Contrast Film and OD

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)