Definition
Illuminance is the photometric measure of the amount of luminous flux incident on a surface per unit area. It quantifies how much visible light falls on a given surface and is expressed in lux (lx), where 1 lux equals 1 lumen per square meter (lm·m⁻²).
Overview
Illuminance is a fundamental concept in lighting engineering, architecture, photography, and vision science. It is used to assess the adequacy of lighting for tasks, to ensure compliance with safety and ergonomic standards, and to evaluate the performance of lighting fixtures. Because it is a photometric quantity, illuminance accounts for the human eye’s sensitivity to different wavelengths of light, weighting radiant power by the standard luminosity function.
Etymology / Origin
The term derives from the Latin illuminare (“to illuminate, to light up”) combined with the suffix ‑ance, which forms nouns indicating a state or quality. The modern scientific usage emerged in the late 19th and early 20th centuries alongside the development of photometry as a distinct discipline from radiometry.
Characteristics
| Aspect | Details |
|---|---|
| SI Unit | Lux (lx) = lumen per square meter (lm·m⁻²). |
| Related Units | Foot‑candle (fc) = 1 lumens per square foot ≈ 10.764 lux. |
| Measurement Instruments | Illuminance meters (lux meters) equipped with photodiodes or silicon phototransistors calibrated to the photopic luminosity function. |
| Dependence on Geometry | Varies inversely with the square of the distance from a point source (inverse‑square law) and directly with the cosine of the angle of incidence (Lambert’s cosine law). |
| Spectral Sensitivity | Weighted by the CIE photopic response curve, peaking at 555 nm (green light). |
| Typical Values | • Domestic living rooms: 100–300 lx • Office workspaces: 300–500 lx • Surgical operating rooms: 10 000 lx or higher |
| Regulatory Standards | Defined in standards such as IEC 62471 (photobiological safety) and various national building codes that prescribe minimum illuminance levels for specific spaces. |
| Relation to Other Photometric Quantities | • Luminous flux (lumens) is the total light output of a source. • Luminous intensity (candelas) is luminous flux per unit solid angle. • Luminance (candela per square meter) describes light emitted or reflected from a surface in a given direction. |
Related Topics
- Luminance – the photometric equivalent of radiance, describing perceived brightness of a surface in a particular direction.
- Luminous Flux – the total amount of visible light emitted by a source, measured in lumens.
- Photometry – the science of measuring visible light in terms of its perceived brightness to the human eye.
- Radiometry – the measurement of electromagnetic radiation, inclusive of all wavelengths, without weighting for human visual response.
- Illuminance Meter – a handheld instrument used to measure illuminance in various environments.
- Lighting Design – the practice of planning and implementing lighting systems to achieve desired illuminance levels and visual comfort.
- CIE (International Commission on Illumination) – the organization that defines standards for photometric and colorimetric measurements.