Luminance

Luminance is the photometric measure of perceived brightness, expressed in candelas per square meter (cd/m²) or nits in display contexts. It quantifies how bright a surface appears to the human visual system, weighted by the luminous efficiency function (the eye is most sensitive to green wavelengths around 555nm). In post-production, luminance underlies several core concepts: the Y channel of YUV/YCbCr color spaces (which separates brightness from color); the contrast and gamma curves that control midtone luminance; HDR display peak luminance specifications (1000 nits, 4000 nits, 10000 nits); SDR Rec.709's 100-nit assumed peak; and dynamic range calculations (stops of luminance from black to peak white). Luminance is distinct from light radiance (which is wavelength-uniform) and from value or lightness (which are perceptual). On the waveform monitor, the Y trace shows luminance distribution across the frame; on the parade, R, G and B traces show individual channel luminance contributions. Photometric measurements at acquisition use luminance meters (Sekonic, Konica Minolta) calibrated in nits. The PQ HDR transfer function is defined directly in nits of absolute display luminance, unlike Rec.709 gamma which is signal-relative. Understanding luminance is essential to grading HDR content correctly — a midtone gray at 50 nits in SDR maps differently across HDR formats.