, the
process, the film type, the film speed and the film length. In the
photofinishing labs, it permits automatic sorting for the specified
processing. Between the film slit and the bar code is a visually
readable numerical code with the same content as the bar code.
Information area on the film cartridge:
Film type, film length and speed are indicated on a particular area of
the film cartridge. Suitably equipped cameras display this information
in the control window.
Bar code at the edge of the film:
AGFACOLOR films have a digital bar code exposed into them at the edge
of the film below the perforations. Photofinishing labs use this
information for automatic product recognition.
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