Can be confusing because most people mistakenly believe that if a bulb is lit, then it is producing UV. That isn't the case.

To produce ultraviolet, lamps require very different phosphors than regular lamps. (Phosphors are the chemicals on the inside of the clear glass that make it look milky.) Because UV is a range of very small, high energy wavelengths, these phosphors break down much faster than phosphors used for common, everyday light. A lamp might have a rated life of 1000 hours, but it will continue to burn for 3000 or 4000 hours. It simply is producing very low amounts of ultraviolet during this time.

Rated life also depends on how you power the lamp. If you take a 4 foot SG series lamp and run it for long periods of time at 40 watts, you might get up to 2000 hours out of it. If you power it at 80 watts and turn it on and off every 15 minutes, you might get 800 hours. Doubling the watts cuts the life in half, although you are still getting the same amount of UV. You are simply getting it twice as fast. And turning any fluorescent bulb on and off a lot will reduce the life.

We have to guess that you will cycle the lamp a fair amount and run it at basic power levels. We call the "life" of a lamp to mean when the lamp is still putting out at least 70% of the original strength in the UV spectrum. Many people will continue to use them well beyond the rated life, other will change them at 80%, it just depends on your needs.

If you need precision amounts of UV, then you should invest in a UV meter for the right spectrum you are looking for. They cost around $300 and will provide you with reasonably accurate information on how powerful your lamps are at any given stage of their life.