I'm sure there are more detailed and scientific versions than this... but a few things I've learned...
The UV effects of a UV bulb start to erode the moment it is first turned on and continues to erode every minute it is in use. Thus the bulb will be at it's strongest when it is first put into use and will become slowly less effective from that moment forward...
Thus running the bulb on an on'off cycle seems logical... at least at first glance...
One fact to keep in mind though, is the "moment of illumination" puts far more wear and tear (stress) on the bulb and erodes the UV potency far more than several hours of use...
After a long conversation about UV lights with a 40+ year Union Electrician (my father) and a PE Electrical Engineer (my uncle) I was offered the conclusion that running the bulb at 12 hours on / 12 hours off will slightly extend the life of the bulb... but running it 6 hours on / 6 hours off will actually reduce the life of the bulb...
Something else to keep in mind, the bulb will continue to emit light long after the UV potency is completely eroded...
I replace my bulbs once per year (Jan 1st at my house) utilizing the above mentioned formula and am thoroughly convinced I am destroying parasites throughout that entire year...
