Hi,
I don't know what you mean by "cool" but as some contributors to this thread have pointed out, UV produces vitamin D in the skin. Prolonged UVB is dangerous and affects the eyes - direct sunlight should be avoided. There's enough UVB coming from the rest of the sky to satisfy this - atmospheric dust scatters it. Indoors you need an artificial source. I use a PHILIPS face tanning lamp bounced off a warderobe mirror which is on 6 hours a day. (the glass attenuates the dangerous short wavelenghs). It also sterilises the air. I used to breed budgies in an attic and had a lot of sickness. Since using this lamp I've had none. The lamp emits UVA and I have found this to be safe and effective. Any artificial source which emits strong UVB should be avoided.
BYTEMAN
http://www.avian-world.net/