| The sensitivity of a film-screen combination
depends on the film, the screen, the film processing, and the
beam quality, i.e. the spectrum of the X-rays exposing the film
screen combination. This explains immediately, why the
sensitometry of a film-screen combination with X-rays is a lot
more complex than the sensitometry of a film with light, and
therefore is hardly ever done outside the manufacturer's
laboratory:
1. The film-screen combination has to be exposed with a
standardized spectrum. This requires the use of a specified high
voltage value, a specified high voltage waveform (usually DC), a
specified target composition, a specified filtration, all
resulting in a specified half-value layer.
2. While the film-screen combination has to be exposed with
different dose values, the operating parameters of the X-ray
source (tube voltage, tube current, and exposure time) must not
be changed, as this is the only way to avoid measurement errors
due to spectral changes and due to the reciprocity law failure.
Therefore, the dose can only be varied by changing the distance
between source and film-screen combination.
3. The film has to be processed under standardized
conditions.
The speed of a film-screen combination is stated as the
inverse of the dose (in  Gy)
needed to obtain a film density of one above base plus fog,
multiplied by 1000 Gy:
1000 Gy
SPEED = -------------------------
Dose for D = 1+Base+Fog
The speed is the quotient of two dose values, it does not
have a dimension or unit name attached to it. As the speed is
inverse proportional to the dose requirements of a film-screen
combination, twice the speed is equivalent to half the dose and
vice versa.
With this definition, the standard or universal film-screen
combinations with calcium tungstate phosphor used to have a
speed of 100. With the modern rare-earth systems, the speed of
the standard screen is usually 200, i.e. the film-screen
combination for universal application requires 5
Gy (approximately 0.5 mR) for a
film density of one plus base plus fog. The speed values of the
high resolution ("detail" or "fine") resp. the high sensitivity
("high speed") film-screen combinations of one and the same
product line differ from the speed (and thus, dose requirement)
of the standard combination by a factor of two in either
direction. Thus, a rare-earth "detail" film-screen combination
has a speed of 100, and a rare-earth "high speed" film-screen
combination has a speed of 400. These are typical values, but
for special applications screens with lower and higher speeds
are available. |
