They don't put out a lot of light, but there is some visible light given off. UTH is just not cutting it to keep the temp up in the enclosure. I currently have 1 75 watt ceramic heater and it helps just not enough. Thinking of returning the $40 ceramic thing and adding 2 infared heat lights instead. 100 watt on the warm side and 75 watt on the low side and adjust with a dimmer to get the right temp.