Operating Manual

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7.4 Characteristic curve (density curve)
The characteristic or density curve indicates the relationship between increasing exposures
and resulting density. By exposure (E) is meant the radiation dose on the film emulsion. It
is the product of radiation intensity (I
o
) and exposure time (t), therefore: E = I
o
.t
The ratio between different exposures
and related densities is not usually plotted
on a linear scale but on a logarithmic
scale; i.e. density D versus log E.
The curve is obtained by applying increa-
sing exposures to a series of successive
areas of a strip of film, whereby each follo-
wing exposure is a certain factor (for
example 2) greater than the previous one.
After development, the densities (D) are
measured by means of a densitometer and
then plotted against the logarithmic
values of the corresponding exposures
(log E). The points obtained are then joi-
ned together by a continuous line. It is not
necessary to know the absolute exposure
values; relative values can be used, so at a
fixed X-ray intensity only exposure time
needs to be changed.
Density (D) of a photographic emulsion does not increase linearly with exposure (E) over
the entire density range, but has a shape as in figure 2-7. The lower part of the curve (a-b)
is called the “toe”, the middle part (b-c) is called the “straight line (linear) portion”, and the
upper part (c-d) is called the “shoulder”. Usually the characteristic curve of industrial X-ray
films shows an S-like shape.
The shoulder of a characteristic curve relating to industrial X-ray film corresponds to densi-
ties higher than 4. Since such densities are too high for normal film viewing, the curve from
density D = 3.5 upwards is shown as a broken line.
It should be noted that the straight-line portion (b-c) is not truly straight, but slightly
continues the trend of the toe of the curve.
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Developing the latent image
Development is the process by which a latent image is converted into a visible image. This
result is obtained by selective reduction into black metallic silver of the silver halide crystals
in the emulsion. These crystals carry traces of metallic silver and in doing so form the latent
image. Several chemical substances can reduce the exposed silver halides to metallic silver:
these are called “developing agents”.
7.3 Characteristics of the X-ray film
The use of X-ray film and the definition of its characteristics call for an adequate knowled-
ge of sensitometry. This is the science which studies the photographic properties of a film,
and the methods enabling these to be measured.
The density (or blackness) of the photographic layer, after development under closely def-
ined conditions, depends on exposure. By exposure is meant a combination of radiation
dose striking the emulsion, that is to say intensity (symbol I) and the exposure time (sym-
bol t). In sensitometry, the relationship between exposure and density (I.t) is shown in the
so-called characteristic curve or density curve.
Density (optical)
When a photographic film is placed on an illuminated screen for viewing, it will be obser-
ved that the image is made up of areas of differing brightness, dependent on the local opti-
cal densities (amount of silver particles) of the developed emulsion.
Density (D) is defined as the logarithm to base 10 of the ratio of the incident light I
o
and
the transmitted light through the film It, therefore: D = log (I
o
/ I
t
) . Density is measured
by a densitometer, see section 9.2.
Industrial radiography on conventional film covers a density range from 0 to 4, a differen-
ce corresponding with a factor 10,000.
Contrast
The contrast of an image is defined as the relative brightness between an image and the
adjacent background.
The contrast between two densities D
1
and D
2
on an X-ray film is the density difference
between them and is usually termed the “radiographic contrast”.
Film contrast, or emulsion contrast, are rather vague terms used to describe the overall
contrast inherent in a particular type of film. When an emulsion type shows most of the
image contrasts present, the film is said to be “of high contrast” or “hard”.
For the measurement of film contrast, the term “ film gradient” is used, for which the sym-
bol is G
D
. Suffix
D
indicates the density at which G is measured.
a
Fig. 2-7. The characteristic curve for an
industrial X-ray film
log E (exposure) – relative units
density D