Lots of good feedback, thanks!
Cheaply quantifying spectral energy, not really possible. Doing it below 300nm, another order of magnitude more expensive at least. Without an accurate and stable spectral output the cheap methods to quantify energy are all useless.
Yea... That is what I figured, thus my inclusion of the "

" emoji next to the word "ideally" in the OP.

A spectrum analyzer or VNA is on my to-buy list, but wasn't intending on picking one of those up until next year. I think I read somewhere that SAs are used with a special probe for wavelength measurements? I have a scope and a miscellaneous assortment of bits n pieces, had hoped cobbling together a makeshift probe for use with my existing scope might have been possible, but I really did not expect it.
@thm_w
Thanks for the info. Prior to pandemic, I bought a floor-standing air purifier with UVC. At least for the model I bought, if you pop off the cover and check out how it works, only a tiny percentage of the total volume of air passing through the unit gets any UVC exposure at all, and the duration of exposure is very short. For the replacement germicidal bulbs, the only numbers I have seen are power consumption, but I need the
W/cm2 of the UVC/germicidal emission to relate back to virus-killing professional studies that use numbers like 0.1J/cm2 or 4000 uW/cm2 for 15 minutes, etc. I saw various photodiodes on digikey, I'll have to look at those a bit more in depth.
I bought some UVC LEDs from Digikey for the same purpose you describe. Yes, they were expensive, and yes, I do realize with my napkin math that with 60mW output power, the disinfection rate of air will be something between maybe 70% and 99% but definitely no more. I'm combining it with a HEPA filter (I happen to have quite a few from a previous project), though. Proper mechanical filtering is what all those masks are doing, and as you may know, proper masks are really good stopping viruses, no UV needed.
Very cool! The direction I was going is mostly for sterilizing exhaled-and-aerosolized viruses from sick people who need to use a CPAP/BiPAP, whether because a ventilator is not available or if they normally use a CPAP on a regular day but can't now without broadcasting nasty bits all over the place. There are a massive number of people out there who have CPAP or BiPAP machines at home. With enough UVC exposure, shouldn't need a filter, and with the various shortages of medical supplies, having an all-electric solution would be a good thing. And, as you pointed out, you can always add a filter later. Might be able to do something similar with inhaled air, but that's not my main objective.