Leuchtdichten

Leuchtdichte, in English luminance, is the photometric quantity that describes how bright a surface appears to the human eye when viewed from a given direction. It represents the luminous intensity per unit area emitted, reflected, or transmitted by the surface into a specified solid angle. The SI unit is candela per square meter (cd/m^2); the unit is also commonly called a nit, where 1 nit equals 1 cd/m^2.

Luminance is a directional quantity and is related to the spectral power distribution of the light and the eye’s sensitivity. In practical terms, it depends on the surface properties (emission, reflection, transmission), the illumination, the geometry of observation, and the viewing angle. The concept is distinct from illuminance (lux), which measures incident light on a surface rather than light leaving or reflected from it. For a diffuse Lambertian surface, luminance L is approximately ρE/π, where ρ is reflectance and E is illuminance.

Measurement of luminance is performed with luminance meters or calibrated cameras that detect light in a defined viewing direction and apply the human-eye weighting function. Measurements are reported in cd/m^2.

Applications and relevance: Luminance is central in display technology to specify screen brightness, in signage and architectural lighting to ensure visibility, in automotive dashboards, and in photography and film for exposure planning and quality control. Typical consumer displays range from a few hundred to around a thousand cd/m^2, with higher-end HDR displays exceeding 1000 cd/m^2. Luminance is a perceptual quantity influenced by adaptation level and viewing conditions, and it is often considered together with luminance contrast in visual tasks.