How do you calculate the par?
I don't know of any way to calculate the PAR, but I may try to figure out a way to do it starting with lumens. I haven't tried that yet. You can buy a lux meter from Amazon or Ebay for around $15 plus shipping, and use that to measure the light intensity in lux at the same distance as your bulb will be from the substrate, with the light sitting across a couple of chairs. Divide the lux reading by 61 and you are very close to the PAR at that distance.
EDIT: No you can't calculate PAR. If you had a point source of light, mounted at exactly the focal point of a perfect and huge parabolic reflector, you could calculate the PAR. But, we never have either a point source of light nor a perfect parabolic reflector. The problem is that so much of the light is not directed to the aquarium substrate, and the light that is directed to the substrate isn't evenly distributed over a known area. No matter how much data you get you are always short of data to calculate the PAR or lux at all accurately. We are stuck with having to measure it if we want an accurate number.
Last edited by Hoppy; 08-30-2012 at 10:53 PM.
Reason: add more info