In video, which photometric measure represents the brightness in an image?

Luma is the correct measure representing the brightness in an image within video content. It quantifies the brightness level of a pixel and is crucial in video processing because it focuses solely on the intensity of the light, independent of color information. This makes it effective for applications where the perception of brightness is vital, such as in visual displays or during color grading.

In video systems, luma is often expressed in a range that corresponds to how bright a signal is perceived. It plays a significant role in various functions, including display calibration and compressing video signals. Understanding luma allows professionals to manipulate image brightness effectively, ensuring that the final output meets visual standards and viewer expectations.

Chroma refers to the color information in an image, which does not directly correlate with brightness, while brightness is a general term that doesn't have a standardized measurement in the context of video. Contrast pertains to the difference in brightness between the lightest and darkest parts of an image, but it does not directly measure brightness itself.