I've had an update email from one of the suppliers that we contacted when we were seeking UV-C dosimeter badges. These work by changing colour when an appropriate level of exposure to UV-C radiation hits the surface. More detail can be found here: https://uvcdosimeters.com/uvc-100-dosimeter/
We use this type of tool when we use our UV-C lamp to sterilise the rooms in our holiday cottage after guest departure. The photochromic patches can be used to check the effectiveness of a UV-C lamp if you do not possess an appropriate UV-C meter. Given the large number of fake Chinese LED UV-C lamps on the market it is worth running a check if you buy one from Ebay or Amazon.