We are used to units of illumination and irradiance that involve human vision, such as footcandle, lumen, and lux. [7] But plant biologists of various kinds have been interested in measuring photosynthetically active radiation (PAR) from 400-700 nanometers. Evidently the push for this was stronger in Europe than in North America. [1] Another distinction with PAR, is that we are interested in irradiance (amount of energy striking a surface per unit area per unit time) rather than brightness (energy output of the bulb or light source).
A breakthrough was the demonstration in 1972 by Keith McCree to show that photosynthesis was better predicted by counting photons in radiation than in using footcandle measurement meters, leading to the development of devices called quantum meters. And the price of these meters has come way down and is much less costly than the more accurate spectroradiometer for measuring PAR.
All this lead to the designation of the "Einstein" as a unit of irradiance. A whole Einstein is defined as 1 mole of photons. Recall that one mole is 6.02x10^23 atoms (Avogadro's number). The unit used for PAR irrandiance is a millionth of an Einstein per square-meter per second, or 6.02x10^17 photons per square-meter per second, the microEinstein (µE). [4] Note, however, that the Einstein is not an official SI unit of measure [3].
See [5] for an excellent discussion of all this with respect to greenhouses. They also have some conversion factors such as
5 microEinstein = 1 footcandle
Some measures may be expressed as an integration over a longer time period, such as for the Langley unit sometimes used in climatology
10.45 Einstein/day = 1 Langley/day
Climatologists and other atmospheric scientists are sometimes interested in expressing solar irradiance at the top of the atmosphere, before any absorption. See [6] for a graph and explanation of the unit of measure: Watts per square-meter per inverse-centimeters per steridian!
For additional units conversion data, see the tables at the ends of [1] and [3]. To be precise about conversions, either from PAR units to illuminance units or vice versa, you also need to know the type of illumination source (e.g., kind of bulb).
Q.E.D.
