Initial testing of the SQ-500 vs the SQ-120, as read by a Fluke 87V in high res mode:
The sensor, on its side with the protective cap installed:
For these tests, I zipped tied the two sensors together and kept their positioning fixed.
First tes, a 14k 250MH electric ballast used Hamilton 14k MH bulb in an average reflector:
SQ-120 PAR: 146.7
SQ-500 PAR: 145
Very close
Next up is a 100W Royal Blue multichip on my bench, there due to other testing:
These results are... very different.
SQ-120: 860 PAR
SQ-500: 670 PAR
Yes, backwards from what I expect...
I did a few other tests, on my LEDBrick units installed. This is a violet + RB + blue + cyan + coolwhite + (very limited PC amber, green, red).
SQ-120 PAR: 960 (at the water line, its very diffuse and a nice 250 midway)
SQ-500 PAR: 820
Again, different direction than I would expect.
Its dark outside so I can't baseline it against the sky.
If the SQ-500 was consistently low I'd expect something, but the MH result being a good match doesn't imply its direct measurement error. Ambient light in all cases was off or minimized.