When it is necessary to adjust the density of a radiography, a simple ratio can be used to estimate the exposure necessary to produce the change. In the straight line portion of the characteristic curves of many films, it can be seen that doubling the exposure will produce a doubling of the film density. Therefore, the following equation can be used to estimate the change in exposure needed to produce a change in the film density. A more accurate calculation can be made using the film characteristic curve and the characteristic curve must be used when one exposure is outside the straight line portion of the curve.
Where: E1= Exposure 1
E2= Exposure 2
FD1= Film density at exposure 1
FD2= Film density at exposure 2
If a exposure of 6.2 mA-minutes produces a film density of 1.5, what exposure will produce a film density of approximately 2.5? Assume that both densities fall on the straightened portion of the film characteristic curve.
Solve the equation for E2, substitute in known values and solve for E2.
Procedure for Using the Film Characteristic Curve to Adjust the Exposure
- Locate the measured density (Dm) on the characteristic curve of film being used.
- Record the relative exposure corresponding to this density. Call this value Em.
- Record the relative exposure that would produce the target density. Call this value ET
- Compute the ratio R=EM/ET. This is the amount the actual exposure needs to be adjusted to produce the target density.
- Compute the adjusted exposure by dividing the actual exposure used to produce the initial radiograph by this ratio (Ea = Ei/R).