I forgot to mention the thermostat is set on dimmer so when the doors are closed and the temp is up to 89 the light bulbs are almost off or at a very low glow. I guess I could also use a ceramic heat element but I haven't had a need to change. Don't fix it if it's not broken.
I built my incubation in early 2000 and I'm still using the original four 100 watt light bulbs.