DANB Radiation Health and Safety (RHS) Exam 2025 – 400 Free Practice Questions to Pass the Test

When the target source film distance (TFSD) is doubled, the exposure time must be adjusted based on the Inverse Square Law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source.

As the distance increases, the intensity of exposure decreases, and to maintain the same level of exposure to the film, the time must be increased. Specifically, if the distance is doubled, the intensity at the new distance is one-fourth (1/4) of what it was at the original distance. Therefore, in order to achieve the same exposure level, the exposure time must be increased by a factor of 4.

Hence, when the target source film distance is doubled while using a 16 inch PID, the exposure time needs to be multiplied by 4 to ensure the film receives adequate radiation exposure for proper imaging.