The Planted Tank Forum - View Single Post - Convert Lux meter to PAR meter?
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post #175 of (permalink) Old 12-22-2013, 08:43 PM Thread Starter
Planted Tank Guru
Join Date: Dec 2005
Location: Sacramento, CA
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Originally Posted by Aquaticz View Post
How do you convert Lux to par - please
Lux and PAR are measurements of the same thing - light intensity at a surface. "Lux" is the units used when measuring the light, with the spectrum weighted to match human eye sensitivity. PAR is the units used when measuring light over the complete 400 to 700 nm wave length range, with all parts of the spectrum treated equally. Because lux involves suppressing wave lengths not easily seen by humans, the two sets of units can't be directly converted from one to the other, without using the spectral distribution of the light source to adjust the lux measurement to one that treats all wave lengths equally. So, the conversion factor for going from lux to PAR is different for different types of light sources - fluorescent, the sun, LEDS, CFLs, T8 bulbs vs T5 bulbs of different color temperatures, etc.

Fortunately, for our use, we don't need to know PAR to within 1% or better. If we know it to within 10% that works perfectly well. So, it is possible to use a lux meter with the reading divided by about 60 - 80 to convert the measurement to the equivalent of PAR. Or, you can use various filters on a lux meter to tweak the frequency sensitivity of it, and the overall sensitivity of it, and make it read in PAR units, accurate to +/-10%, and possibly better, for the commonly used lighting types on planted tanks. That is what I was doing in this thread.

I have stopped making these "PAR meters", out of boredom, mostly. But, I think I can just slightly modify a lux meter like the ones I used, so it reads in PAR units almost as accurately as those I made. I'm making good progress towards doing so. That would make it an easy DIY project for anyone, or it would make selling modified units for $25 and making some profit for little work, possible. If this works out, I will start a new DIY thread about how to do it.

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