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Phot

Measure of illuminance

Wikipedia contributorsAdapted for Fathomly · View authors
1 min read
Source updated November 21, 2023

A phot (ph) is a photometric unit of illuminance, or luminous flux through an area. It is not an SI unit but rather is associated with the older centimetre-gram-second system of units. The name was coined by André Blondel in 1921.

Metric equivalence:

1\ \mathrm {phot} =1\ {\frac {\mathrm {lumen} }{\mathrm {centimetre} ^{2}}}=10,000\ {\frac {\mathrm {lumens} }{\mathrm {metre} ^{2}}}=10,000\ \mathrm {lux} =10\ \mathrm {kilolux}

Metric dimensions:

Illuminance = luminous intensity × solid angle / length2
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Sources and credits

This article is adapted from the Wikipedia article “Phot”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.

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Illuminance

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. 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.

Lumen (unit)

The lumen is the SI unit of luminous flux, which quantifies the perceived power of visible light emitted by a source. Luminous flux differs from power (radiant flux), which encompasses all electromagnetic waves emitted, including non-visible ones such as thermal radiation (infrared).

Lux

The lux is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre.