What is the purpose of an 18% gray card in lighting and exposure metering?

• A. It measures color temperature.
• B. It provides a standard reflectance to help meters set correct exposure; it is used as a neutral target rather than a bright white.
• C. It calibrates white balance.
• D. It functions as a light source.

Answer: (B)

A gray card provides a known neutral reference for exposure meters. In reflective metering, cameras are calibrated so that an average scene is thought to reflect about 18% of the light it receives. By placing the 18% gray card in the shot, you give the meter a target with that exact reflectance to read, so it can set exposure that renders the card as a midtone in the final image. This helps keep your exposure balanced and prevents bright whites or deep blacks from fooling the meter.

It’s not used to measure color temperature or calibrate white balance, and it isn’t a light source. Those tasks use different references or tools. The gray card is specifically about giving the meter a neutral, standard reference for accurate exposure.