After posting I started to research lumen (via Wikipedia) which led me to CIE XYZ color (human color sensitivity), color temperature, then back to PAR, etc etc. Now I have far too many facts about light. :-)

(Side question: Does color temperature have any practical meaning to PAR when we are buying light bulbs for our aquariums?)

Okay, we know that PAR and WPG are not directly comparable. We know that WPG was created as an easy, shorthand, rule-of-thumb way to help aquarists determine how much light they have over their tank. We know that different light technologies (PCF, T8, T5HO, MH) offer tradeoffs of electrical efficiency, lumen output, color temperature, PAR output and heat generated.

So, I have an Apogee PAR meter. I want to be able to figure in my head that if someone here posts about a tank with 3 WPG, that would likely mean they would have PAR readings at the water surface of X µmol m^-2 s^-1. Then, for example, if I take my PAR meter to a tank that has the same light, but different bulb, and take a reading and see Y µmol m^-2 s^-1, I could generally (roughly) compare X to Y and get an idea how our tanks “metabolisms” are similar or differ. Or perhaps I could equate their 3 WPG tank to something I might be trying out with a suspended light.

-Jason