It is a good idea to expose your tortoise to some real sunlight even if you have UVB bulb. No UVB bulb can compare to the sun. Tortoise needs the sun and vitamin D to prevent Metabolic Bone Disease (MBD). I have also heard that the sun exposure can accumulate. That means if a tortoise should bask 1 hr a day, it is the same as giving in 3 hours on two days and 1 hour another day in a week. I am not sure about if this is true though.
The following site is a very good one and it clears up my confusion on lighting. Please take your time to read it.
http://www.tortoisetrust.org/articles/lighting.htmlDuring my visit to the vet, the vet told me that most UVB bulbs are not doing what they claimed on the labels. I also came across a site testing the bulbs.
http://www.uvguide.co.uk/Their reports on diff UVB light are very long. I never read them all. I just asked my vet to see if my light is ok. He got a meter to measure too. So I just stick to that brand. It is better to use a good brand. Also those mercury light bulbs that claim to provide heat and light are no good. They are not durable and will die after you turn on and off frequently. I bought many before knowing that and they only lasted for around a month.
So get a good brand for the UVB light bulb and have a separate heat source. Ceramic heating bulbs work really well and they last for a long time. Some people have thermostat to control the temperature in the tank.
You may use a timer to set the light to mimic the sunlight. I turn on my light at 7 am and shut off around 7 pm. Give them 8 – 12 hour of UVB exposure. Half of the time, Tun Tun stay in her cave though.
I have a UVA bulb for Tun Tun too.
More info:
http://russiantortoise.org/uvb.htm
http://zoomed.com/Library/ProductDBFiles/Reptiles%20and%20UVB.pdf (info on UVA and UVB)
http://www.hkherp.org/newforum/viewthread.php?tid=3499&extra=page%3D8 (Chinese 紫外线的波段-UVA/B/C/D)