We are used to units of illumination and irradiance that
involve human vision, such as footcandle, lumen, and lux. 
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. 
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). 
Note, however, that the Einstein is not an official
SI unit of measure .
See  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  for a graph and explanation of the unit of
measure: Watts per square-meter per inverse-centimeters
For additional units conversion data, see the tables
at the ends of  and .
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).