luminansi

Luminansi, or luminance, is a photometric quantity that describes how bright a surface appears to a viewer in a specific direction. It represents the brightness of the light leaving a surface and is independent of distance, depending instead on the surface's emission or reflection properties and the observer’s viewing angle. Luminansi is used to characterize displays, painted walls, and other illuminated or self-luminous objects.

The international standard unit for luminansi is candela per square metre (cd/m^2), with the symbol L. It is defined by the rate at which luminous flux reaches the eye from a surface per unit area per unit solid angle: L = dΦv / (dA cos θ dΩ), where Φv is the luminous flux, A is the surface area, θ is the angle between the surface normal and the viewing direction, and dΩ is the differential solid angle. In practice, luminansi describes the brightness an observer perceives.

Luminansi is related to illuminance (the incident light on a surface) and to radiance in radiometric terms, but uses human-eye weighting through the luminosity function V(λ). For a diffuse Lambertian surface, luminansi is approximately L ≈ (ρ/π) E, where ρ is the surface reflectance and E is the illuminance.

Measurement involves luminance meters or spectroradiometers, often calibrated against standard sources. Goniophotometers may be used to map angular luminance distributions. Applications span display technology, photography and film, architectural lighting, and vision research, where consistent luminance values are essential for appearance, safety, and imaging fidelity.