± 5nm: Does that mean you actually get 60 different readings, one for every 5 nm measuring point? Or does it mean you get to pick a particular wavelenght accurate to 5 nm, and get one reading. Or does it sweep all the wavelengths, multiply the intensity it found at each measuring point by a table value, and then sum everything? All the papers I have read about PAR do it like that.
These meters I see advertised look like they have a big LCD display and they show you one number. So that would make the big sweep and average method seem more likely. But it that’s all they do then a double bright blue LED could have exactly the same PAR reading as a better light that puts out red to blue balanced light. So I suspect PAR meters are not so useful when looking at LEDs, and look: Some of the papers suggest exactly this point.
I think their basic assumption is that you will have a light spectrum that is like the sun or incandescent lamps, which give you a pretty smooth black-body spectrum. When you only have simple LEDs, the light will be at a very narrow peak and that’s it. White LEDs mostly use blue light to stimulate a mix of phosphors that supply light at longer wavelengths. But those wavelengths may not match the blackbody spectrum the PAR meter expects.