The coefficient of variation for phototime reproducibility is measured on 4-6 consecutive exposures. What maximum variation is allowed?

• A. +/- 3%
• B. +/- 5%
• C. +/- 7%
• D. +/- 10%

Answer: (B)

For phototime reproducibility, we assess how consistently the exposure timer delivers the same exposure across several consecutive attempts. The measure used is the coefficient of variation, which is the standard deviation of the phototimes divided by their mean, expressed as a percentage (CV = (SD/mean) × 100%).

A maximum CV of 5% means the spread of the exposure times can be at most five percent of the average time. For example, if the average phototime is 100 ms, the standard deviation should be no more than 5 ms. This level of reproducibility helps ensure consistent image density and patient dose across repeated exposures.

A tighter limit like 3% would be more stringent than typical QA practice, while larger values such as 7% or 10% would permit too much variability in exposure.