What does illuminance mean?
Illuminance is the technical term for the amount of visible light falling on a surface. It is expressed as luminous flux per unit area, with the SI unit being the lux; one lux represents one lumen of light distributed across one square metre. The term is widely used in lighting design, architecture, photography, and the physical sciences. It measures the light arriving at a location rather than the brightness perceived by an observer or the light emitted by a source. For example, an outdoor reading area may receive thousands of lux, while a poorly lit room may provide only a few. Because illuminance concerns incident light, it is especially useful when comparing how effectively different areas are lit and when determining suitable exposure or lighting levels.
nounThe amount of light falling on a surface, measured in lux. In the SI system, one lux equals one lumen of luminous flux distributed over an area of one square metre.
- The quantity of visible light incident on a surface, expressed as luminous flux per unit area and measured in lux.
"The room's illuminance was too low for comfortable reading."
"Daylight can produce an illuminance of tens of thousands of lux outdoors."
"The sensor adjusts its exposure when the illuminance changes."
Usually used as an uncountable singular term when referring to a measurement; the plural is used when comparing distinct illuminance values.
"The illuminances at the two workstations were noticeably different."
Illuminance is the photographer's measure of how much light actually lands on the scene, not how bright the scene looks in the finished image.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does illuminance come from?
Illuminance was formed in English from the Latin root associated with light, including Latin lumen, meaning “light,” and the verb illuminar(e), meaning “to light up.” The word entered English in the 19th century as a technical term patterned on the Latin-derived family of illuminate, illumination, and luminous. Its development reflects the broader use of Latin scientific terminology in English during that period.
Related word forms
How illuminance is actually used
Illuminance is primarily technical, especially in lighting design, photography, architecture, and the physical sciences. It is distinct from brightness or visual perception and from luminance, which concerns light emitted or reflected by a surface.
Easily confused with illuminance
Illuminance describes the light arriving at a surface, whereas luminance describes the light emitted or reflected by that surface.