I was in a Pt/Pd workshop a couple of weeks ago with Tillman Crane. He recommends just leaving the box on throughout the printing session.
Originally Posted by cperez
There are two reasons to want to NOT turn off the light source. With traditional fluorescent tubes with magnetic ballasts, there is a problem that some of the tubes may not start when they unit is turned on. Most box designs call for a metal plate that is supposed to correct this problem, but I don't think is an absolute guarantee. I know that Tillman has a problem with his boxes and has to tinker with them a bit to make sure that all of the tubes come on. And once they are on, you don't want to turn them off again.
CFL's (spirals) have a very pronounced warm-up characteristic, but traditional tubes also have this characteristic. Some months ago I found a report from a lighting technology research group at RPI in which they measured the warmup times - unfortunately, I failed to save it and now I can't find it again - but my recollection is that their conclusion was that while the warmup actually took 2-4 minutues, the light output was reasonably close to the maximum after the end of the first minute. With exposures of 4-12 minutes for various alternative processes, a variation during the first minute probably won't make a big difference. I believe that Sandy King did some tests that confirmed that conclusion. But again, just leaving them on avoids having to worry about warmup.
The other side of the coin is why would you want to turn them off between exposures. Fluorescent bulbs have a fairly long life expectancy, and their energy consumption is rather low. So economics are not a concern. I suspect that the two valid reasons have to do with the potential risks of leaving the UV source on continuously. One risk is that spillage from the lights could cause paper to be fogged before it is intentionally exposed - having a bare bulb left on continuously in the area where you are coating paper woudl be a bad idea. The other is that you want to minimize exposure of your eyes to UV - unless you like the idea of cataract surgery at an early age. Both issues can be addressed by have a UV box design with a door that can be opened only to insert and remove your printing frame, and left closed at all other times.