luminance
The luminous flux emitted in a given direction divided by the product of the projected area of the source element perpendicular to the direction and the solid angle containing that direction (i.e. luminous intensity divided by unit area), measured in stilbs or apostilbs.
luminance: brightness per unit area in a given direction
Luminance is the measure of how bright a surface or light source appears when viewed from a specific angle. It quantifies luminous intensity per unit projected area, expressed in candelas per square meter (cd/m²), stilbs (sb), or apostilbs (asb). Unlike illuminance, which measures light arriving at a surface, luminance describes light leaving a surface and reaching an observer's eye. A neon sign, an LED display, a painted wall lit by sunlight, and the sun itself all have measurable luminance values.
The definition hinges on "projected area": the apparent size of the surface as seen from the viewing direction. A glossy surface viewed edge-on has negligible projected area and therefore very low apparent luminance, even though the surface itself reflects the same photons. This is why a cinema screen remains bright when viewed head-on but dims dramatically from extreme angles. Measurement always requires specifying direction, making luminance a directional quantity unlike illuminance, which is omni-directional.
In practical work, luminance appears critical in display technology, photography, and vision science. Monitor manufacturers specify peak luminance (often 300 to 500 cd/m² for office displays, up to 1500 cd/m² for high-brightness outdoor screens). Human perception adapts strongly to luminance levels: a 100 cd/m² surface feels equally bright whether the surrounding scene is at 50 or 200 cd/m², but the absolute value determines whether glare, discomfort, or visibility hazards occur. This adaptation is why headlight luminance standards exist: above roughly 1000 cd/m², oncoming drivers experience disabling glare.
Common pitfalls and measurement reality
Confusion between luminance and illuminance is endemic. Illuminance uses lux (lm/m²); luminance uses candelas per square meter. A surface at 1000 lux illuminance could have wildly different luminance depending on its reflectance: a matte white wall might reach 300 cd/m², while a glossy black wall under the same 1000 lux might measure only 30 cd/m². Measuring luminance requires a luminance meter that looks at the surface from a fixed angle, typically within a narrow viewing cone. Smartphone light meters measure illuminance, not luminance, and yield useless numbers for display or lighting design.
Luminance also depends heavily on surface finish. A perfectly diffuse (Lambertian) reflector maintains constant luminance regardless of viewing angle; real materials depart from this ideal. Specular (mirror-like) surfaces show luminance only in the direction of reflected light. Most industrial surfaces fall between these extremes. This directional behavior matters for inspection lighting, where luminance must be measured at the actual viewing angle used by human inspectors or imaging systems.