What device is used to measure the luminance of a monitor?

• A. Photometer
• B. Radiometer
• C. Photodiode
• D. Spectrometer

Answer: (A)

Luminance is a photometric measure—it's about how bright a surface appears to the human eye. A photometer is built to respond to that perceived brightness and is calibrated to express the result in luminance units (cd/m^2), which is exactly what you need when checking a monitor’s brightness. That makes it the best choice for measuring how bright a display looks.

A radiometer, by contrast, measures radiant power without weighting for human vision, so converting its reading to luminance requires extra steps and calibration that aren’t built in. A photodiode is a light sensor that produces an electrical signal, but on its own it isn’t tied to luminance units without a calibration setup. A spectrometer analyzes the light’s spectrum, revealing color and wavelength information, not a direct measurement of brightness as perceived by the eye.

So the photometer directly provides the luminance measurement you need, which is why it’s the most appropriate tool for this task.