The first cold light head I had was an Aristo and it had a heater in it. I would turn it on about 15 minutes prior to starting to print. I don't think I had an issue with that head. I might not have known enough in those days either. I have used Zone VI heads where it had a green and red light on it. when the green light was on you could print the photo and when the red light was on you needed to hit the print button and turn on the light for a second or two. The Aristo VCL 4500 that I now have is hooked to a Metrolux II timer that has a probe that keeps the time accurate relative to the amount of light that is used. A closed loop if you will. It is nice but not technically necessary.