Since UV lights are important to reptiles, and I've heard it said that they lose their UV output after about 6 months even if they haven't burnt out, is there a way to measure the UV output of a light? That way I can get the most out of my UV lights, only replacing them when the UV output stops.
These lights/tubes produce UVA and UVB. The same type of tubes are used in sunbeds and marine tanks
To produce UVA, mercury inside the tube reacts with the gas and electricity. This is why you should never shatter tubes because you can get mercury poisoning and they have to be disposed of properly. The mercury will slowly stop to react as expected - as a result, the tube will produce more UVB. We do not need UVC to be emitted from the tubes because in nature most of UVC from the sunlight gets stopped by the ozone and atmosphere. UVC is used in germicidal lamps and has no benefits to living matter.
This is very noticeable in the sunbed sector, when old lamps start to burn skin instead of tan. It is very unhealthy for all living things as it directly damages DNA.
To accurately measure the tubes you need a UVA and UVB meter. Usually sold separately and cost in the region of 100USD each for low quality ones. Anything cheaper is junk that just makes up numbers.
If you run your tubes/lights 8 hours a day then it is typical to replace them after 6~8 months, as this will keep your animals and or plants much healthier. You will be better off investing the money in new lights rather than in testing equipment you will use a few times only.