# Is Umol the same as PAR?



## thegr8wendt (Dec 11, 2012)

Take for example this chart:










Is Umol the same as PAR?

And how does the Spectral Output look from this lamp?


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## Seattle_Aquarist (Jun 15, 2008)

Hi thegr8wendt,

Yes the umol is the PAR rating. The output of this lamp/led seems weighted to the blue end of the spectrum. The output spectrum looks good, maybe a little heavy on the 'blue' end. Possibly it is geared toward marine/reef/coral applications. Because blue light penetrates water better that would account for the higher umol ratings at the deeper depths.


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## Darkblade48 (Jan 4, 2008)

The advertising is a little too good to be true too. 22 umol per square metre of PAR at 120 cm (almost 48 inches)? From a single bulb?

A little hard to believe...


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## flc (Sep 10, 2012)

A micromole, or umol, is the unit of measurement associated with PAR. The u is used because most web apps can't handle multi-script encoding like english combined with the greek letter mu.

A mole is a fixed number of things, or elementary entities, if you want to be precise. One mole is composed of 6.02 x 10^23 things. That's 6.02 times ten followed by 23 zeros. The term micro refers to 10^-6 or 0.000001 units of something. If you multiply the two things together, a micromole comes out as 6.02 x 10^17 units of something. 

Like everything else, plants have to obey physics, in this case quantum physics. (The transfer of energy at small scales happens in discrete, exact amounts, or quanta.) Chemical bonds in plant pigments act like antenna which receive light. A chemical bond used to receive light to that's useful for photosynthesis can only use photons with a discrete, very specific amount of energy. The amount of energy of a photon is inversely proportional to its wavelength- which is why plants need specific wavelengths. Photons at any other energy level (wavelength) are ignored. Photosynthetic reaction centers have ways to get around that, but that's too much detail to casually explain in a forum post.

*Take home lesson:* 1 umol PAR corresponds to 6.02 x 10^17 photons with an energy level (or wavelength) plants can use for photosynthesis.


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