Einstein (unit)

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 might be measured in einsteins per square metre per second, if the frequency is well defined, as for a monochromatic source. If the optical frequency is not well defined this is a sloppy misuse of the term irradiance, which is defined in terms of power per unit area.

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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 photosynthetically active photons (about nine photosynthetically active einsteins per mole of oxygen formed).

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.