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Radiographic Inspection - Exposure-Density Relationship

When it is necessary to adjust the density of a radiography, a simple ratio can be used to estimatethe exposure necessary to produce the change. In the straight line portion of the characteristic curve

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 produc

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

Example Calculation

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 filmcharacteristic 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

1. Locate the measured density (Dm) on the characteristic curve of film being used.

2. Record the relative exposure corresponding to this density. Call this value Em.

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3. Record the relative exposure that would produce the target density.

Call this value ET4. Compute the ratio R=EM/ET. This is the amount the actual exposure

needs to be adjusted to produce the target density.

5. Compute the adjusted exposure by dividing the actual exposure

used to produce the initial radiograph by this ratio (Ea= Ei/R).

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