# Re: [APD] light and the w/gal rule

```thats what my other post was about.  watts is just not the way to go.
not all watts are equal is what i'm saying. bulb effciency is a big part
of it and as wright was pointing out to me that includes wasted light in
the infrared and non useable uv spectrums. the numbers all around just
arent for our use and i think till this point there wasnt a need for
them, unless the exsist in some science journal somewhere.

Jerry Baker wrote:

>S. Hieber wrote:
>
>
>>Well, if you want to go whole hog, use Pearson's R to do a
>>partial regression on all the relevent factors, and don't
>>forget to take into account the spectral shift that occurs
>>with age and variations that occur coming off the
>>production line, etc. When you're all done, most folks will
>>find that they wpg still works well, is close at hand and
>>convenient, and works as anything else. It's not like there
>>is some critical value of light that has to be obtained or
>>else plants won't grow well.
>>
>>
>
>There is such a value, but it's probably different for individual plant
>species and aquarium conditions.
>
>One neat thing about regression analysis is that it doesn't matter *why*
>something is correlated, it just is or isn't. I can explain 96% of the
>PAR output using lumen data alone, and I will be within 3.5 PAR of the
>true PAR value 89% of the time. That's pretty good to me. It doesn't
>really motivate me to go find the source of that other 4%.
>
>Your suggestion gave me an idea. Since lumens are based upon almost the
>same spectrum of light that PAR is, but uses a weighted curve of
>perceived brightness as seen by the human eye, I wondered if lumens and
>watts taken together would make a more accurate predictor of PAR than
>lumens alone. Thank you for the idea. I now have a logarithmic equation
>with an r-squared of 0.98!!! Here's the equation:
>
>PAR = 0.13 x sqrt(lumens x watts)
>
>So, for an example let's use an AH Supply 55W bulb. We know it's 55
>watts and has about 4700 lumens. That comes out to a PAR of 66. Who
>knows how true this is?
>
>
>
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