luminancën

Luminance is a photometric quantity that describes the perceived brightness of a surface when viewed from a particular direction. It represents how bright a surface appears to the human eye, taking into account the directional distribution of light and the eye’s sensitivity to different wavelengths. Luminance is defined as the luminous flux per unit area per unit solid angle that leaves or is reflected from a surface, weighted by the eye’s response to color and brightness.

In SI units, luminance is measured in candela per square meter (cd/m^2). The term “nit” is commonly used in display technology as a practical synonym for 1 cd/m^2. The formal relationship involves the luminous flux, which is the radiant flux weighted by the photopic luminosity function V(λ); thus luminance reflects perceived brightness rather than raw radiant power. Luminance can describe light emitted by a surface, reflected by it, or transmitted through it, for a specific viewing direction.

Luminance is distinct from related quantities such as illuminance and radiance. Illuminance measures the amount of luminous flux incident on a surface per unit area (lux, lx). Radiance is a radiometric measure, describing the directional distribution of radiant power per unit area; luminance is the photometric counterpart that incorporates human visual response. In practice, luminance is used to quantify how bright a scene or display appears to observers, and it is defined for particular directions and viewpoints.

Measurement and applications: Luminance is measured with luminance meters or spectroradiometers that report values in cd/m^2. It is central to display design, photography, lighting engineering, and computer graphics. Typical outdoor daylight luminances reach thousands of cd/m^2, while computer monitors often range from tens to hundreds of cd/m^2, and night scenes can be well below 1 cd/m^2. Luminance underpins perceptual imaging, HDR workflows, and visual ergonomics.