Final answer:
Brightness gain degradation in an image intensifier is typically assessed by measuring the total brightness gain, which is the most direct indicator of the intensifier's performance.
Step-by-step explanation:
Brightness gain degradation in an image intensifier is an important measure of the device's performance over time. Among the options provided, a periodic check on an image intensifier's total brightness gain is the most direct way to assess any degradation in performance. This is due to the total brightness gain being a measure of the increase in brightness from the input to the output of the image intensifier, indicating how much the image is amplified. It incorporates both the conversion factor, which is the ability to convert photon energy into an electrical signal, and the efficiency of the intensifier. Quantum mottle factor and conversion efficiency are also related to image quality, but the total brightness gain is a comprehensive measure that includes the effects of these factors.