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Radiographic Inspection - Exposure-Distance Relationship

Where:

 E1 = Exposure at D 1 E2 = Exposure at D2 D1 = Distance 1 from source D2 = Distance 2 from source

When producing radiographs, it is sometimes necessary to change the source-to-film distance.  Since the intensity of the source varies inversely with the square of the distance from the source, the exposure must be adjusted.  When the exposure at one distance is known, this information can be used to calculate the new exposure with the equation above.  Since exposure is the product of time and amperage, either of these variables can be substituted directly for exposure in the equation.

Example 1)  An exposure of 560 milliampere – seconds produces an acceptable radiograph at a source-to-film distance of 30 inches. What would the exposure need to be if the source-to-film distance is decreased to 24 inches?

Solve the equation for E2, plug in known values, and solve.

Example 2)  An exposure time of 1.86 minutes and an amperage of 5.6 mA produces an acceptable radiograph at a source-to-film distance of 30 inches. What would the exposure time need to be to produce a similar radiograph at a source-to-film distance of 24 inches.

Solve for E2 or in this case T2 since only the exposure time will be adjusted.  Then plug in the known values and solve for the new exposure time.

Example 3)  An exposure of 5.6 milliamperes with a 30 inch tube to film distance produced a good radiograph. What would the milliamperes need to be, if the tube to film distance is changed to 24 inches?

Solve for E2 or in this case C 2 since only the exposure current will be adjusted.  Then plug in the known values and solve for the new exposure time.