We bought a Hydrofarm LGBQM quantum meter to do some PAR testing.
PAR is photosynthetically active radiation, the visible light spectrum, and is measured in photosynthetic photon flux, or PPF. The unit of measurement is µMol/m²/s. It describes the number of photons passing through a square meter, per second.
There are a couple of better PAR meters, namely from the LiCor and Apogee brands, but Hydrofarm's meter is pretty decent, especially for the price. It doesn't have cosine correction, which means it doesn't pick up the light coming in at an angle correctly. So the edge measurements are minimum values.
PAR is not an ideal measurement, but it's related to plant response, unlike a lux meter, which is weighted to a model of human sensitivity to wavelengths. We evolved to detect differences in green, back in the jungle, so green-heavy lights will have more lumens. But plants can use any of the photons from 400nm to 700nm, so PAR is a better measurement.
Then there is YPF, the yield photon flux, which weights photons based on the photosynthetic response at each individual wavelength of the spectrum.
But PAR, i.e PPF, is the standard measurement in the grow light industry, so that's what we're trying to measure, in (µMol/m²/s).
i.e. A quantity of photons. So that's what we are looking at, with a PAR meter.
The red wavelengths hit the surfaces, the yellow wavelengths will go a bit deeper into the leaf, and so on, green hitting the underside of a leaf, mostly, and blue wavelengths finding a chloroplast deeper in the canopy, on the next leaf, perhaps.
We set up an experimental light rig, with a distance of 27cm between the LED(s) and the surface of the sensor, then drew an 'X' below. We recorded measurements at the center, and the average of the four corners (which are separated from the center by 15cm). This gives us a fairly basic idea of the efficacy of the 'light cone' produced by an LED.
The most interesting thing we discovered was the prominent effect of the inverse square power law. That is, a few centimeters can make a big difference. Lenses also make a big difference.
To give an example, 1500 µMol/m²/s is similar in effect to the sunlight hitting a square meter, at noon on a hot day. So you don't want to bleach your plants by putting a light closer than they should be.
The value of these measurements is to have a better idea of how far away to place the light. Lenses focus a lot of light on a smaller area, and you want to aim for 600 to 1000 µMol/m²/s on as much of the canopy as you can, for steady growth.
Warning: Do not look directly at LEDs.
Brand Name COB tests
We tested Cree and Bridgelux COBs, using the Meanwell LPC-60-1400 driver.
Bridgelux Vero29 Gen7 4000K 80 CRI
Bridgelux Vero29 Gen7 4000K 80 CRI with 90 degree reflector
Cree CXB3070 5700K 80 CRI with 90 degree reflector
Cree CXB3590 3500K 80 CRI
Cree CXB3590 3500K 80 CRI with 90 degree reflector
Reflectors make a big difference. Otherwise, Cree and Bridgelux COBs are pretty similarly matched, under these test conditions. This link is probably still the best comparison between these COBs.