Measuring PAR for LED lights???

[chuckdallas]

I have read a few posts back and forth about trying to use a PAR meter to test LED light intensity. One camp says it's okay and you can get good readings and I've also read that you can't compare Metal Halide readings with LED readings using a PAR meter. I'm an electrical engineer but no physicist.

Does anyone know what the real story is? I have a feeling that the "die hard MH'ers" post here to sway people to their side. I have heard from a LFS that white LEDs are just as intense as blue LEDs, which is not the case for T5s (as far as I've been read on RC) but if a PAR meter can't accurately measure LEDs, then how will we ever know?? I'd contact some of the more reputable LED light fixture makers, but I think they would use their sales techniques to sway the vote. Any non-biased opinions out there?

Thanks in advance and P.S., I already own 2 LED fixtures, after using T5s for half a year, so I'm not trying to determine whether to buy LEDs. I just want to know if my LEDs are TOO bright for the 24 inch deep tank, even though they are 18-24 inches over the top of the water. The zoas sitting 6 inches below the top are doing GREAT and the mushrooms down near the sand in the corner away from big flow are doing great, but I can't keep a chalice (3 different ones down low near the sand) and the oxypora died too. The phos is measured with a Hanna Phosphorus meter and has 0-3 ppm each time, using my sump with chaeto and the media reactor with GFO in it.

[dzhuo]

This is a general deficiency with the Apogee quantum sensors. It underweights blue and overweights red. Technically, it has nothing to do with LED vs. MH or T5 although lots of commercial LED fixtures produces lights in the blue spectrum heavily so it's being affected the most. You can still use this particular sensor for a general guideline of PAR output. Here is Apogee's official response:

"Apogee quantum sensors underweight blue light, and as a result, photon flux measurements for blue LEDs will be too low. Also, the quantum sensors overweight red light up to a wavelength of approximately 650 nm, above which they do not measure, and as a result, photon flux measurement for red LEDs will either be too high (if the LED output is all below 650 nm) or too low (if a non-negligible fraction of the LED output is above 650 nm). Our quantum sensors will likely provide a reasonable measurement for white LEDs because they are broadband, and because the sensors are calibrated under CWF lamps. However, because of the diversity of LED lighting systems the precise errors have not been quantified. The current spectral response of our quantum sensor can be viewed on our website (http://www.apogeeinstruments.com/qua...lresponse.html). We are currently working on better filtering in order to achieve a sharp cutoff at both the 400 and 700 nm wavelengths, but this improvement is still a few months away.

That being said, Apogee quantum sensors can be used to measure the relative output of an LED or bank of LEDs, in order to track variability in output with time or temperature for example. However, quantum sensors should not be used to characterize the absolute output of LEDs (except for the possibility of white LEDs), to compare one LED to another, or to determine photon flux for plant growth for example."

The more expensive Li-Cor sensor performs similarly to Apogee: Product Review: A Comparison of Two Quantum Meters - Li-Cor v. Apogee