@Webb.Peter There are a few things people might do…
- Participate in the Public Lab Spectral Workbench community. They’ve got what looks like a good system for qualitative analysis of spectra with CFL based calibration. Their tools would probably be adequate as-is for measuring the peak wavelength of an unknown LED or identifying the color temperature of a CFL bulb. If you combined that with a table of lux to PAR conversion factors–I don’t know where you’d get that, but Dr. Kubota implied such things exist–you could probably use a cheap light meter to estimate PAR.
Somebody with a PAR meter, a lux meter, and a spectrometer could measure different kinds of lights and publish lux to PAR conversion factors. Somebody else with just a lux meter and a spectrometer could look at the spectrum of an unknown light, match the spectrum chart to the closest lux to PAR conversion factor, and use that with their lux meter to estimate PAR.
Somebody could try building a combo lux meter and automatic stepper-motor driven monochromator. If you used the right sensors and got the math right, it might be reasonable and affordable to measure photon flux vs wavelength and calculate PAR for real.
I’m not familiar with the specifics of any particular quantum PAR meters. But, according to my admittedly hazy understanding of the physics behind it, they would have to assume a spectral distribution of the light hitting the sensor. Also, I think Dr Kubota made some comments about how PAR meters intended for Mercury Vapor lights can be way off for LEDs–presumably because of the vastly different spectral energy distribution (incandescent=smooth vs. LED=spikes).
#6 is a really blue looking, higher color temperature, bright white fluorescent, so the blue-green-indigo-violet stuff is present in the light. It looks different from the other CFL spectra because there’s other stuff in the tube to give off the additional wavelengths.
Color temperature is weird. This is stretching my limited physics knowledge to the max, but I think color temperature ratings for lights are essentially a way of saying, “If you look at this light, your eyes will think it looks like the radiation coming from a black body heated to the rated color temperature”. The comparison gets really weird because of different spectral distributions between how our eyes respond to light, and how LEDS, CFLs, and incandescent sources emit light. See https://www.e-education.psu.edu/astro801/content/l3_p5.html for more about black body radiation and a chart of color temperatures and spctra.
To convert PAR from lux–according to Dr. Kubota’s lecture–what you want is a lux to PAR conversion factor for that specific type of light source prepared by somebody who knows what they’re doing. I think she might have rattled off a couple conversion factors during one of the lectures.
[edit: I mentioned lux instead of lumens because lumens measure the amount of luminous flux coming out of the light at all angles. What you need is the flux passing through an area at leaf level. So, getting from lumens to PAR would require mathematical gymnastics maybe kinda like ray-tracing plus some spectral conversions. I think that developing a software tool for doing this might be fairly reasonable. I’ve wondered if you might even be able to do it directly with Blender by learning to configure light sources properly. That said, buying a cheap lux sensor or meter seems like a much more practical option.]