What is the primary function of the back-up timer?

The primary function of the back-up timer is to prevent over-exposure to the patient. In imaging procedures that use ionizing radiation, such as X-rays, the back-up timer serves as a failsafe mechanism. It limits the amount of radiation the patient is subjected to during an exposure session, especially in situations where automatic exposure control (AEC) is in use.

If the AEC system does not terminate the exposure after a certain threshold of time, the back-up timer will automatically turn off the radiation after a predetermined period. This helps to mitigate the risk of exposing the patient to unnecessary radiation, which is crucial for patient safety.

Other choices, while related to exposure, do not directly address the primary function of the back-up timer. For example, while preventing over-exposure to the receptor is important, the focus of the back-up timer is specifically on protecting the patient.