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Unit systemCGS units
Unit ofIlluminance
1 ph in ...... is equal to ...
   SI derived unit   10000 lx
   SAE units   929 fc

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.[1]

Metric equivalence:

Metric dimensions:

Illuminance = luminous intensity × solid angle / length2

See also[edit]


  1. ^ Parry Moon. "System of photometer concepts", in the Journal of the Optical Society of America, volume 32, number 6 (June 1942), doi:10.1364/JOSA.32.000348. – Page 355: "The lumen was proposed by Blondel in 1894 and is now universally accepted. The names, phot and stilb were likewise coined by Blondel (1921) and are in general use on the Continent."

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