A film used in an X-ray cassette must have a spectral sensitivity
that is matched to the emission spectrum of the intensifying screen.
Light emitted from an intensifying screen in general can be either
of two types; a continuous spectrum, as in the case of CaWO0, or a
band spectrum, as in the case of Gd2O2S:Tb (Lanex screens) (Fig.1).
A standard silver halide film will be sensitive to light up to a
wavelength of 520 nm, but will be almost insensitive to most of the
light emitted by a gadolinium oxisulfide screen (540 nm).
For this type of screen another type of film must be used. This type
is called orthochromatic film and is made sensitive to the green
light from the screen by a sensitizing dying agent in the emulsion
that absorbs the green light and then transfers the energy to the
silver halide grains.
The spectral output of the phosphor must be matched to the
response of the film (Fig. 2). Calcium tungstate screens emit blue
light of continuous spectrum with a peak wavelength at about 430 nm.
The term "blue screen" refers both to the screen itself and to the
blue sensitive film used together with the CaWO4 screen. Rare earth
screens emit light in narrow lines with strong peak(s) in the green
part of the spectrum but smaller ones also in the blue, blue-green
and yellow regions. The term "green screen" may be used. It is
absolutely necessary to use green sensitive film with these screens
to make sure that useful transmitted radiation is not lost.
Types of film and their sensitivity spectra
||300 - 500 nm
||300 - 520 nm
||300 - 580 nm
||300 - 615 nm
sensitive to all wavelengths
||300 - 760 nm