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What are the differences between Quantum Light and Solar Radiation?
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Last Updated
10th of January, 2012

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The range of wavelengths that plants use is called Photosynthetic Active Radiation (PAR). This is measured in units of micro-mol per metre squared per second (umol/m2/s) which is usually called Photosynthetic Photon Flux Density (PPFD).

At midday in mid summer the sun can reach around :-
2000uE = 2000umol/m2/s = 9800FC = 1060W/m2 = 106000LUX.   
Of course, this depends on the latitude where you live.

From the above you might imagine that you could calculate conversion constants between them.  However, this cannot be done.  The trouble with comparing watts/m2, lumens, lux and umol/m2/sec (uEinsteins)
is that it is like comparing apples with bananas with pears.  Each system of units refers to different portions of the total light spectrum. So, to produce a set of conversion constants is misleading to say the least.

The total solar spectrum is measured with a pyranometer in units of watts/m2 relating to light in the 400 to 1100 nanometer wavelength. 

Photosynthetically active radiation (PAR) is measured with a quantum sensor in units of umol/m2/sec  relating to light in the 400 to 700 nanometer wavelength. 

Photometry is measured with a light meter in units of lumens or lux relating to light in the 380 to 770 nanometer wavelength. 

As a matter of interest one mol/m2/sec is equal to Avogadro's number in photons,
6.02  X 10 23.

This means that 1 umol/m2/sec relates to 602,200,000,000,000,000 photons reaching the earths surface in each square metre every second.  That an awful lot of photons - maybe keep that number in mind next time you contemplate sunning yourself.

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