What device is used to measure the optical density of an exposed and processed radiographic film?

• A. Photometer
• B. Spectrophotometer
• C. Densitometer
• D. Geiger counter

Answer: (C)

Optical density on a processed radiographic film is measured with a densitometer. This device shines a steady light through the film and uses a detector to measure how much light passes through. The reading it provides corresponds to the optical density, defined as the logarithm of the ratio of incident light to transmitted light. Because exposed areas of film become more opaque as silver is deposited during development, they transmit less light and yield higher density values. Densitometers are specifically designed for this purpose, giving precise, reproducible density measurements that are essential for quality control and evaluating exposure and processing consistency. While a photometer also measures light, it’s a more general instrument, a spectrophotometer analyzes light across wavelengths and is not the standard tool for film densitometry, and a Geiger counter detects radiation rather than optical density.