CC – shows relationship between exposure
and optical density or optical brightness (for digital rad.)
The CC provides information about fim
contrast; speed (sensitivity) film; latitude – dynamic range
Film contrast:
·
Depends on:
o
Shape/ slope of the film’s Characteristic Curve –
tells us how much change in density will occur as exposure changes
§ Gamma = D2-D1/ logE2-logE1
§ Average gradient (gamma using density of 0.25 – 2 [useful density range]
above base density and fog
§ Ave grad >1 – exaggerates contrast and the higher the ave grad he
higher the exaggeration.
§ Ave grad = 1 – no increase in contrast
§ Ave grad < 1 decreases subject contrast
o
Film density
§ The ratio of the difference of the log relative exposure is
determined by the kVp
§ Changes in mAs (expoure factors) determine the location on the log
relative exposure axis
§ In toe and shoulder regions very little density change for mAs
o
Screen vs direct exposure
§ At the same density contrast is always lower on a film exposed to XR
alone than for the same film exposed to light from a screen
o
Film processing
§ Increasing time and temperature:
·
Increases the ave gradient
(increase film contrast) – to a point
·
Increases film speed (increased
density for a given exposure)
·
Increases fog (decreases film
contrast)
