I know it seems counterintuitive to use a sunlight calibration model for electric light sources, and I spoke with a tech at Apogee a few months ago and he recommended an electric calibration model for electric light sources just like he did for you.
But the recommendations for our reefs are based on an analysis done by Dana Riddle. Did you read the comparison article from Advanced Aquarist I linked to in my post? Here it is if you didn't:
http://www.advancedaquarist.com/2005/7/review/view
Dana Riddle compared the Apogee unit (his has both Sunlight and Electonic calibration in one unit) as a low cost alternative to the de facto Standard in Quantum Meters, the unit made by Li Cor (about $1200 in cost). In the course of his comparison of accuracy of the Apogee unit in Sunlight Calibration mode to the Li Cor unit and the same Apogee unit in Electronic Calibration mode to the Li Cor unit, he found that the Sunlight Calibration mode had a lower percentage deviation from the Li Cor unit than the electronic calibration in all Kelvin ranges except the 4000K range, which no Reefer I know of uses anyways.
So which Apogee (Sunlight or electronic calibration) model would you prefer to buy? My guess is most Reefers will buy the one that is the closest in accuracy to the industry standard meter (Li Cor) that was tested under water with the Kelvin ratings we use in the hobby.
Pay attention to the comparison tables 1 and 2 at the end of Dana Riddle's article.
The difference in the accuracy of the Sunlight vs Electronic Calibration models may be related to, and this is just a guess, the fact that we are measuring light that is passing thru a liquid medium, whereas the primary purpose of these Apogee Meters is to measure PAR of light in Agriculture and Hydroponics that is traveling thru air instead of water.
For whatever reason, when measuring PAR in a reef tank, the Sunlight Calibration model is closer to the Li Cor readings than the Electronic Calibration model.
You can use either model, but the Apogee Sunlight Calibration unit readings are closer to the Li Cor unit readings.
Hope this helps. The Dana Riddle article is pretty self-explanantory. You may also want to do a search of Reef Central under "Apogee" or "Quantum Meter" or "PAR Meter" and you'll find the same recs there.
