2. Spectral Matching
The light emitted must be of proper wavelength(color) to match the sensitivity
of the x-ray film. This is called spectral matching.
Phosphor- emits light during stimulation by x-rays. Phosphor layers vary in
thickness from 50-300µm depending on the type of screen.
The active substance of most phosphor before about 1980 was crystalline calcium
tungstate embedded in a polymer matrix.
The term rare earth describes those elements of group
all in periodic tablet that have atomic numbers of 50-71.
The used in rare earth screens are principally gadolinum, lanthanum and
yttrium.
3. Rare earth radiographic intensifying screen must be used only in conjunction with film
emulsions whose light absorption characteristic are matched to the light emission of
the screen. This is called spectrum matching.
Spectral Matching
Conventional x-ray film is sensitive to blue and blue violet light and is rather insensitive
to light of longer wavelengths.
Blue sensitive films are used with calcium tungstate screens because their absorption
spectrum matches the emission spectrum of calcium tungstate.
Green sensitive film must be used with rare earth screens. If a green emitting screen
were used with blue-sensitive film, the strong emission in the green region would go
undetected and system speed would be sharply reduced.
4. Safelights
Green sensitive film creates problem in the darkroom. Safelights filters that are
satisfactory for regular x-ray film fog film manufactured for used with earth screens. Rare
earth screen-film requires the used of safelights that are colored even more toward the
red portion of the spectrum.
Phosphor used in manufacture of screens have one characteristic in common- they are
capable of luminescence which means “the ability to give of visible light”
The glow of the light from the screens is called fluorescence.
Important characteristic of the phosphor used in screens is minimal afterglow.
Afterglow of phosphorescence is the glow of light after the exposure has stopped.
5. Rare earth
Rare earth phosphors are more efficient in converting x-ray energy to light.
Rare earth phosphor need less x-ray energy to provide the same energy conversion as
calcium tungstate.
Rare earth phosphor have an x-ray to light conversion of 18-25%, which is four-five
times greater than that of calcium tungstate.
Film and screen types should be matched to provide maximum speed efficiency
Types of screen type of film relative
speed value
Calcium tungstate(medium) blue sensitive(regular) 100
Calcium tungstate(slow) blue sensitive(regular) 30-50
Calcium tungstate(high speed) blue sensitive(regular) 200
Rare earth(slow detail) green sensitive(regular) 100
Rare earth(medium) green sensitive(regular) 300-500
Rare earth(fast) green sensitive(regular) 800-1200
Imaging system screen-film comparison
6. Crossover
Crossover can reduced or eliminated by the use of radiographic intensifying screens that
emit short wavelength light (blue or ultraviolet).
the exposure of an emulsion caused by light from the opposite radiographic intensifying
screen is called crossover.
Device in an automatic processor that transports film from one tank to the next is called
crossover rack
Three critical characteristics
The addition of light-absorbing dye in a crossover control layer reduces crossover to
near zero
It absorbs most of the crossover light.
It does not diffuse into the emulsion but remains as a separate layer.
It is completely removed during process.
7. Crossover control layer
Crossover occurs when
screen light crosses the base
to expose the opposite
emulsion
Crossover is reduced by
adding a dye to the base; this
is called a crossover control
layer
