A shutter controlling light from an always-on lamp would solve the problem, but that's not how it's normally done. I think overheating of the tube causes its life to be shortened. In my Aristo VCL4500, admittedly with two tubes, not one, after 10 minutes continuously on, the air in the enclosure was 70degC (158degF) and still rising. Maybe they should be called "not-so-cold-lights."
Just to elaborate on my post above, the Zone VI Compensating timer allows the tube to run at full output (which varies with temperature) and the timer shortens the units of time, "nominal seconds" as the unit gets brighter. An audible tick marks the passing of each "nominal second" and the acceleration in the ticking on the old one-tube ZoneVI unit is very dramatic (the lamp unit bought in the mid 1970s). Interestingly, this does not happen nearly as much in my Aristo VCL4500, where the rising in temperature causes some (not a lot) increase in brightness which levels out and actually can decrease with much higher temperatures (which I don't reach because the unit is bright and exposure times don't need to be inordinately long.) I wonder if it's a deliberate design improvement or just an artefact of different materials etc used. This leads me to wonder if the newer tubes for the one-tube unit made with a wider spectrum for VC might also exhibit less intensity variation with temperature, but that is pure conjecture.