Both Argentine and Colombian tegus require 12-14 hours of daily exposure UVB for their health. In captivity, this should be supplemented by opportunities to bask in “real” sunlight as often as weather permits.
UVB is a form of radiation produced by the sun. All animals produce vitamin D3 via exposure to these rays, including humans (although when we bask in them for too long, we get a sunburn because we don’t have scales to protect us). Vitamin D3 is important for metabolizing calcium. When tegus don’t get enough UVB, they can become D3 deficient, which leads to Metabolic Bone Disease. Although ingesting vitamin D3 in food is often enough to stave off MBD, it’s best practice to use appropriately placed, high-quality UVB lighting instead to reduce the risk of overdose or underdose.
Tegus are classified under Ferguson Zone 3, according to The UV Tool publication by Dr. Frances Baines et al. If you have a Solarmeter 6.5 (recommended), the UVI on the basking surface should be between 3.0-4.0. UVI should be highest at the basking spot and lower in other areas of the enclosure so your lizard has options to choose from. If you don’t have a Solarmeter, here’s a fair guide to distancing with each bulb type. The given distances are the distance between the lamp and the tegu’s back while standing on the surface closest to the UVB lamp:
Arcadia T5 HO Desert 12% — 17-21″ / 43-53cm
Zoo Med T5 HO Reptisun 10.0 — 17-21″ / 43-53cm
Arcadia T5 HO Dragon 14% — 18-22″ / 45-55cm
Your tegu’s UVB bulb should be 50% of the enclosure’s length, mounted over the basking area with the heat lamps. Use a fixture with a reflector so you don’t accidentally waste any of the UVB — the above measurements are consistent with the Arcadia ProT5 and Vivarium Electronics fixtures.
The above estimations assume that there is no mesh between the tegu and the UVB lamp. If there is some kind of screen present, see the Facebook group Reptile Lighting > Guides > Guide 1: Using T5-HO lamps above a Mesh Screen for recommendations on adjusting basking distance appropriately.
It takes a lot to light a large enclosure, and a set of heat bulbs plus a UVB bulb isn’t nearly enough to provide the bright light that a diurnal reptile needs for simulating natural sunlight.
The brightest reptile light fixture currently on the market is the Arcadia Jungle Dawn LED Bar. They’re expensive, but the sheer output of bright, beautifully white light makes them worth the investment. I highly recommend installing one or even two in your tegu’s enclosure. Use this to actively light at least half the length of your enclosure.