By scientific definition of PAR - Photosynthetically Active Radiation is any light wavelengths that fall between 400nm to 700nm. If its outside of this range - it shouldn't get counted and in some products it does - because they don't have the ability to ignore it. We can.
The sensor that Apogee have developed and published their response curve for - shows it picks up light outside of this range. You can check this with a a cheap UV black light off of amazon . There will be some visible light and PAR (we can see it) but the meat of the light that it outputs we cant as its outside of our visual range which also ends around 400nm.. So there shouldn't be very much PAR at all. This is of course only relevant if your light fixture actually outputs light outside of the PAR range.
When we tested this we also verified this with a lab spectrometer.
I think it boils down to that the technological advantage that we have with our sensor, we can completely ignore the light that doesn't meet the definition of PAR because we can accurately measure the spectrum and remove what shouldn't be there. We aren't just measuring the total light available through a filter. We hope to build on this in the future, as an example we could offer users the ability to measure the BRS biology band ONLY.. or a wider spectral range. Homework for us, for the future!
Whether light outside of the original scientific definition of PAR its relevant or not to corals is a different matter - that's why you must be able to see the spectrum too. A high PAR level can be 100% irrelevant for the coral if the light is all green, for example!
I also wouldn't rely on a single user feedback that A PARwise is 100 PAR lower than an apogee - even if it was in our favour - the Apogee model, age and device, light source type, position in water, device repeatability are all major factors here - including water parameters. We also have several apogee (MQ-510 included) and they all give us different readings. So take that with a pinch of salt. Our MQ-510 isnt '100' out across a range of lights we've tested.. for example.
We stand by our calibrated results

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Kind regards
Craig