Einstein (unit)

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For other uses, see Einstein (disambiguation).

An einstein is a unit used in irradiance and in photochemistry. One einstein is one mole of photons, regardless of their frequency. Therefore, the number of photons in an einstein is Avogadro's number. Irradiance can be measured in einsteins per square metre per second.

The einstein is used in studies of photosynthesis since the light requirement for the production of a given quantity of oxygen is a fixed number of photons (about nine einsteins per mole of oxygen formed), regardless of the frequency of the light.

It is named in honour of Albert Einstein, who explained the photoelectric effect and introduced the idea of light quanta, now called photons, in a 1905 paper.

Photosynthetically active radiation (PAR) is usually reported in microEinsteins per second per square meter, one microEinstein being one-millionth of an einstein.