What is the purpose of the backup timer in automatic exposure control (AEC) systems?

• A. To calibrate the dose before imaging.
• B. The backup timer prevents excessive exposure if the AEC fails or detectors do not respond.
• C. To adjust the kVp during exposure.
• D. To control the duration of the entire radiographic procedure.

Answer: (B)

The backup timer serves as a safety net for automatic exposure control. Normally, the detectors signal when enough radiation has been detected and the exposure ends automatically. If the AEC system fails or the detectors don’t respond (for example, due to detector misalignment or a fault), the exposure could continue unchecked. The backup timer is set to a maximum duration and will terminate the exposure if that time is reached. This keeps patient dose from escalating and protects the equipment. It isn’t about calibrating dose, adjusting kVp, or controlling the overall procedure length; it specifically prevents excessive exposure in the event of AEC failure.