I did a search on "ultraviolet sensors" in this forum, and came up with only three results, so, it appears, that this has not been asked before.
I have a hand-held ultraviolet light sensor from Extech (Extech UV505) to measure power density from a 300 nm UV LED.
The UV LED is collimated with a lens at NA=0.9, and produces a good spot about 10 mm in diam. The output of the LED is listed as 30 mW at 6V, and yet the sensor reads 3 mW. The sensor is bigger than the spot. For another LED emitting at 365 nm, the sensor shows the expected value (or ballpark). Even though I mostly use the sensor for relative values, I need to have an idea of the power density to compare to published values. E.g. in the past people used UV lamps, and I am trying to replace the expensive 100W $5K lamp with a $200 LED.
Even though the specs of the sensitivity state "290-400nm", I suspect that the response is very much non-flat, and that the sensitivity in the 300/10 nm range is much smaller than at 365. The datasheet does say "peak at 365", that's as much of the spectrum as is available from the manual. I asked for the response curve from Extech, and their answer was "we do not have it", literally (facepalm).
What type of a sensor might this be, so I can look up the curve online? This must be an old technology, since the meter shows a similar number at 365 as an old UVP meter I borrowed from a colleague (the meter says 1991 on it). Interestingly, the UVP unit is still sold, and the design and the pricing both look as if it was still 1991:
Another puzzling feature about the Extech unit is that the sensor looks covered with a translucent plastic window which does not seem to be removable.