Quote:
Originally Posted by TheMikey
PAR meters do not accurately measure LED intensity for a comparison to MH/T5 - it has an issue with the blue LEDs which are what make up the majority of the LED clusters.
You can actually track this revelation back to Reef Central a few years back - People would build these massive LED arrays trying to get to MH PAR readings and they would have disappointing results from LEDs because of colouration and growth issues. The issue, it turned out, was that they were almost frying corals from the intensity.
People are having better success with LEDs now, because they're putting the appropriate intensities over the tank. If you can't get the PAR readings you want, you may just have to set a low intensity and adjust until you get the results you're looking for. Your corals will let you know.
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That really depends on the PAR meter. The meters that us hobbyists usually use (Apogee) are not sensitive to blue wavelengths, and therefore underestimate PAR. Furthermore, the common Apogee sensor is flat, so it only takes into account light coming from directly above the sensor. Spherical sensors, as made by Walz, are more accurate. Licor also makes some highly accurate PAR meters. Unfortunately they are not available to most hobbyists as they are expensive (the Apogee is cheap by PAR meter standards).