# Converting watts to par?



## DennyB (Oct 24, 2014)

Is there a way to convert the light t5/t8 fixtures give off to an approximate par measurement without a par meter?


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## mistergreen (Dec 9, 2006)

nope. Watts measure volts X amps. PAR is micromols per second and square meter (μmol m-2 s-1).

Your best luck is a LUX meter conversion.
http://www.apogeeinstruments.com/conversion-ppf-to-lux/

Not accurate, but close enough.


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## Diana (Jan 14, 2010)

You could look at the color charts- If their peaks are in the right range for plants, then you know they at least are better for plants than some other bulbs. 

But no, there is no way to convert watts to par. 

Have a look here, some research by some of our members about bulbs, and testing with a PAR meter. If your bulbs are listed, this is great. 

http://www.plantedtank.net/forums/showthread.php?t=184634


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## Hoppy (Dec 24, 2005)

One reason you can't convert the bulb wattage to PAR value is that the amount of light a T5 light produces depends heavily on how good the reflector is, and whether or not the ballast is full power or under powered. The PAR from a given bulb can vary by a ratio of more than 2 to one just from the reflector. Also, the PAR from that bulb depends on how far from the bulb you are measuring the PAR. PAR drops with distance from the bulb.


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## mattinmd (Aug 16, 2014)

+1 on Hoppy...

Put another way: 

Par measures surface brightness (ie: how many photons are hitting a given area in a second)... How much brightness a given light puts on a surface depends on how might light is produced, how far away the light is, and how focused it is.

Watts measures energy usage... This has no direct relationship to the amount of light produced, and even less to how bright the light shines on a given surface.

To convert watts to par you'd need:
- some kind of light-production efficiency factor, spectrally weighted correctly to the PAR band. (or at least close, since most PAR meters aren't perfect at this).
- a measure of how focused the light is
- a measure of the distance from the light to whatever it is shining on.
- a correction for re-focusing of the light by the air-water boundary, based on how far the light is from the water surface, if we're talking about underwater plants.

That's a lot of messy calculations.


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