illuminance

{{Short description|Luminous flux incident on a surface per area}}

{{Distinguish|Luminance}}

{{Infobox physical quantity

| name = Illuminance

| unit = lux

| otherunits = phot, foot-candle

| symbols = {{math|E_v}}

| baseunits = cd·sr·m⁻²

| dimension = $\backslash mathsf\{L\}^\{-2\}\; \backslash mathsf\{J\}$

}}

File:Illuminance Diagram.tif

In photometry, illuminance is the total luminous flux incident on a surface, per unit area.{{cite encyclopedia | title=Illuminance, 17-21-060 | encyclopedia=CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition. | publisher=CIE - International Commission on Illumination | accessdate=20 April 2023 | year=2020 | url=https://cie.co.at/eilvterm/17-21-060}} It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception.International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary. [https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-21-060 ref. 845-21-060, illuminance] Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.[http://www.drdrbill.com/downloads/optics/photometry/Exitance.pdf Luminous exitance] Drdrbill.com

In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre (lm·m⁻²). Luminous exitance is measured in lm·m⁻² only, not lux.

International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary. [https://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-21-081 ref. 845-21-081, luminous exitance] In the CGS system, the unit of illuminance is the phot, which is equal to {{gaps|10|000|u=lux}}. The foot-candle is a non-metric unit of illuminance that is used in photography.One phot = {{gaps|929.030|400|001|u=foot-candles}}, according to http://www.unitconversion.org/unit_converter/illumination.html

Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word, such as to mean luminance. "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.

The human eye is capable of seeing somewhat more than a 2 trillion-fold range. The presence of white objects is somewhat discernible under starlight, at {{val|5|e=-5|u=lux}} (50 μlx), while at the bright end, it is possible to read large text at 10⁸ lux (100 Mlx), or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting afterimages.{{Citation needed|date=July 2008}}