Definition
Characteristic curve, a curve used to show the exposure
properties of a film or a film screen system. The characteristic
curve, which was described in 1890 by Hurter and Driffield, is a
representation of how the exposure of the film is related to the
measurable signal, i.e. the blackening of the film, or film density.
The characteristic curve is different for different film types but
has a general shape as shown in Fig.1. The base and fog density is
measured on an unexposed film. The shape of the characteristic curve
tells the user the contrast properties (slope of the linear part)
and the useful exposure range (length of the linear part). It also
will indicate the speed of the film (or film-screen system), which
can be judged from the curve's position along the horizontal axis.
The speed class can also be found from the characteristic curve.
Average gradient,
the slope of the linear part of the characteristic curve of an X-ray
film. This is normally defined from the characteristic curve of an
X-ray film using the density points 0.25 and 2.0 (over the film fog)
to calculate the average gradient as the slope:
(2.0 - 0.25) / (logE2.0 - log E0.25) where Ex is the exposure (or
mAs) needed to produce density x (over the fog level).
Fog,
film density caused by the development of silver halide grains that
are not exposed to light or X-rays in the exposure of the patient.
There are many reasons why fog is present in all X-ray images. Some
of them are:
-- chemical fog
-- storage of the film in warm and humid locations
-- storage of film in locations with a high level of background
radiation
-- contaminated developer solution
-- too high temperature and/or prolonged time in developer
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Useful Link/ Ref: (http://www.medcyclopaedia.com,
Amersham Health).
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