What percentage higher is the back-up timer set compared to the expected exposure?

The correct answer is 150% because the backup timer is typically set to ensure that the exposure does not exceed a specific safety threshold that is calculated based on the expected exposure. This threshold increases the exposure time by an additional 50% over the expected level.

In practical terms, if the expected exposure is considered to be 100%, the backup timer would be set to allow for exposure up to 150% of that expected level. This safeguard helps to prevent underexposure and also addresses situations where the automatic exposure control might not work as intended, providing an added layer of assurance for successful imaging.

Hence, by setting the back-up timer at 150%, it ensures that adequate exposure is achieved while still accommodating potential variability in the exposure control system.