Lighting, Heating & UVB Questions

How much time can a hermanns tortoise spend outside to be the equivalent of a uvb bulb being on for 12 hours?

 

Fran Baines replied

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If you provide a UV Index of 3 in the basking zone indoors, and your tortoise wanders in and out of it all day, he gets UVI 3 whenever he goes into your "indoor sunlight" from dawn to dusk. But much less whenever he moves out of the basking zone.
Outside in the wild in the Mediterranean, a similar tortoise would - only on a clear day, and only in late spring, summer and early autumn - get a "basking zone" - i.e., available full sunlight in a sunny patch - with much lower UVI at first (zero at dawn, rising to maybe 7 or 8 by 11am, so if you AVERAGE OUT the possible exposure it might well be UVI 3) and the temperature will rise steadily so he will retreat into shade for most of the rest of the day when the UVI in a sunlit area is much higher, but in the dappled shade it might well average from zero to about UVI 3 as well...but again, the tortoise is wandering in and out of the sunny patches.
Outside in the UK garden, however, there is much less UVB, even at mid-day, than in Greece, for example. On a cloudy day, far less. However, it is cooler, too. So the tortoise will spend longer out in areas of full sun than he would in, for example, Greece, to get warm. So he will get more UV exposure, but at a lower dose rate, because an "average" exposure of UVI 3 will take more time to achieve in the UK summer sun than in the Greek summer sun....
See, it's ... complicated!