Tortoises raised in improper lighting usually
do not show any clear evidence of problems but may not grow or
reproduce as well as a healthier tortoise. One on-going debate, for
example, is the importance of UV, especially the range known as ‘UVB’
lighting.

Sunlight
Natural sunlight is made up of several elements, many that we cannot see. The important elements for us include…

• Infrared light (IR), a very long wavelength (10,000 to 700 nanometers) ‘below’ the color red, is mostly used for heat. IR
  offers a benefit by warming the item it shines on more than it does the
  air in-between. It also penetrates more ‘deeply’ than many other
  wavelengths.
• Visible light- red, orange, yellow, green, blue, and violet- is a range of
  wavelengths from the longest to the shortest (700-400nm). The human eye
  has three structures that see color and ‘trigger’ at reddish, greenish,
  and bluish. This means that if you shine the right combination of red,
  blue, and green, it will look pretty white to a human.
     
   The problem is that it is not ‘really’ white. Light that looks right to us
  does not always look right to cameras, plants, and other things. It
  does not even really look right to us if we compare it to a true white.
      The problem is a bit worse for tortoises. they have four
  structures in their eyes to see color and are thought to be able to see
  some ultra-violet, like insects and other animals can. What looks white
  to us under standard fluorescent lighting probably looks grey-red to them. This can affect behaviors, food recognition, and more.
  
• Ultra-violet light (UV) is a shorter wavelength (400-100nm) past purple. There is actually a broad range of UV light-
• UVA
  is the longest wavelength of the group (400-315nm) and is closest to
  purple. It is known to have effects on animal behaviors and on the
  pineal gland and other organs- even in humans.
• UVB is the
  middle range of wavelengths (315-280nm). This is the band that is
  needed by most animals to make vitamin D in their skin. (Well, not
  really, but the real process is pretty complicated. You can read it here.)
  It generally does not take a lot- it makes about 1,000 IUs of vitamin D
  a minute with good sunlight, so it only takes a few minutes every
  couple days to get a good dose.
       One problem is that the range of UV that does this is close to 280nm, or almost…
• UVC,
  the shortest and most harmful wavelength (280-100nm). Most of this is
  filtered by the atmosphere, which is good news. UVC is great at killing
  things- cells, germs, etc.

UV light is a mixed blessing.
It helps promote behaviors, makes vitamin D, kills germs, and more. On
the other hand, it causes sunburns, skin cancer, eye damage,
degradation to plastic and cloth, and so on.

Natural
sunlight is by far the best lighting option for tortoises. If a
tortoise can be offered about an hour’s worth of good sunlight (light
that does not pass through glass, plastic, or fine screens and is
strong enough to tan a person) in a week, over several short sessions,
then they only need simple lighting in the habitat.

How much light?
This depends on several things.

• Age/size. Young tortoises spend most of their time hiding from light and predators so would need less light than older tortoises would.
• Natural range- latitude.
  Tortoises near the equator get more direct, stronger light than
  tortoises from outside the tropical zone. Once you get past 40 degrees
  (about Boston in the US), the sunlight is so weak that little UVB
  reaches us in the winter.
       Latitude also affects
  day length. At the equator, days are generally about 14 hours long,
  while further away the days get shorter, and there is more seasonal
  variation. 
• Natural range- climate and overhead vegetation.
  The more overcast or overgrown a place is, the weaker the light hitting
  the ground. Open desert gets plenty of unfiltered light, while a deep forest would only get about 10% on the ground.

No
matter the age or species, all tortoises spend much of their day
hiding, so there should always be plenty of shade, hides, and resting
places.

The best options
There are three main general purpose indoor lighting plans we can use to best simulate natural sun at reasonable costs.

• Florescent bulb option.
  Can be used for small to medium habitats. May require a separate heat
  element. Studies with Panther Chameleons showed this was the best set
  up for reproductive success in that species, but the conclusions seem appropriate here as well.
• Start
  with a low-level UVB-style fluorescent bulb sized appropriately for the habitat
  and hung per directions. Set to run for about 6-8 hours in the middle of
  the day.
• Supplement
  with a warm white, ‘true white’, deluxe white, or ‘daylight’ fluorescent
  bulb or a standard incandescent bulb set to run for 12 hours, or 10 hours in the winter and 14 hours
  in the summer.
• “Four tube” option. Four old-style, thicker, 48″ or longer T-8 bulbs emit
  enough UVB light that no other lighting may be needed. The bulbs should be
  daylight, deluxe white, sunlight  or superwhite, although a mix of
  plain warm and cool white will also work.
• If possible, the lights should come on and go off over a period of time to better simulate dawn and dusk.
• Mercury vapor option.  An inexpensive and simple option for a medium-sized habitat. It can create a lot of heat and light in a smaller habitat. The intensity created may be an issue for any small tortoises or forest tortoises.
• Start with a Mercury Vapor bulb with UVB capabilities.
• Wattage appropriate to the habitat size.
• Hang at least 12in above the tortoise backs- higher if needed to regulate temperatures. 
• Set to run for about 6 hours in the middle of the day period.
  
• Add
  a second bulb to offer more color balance, as well as a dawn/dusk
  simulation. This can be a plant bulb, a warm white (or true white, deluxe white or daylight) fluorescent, or a
  standard incandescent.
• Wattage appropriate to the habitat size.
• Height over the habitat is relatively unimportant.
•  Set to run for 12 hours, or for 10 hours in the winter and 14 hours in the summer. 

Any
of the options is improved by exposure to sunlight, even through a
window, to help offer balance, dawn and dusk, etc. (Although remember- useful UVB does penetrate glass!) Make sure that the
habitat does not overheat because of the sunlight.