I used to use a Zone VI head with the Zone VI compensating timer. The length of the units of exposure vary in real time with the light intensity. An audible "tick" sounds after each unit, about one second. As the lamp heats up, the ticking accelerates markedly, indicating a very significant output increase during the exposure.

The heater element in those units is always on, and goes some way to 1. increase light output and 2. slightly reduce the difference in output between cold and hot. The Aristo VCL4500 (and maybe other models) has a thermostat on the heater, so some regulation is built in. These "cold light" heads can get pretty warm. My Aristo VCL4500 is 80degC (176degF) inside at the end of a longish exposure.

I think I remember reading that the later (current) Aristo tubes are less variable with temperature increases, but I don't remember where I read that. The early Zone VI single tube models can be fitted with a current Aristo tube that is better for VC filter use (wider spectrum- the old Zone VI tubes were rather blue because that's all you need for graded papers)