Grey Card
A physical card of known neutral 18 per cent reflectance, held in the subject's light to give both a reliable exposure reading and a white balance reference.
Measured in
% reflectance
In depth
The card must be lit exactly as the subject is and held roughly perpendicular to the lens axis, tilted slightly to avoid specular glare. Because a good card is spectrally neutral, one frame including it also gives a colour reference that can be applied to every subsequent frame under the same light.
Examples in practice
A product photographer shoots one frame with the card in place, sets white balance from it in raw processing, and syncs that setting across the whole session.