The Zoo Med brand Mercury Vapor bulb is probably the closest thing you can use to mimic natural sunlight within an indoor enclosure. This bulb provides all the daytime heat, light, UVA and UVB that your tortoises need. These are excellent bulbs to use for indoor enclosures, or for temporary indoor enclosures for larger tortoises.
Because of the 100 Watt minimum of these bulbs, we suggest keeping a good eye on the temperature levels under the bulb, and raise and lower it appropriately to get the "surface temperature" where you want it.
*With the way these lights work, they should not be used with a dimmer or a thermostat. One hot, if they are turned off, they will not restart until they have cooled off completely. They cannot run at half or less than full power. Be sure to send in the included warranty card upon receipt in case you have any problems with the bulb.