Luminances

Luminance is a photometric quantity that describes how bright a surface appears to an observer in a given direction. It represents the radiant intensity per unit area projected in that direction, weighted by the eye’s sensitivity to different wavelengths, and is measured in candelas per square meter (cd/m^2). Luminance can arise from emitted light, reflected light, or a combination of both, and it is fundamentally directional: a surface can have different luminance values when viewed from different angles.

In practice, luminance is the quantity most closely tied to perceived brightness. It is distinct from illuminance (the radiant flux incident on a surface) and from radiance (the purely radiometric counterpart before eye sensitivity is applied). The photometric relationship to radiance is mediated by the luminous efficiency function V(λ); for a typical daylight or display scenario, luminance summarizes how bright a scene or element will appear to a human observer.

For many surfaces, especially diffuse (Lambertian) ones, luminance can be related to irradiance and reflectance by L = ρE/π, making luminance largely independent of viewing direction in first approximation. In color and display work, luminance maps are used to quantify brightness and to compute perceptual contrast and grayscale rendering.

Common instruments for measuring luminance include luminance meters and imaging luminance cameras, which calibrate sensors to report cd/m^2 for single targets or for spatially resolved scenes. Luminance is widely applied in display engineering, architectural lighting, photography, and vision science to assess visibility, readability, and visual comfort.