Illuminance
Illuminance measures the amount of light that falls on a surface. It is typically measured in lux (lx) units or foot-candles (fc). Illuminance is the "intensity" of light on a surface and is often used to determine the overall brightness of a room or outdoor area.
Illuminance is a crucial factor in lighting design, as it can affect the visibility, safety, and comfort of the people using a space. For example, high illuminance levels can cause glare and visual discomfort, while low illuminance levels can make it difficult to see or read [IESNA].
Illuminance can be affected by some factors, including the type and number of light sources, the distance between the light sources and the surface, and the reflectance of the surface. As such, it is essential to carefully consider illuminance levels when designing or selecting lighting for a space [CIE].
In photometry, illuminance is the total luminous flux incident on a surface per unit area. It measures how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. In SI units, illuminance is measured in lux (lx), or equivalently in lumens per square meter (lm·m−2) [IEC].
Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance [IEC].
IESNA. (2011). "IESNA Lighting Handbook: Reference and Application". 9th ed. New York, NY: Illuminating Engineering Society of North America.
CIE. (2006). "CIE. Recommended System for Outdoor Illuminance and Luminance Measurement." Vienna, Austria: International Commission on Illumination.
International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary. ref. 845-21-060, illuminance
International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary, luminous existence.