How much exposure to UVB do tortoises need? Two common ways to measure it are the length of time and the intensity of UVB.
1.) Length of time per day
Traditionally, reptiles' UVB exposure was measured in time, but published recommendations varied greatly from a few minutes to hours daily. For example, Dr Hartmut Wilke, a German biologist and zoo director (Schildkröten, 2009 / Turtles, 2010), suggests an exposure of 10-20 minutes. He feels that too much UV is pointless and only breaks down the initial level of vitamin D3. His recommendation is to shine the UV light for about 20 minutes during the morning activity period and another 10 minutes during the afternoon activity period.
On the other hand, Andy Highfield of Tortoise Trust (Promoting Proper Bone Development, 2003), suggests that spending 3-4 hours outside daily is enough UVB exposure to produce adequate levels of vitamin D3 in tortoises if you live in an area that has native chelonians.
Read also the Tortoise Trust (TT) forum thread titled "How much UVB exposure" [offsite] from August 2011. That thread includes posts from Frances Baines, M.A., VetMB, MRCVS, of Reptile UV Guide UK. Frances Baines is a retired veterinary surgeon and well-known for her research on reptile UV lighting.
In that same TT thread, Andy Highfield writes about tortoises: "In captivity, you are really looking for an acceptable, functional, 'average' figure, which going by all the evidence I have so far, is likely to be in the 150-180 uW/sq cm range for 30-40 minutes a day at basking temperatures in the 30-34C [86-93.2F] range. Basking temperatures are critical, because temperature is part of the process in converting UVB to usable vitamin D3 by activation of sterols in the skin."
2.) UVB intensity per Ferguson zones
The newest way to measure the needed UVB output for reptiles when housed indoors is to use a UVI meter and then apply the Ferguson zone for your species. Most tortoise species fall in the zone 3. Scroll down for info.
For zone 3 open sun baskers, like star tortoises, use the "sunbeam method" with the maximum UVI reading of 2.9-7.4 in the basking zone. The highest UVB should be only under the basking bulb and must then fade to nothing over a large gradient. Always provide shady spots in the enclosure as a retreat.
Reptiles magazine's Jan 2017 article by Dr Baines has a good overview of Ferguson zones and reptile UV lights. See links at the bottom.
Solarmeter 6.5 for measuring the UV index (UVI). This is the currently recommended meter for tortoise keepers. There are four versions of this meter. All are the same meter, just labeled differently. Solarmeter 6.5 = Solarmeter 6.5R = Zoo Med Digital UV Index Radiometer (ST-7) = Lugarti Reptile UV Index Meter. Pictured is the original 6.5 that I bought many years ago.
UVB reading on a sunny spring morning at 8 am DST with Solarmeter 6.2 (UVB). Mine is many years old because this meter was popular among reptile keepers in the past. Its use has now been replaced with the Solarmeter 6.5 shown above.
UVB is measured with Solarmeter 6.2 (6.2R) and UV index (UVI) is measured with Solarmeter 6.5 (6.5R).
Currently, the Solarmeter 6.5 UV index (UVI) meter is considered to be more useful for measuring reptile UVB lights than the previously used Solarmeter 6.2 UV meter. Solarmeter 6.2 reads the whole UV spectrum, while Solarmeter 6.5 is sensitive to the specific part of the UVB spectrum that helps create vitamin D3. Both meters are manufactured by Solar Light Company Inc.
Solarmeter 6.5 has several benefits. I can be used to check if a reptile UVB bulb is safe to use and to figure out the correct hanging height. Unlike Solarmeter 6.2, the 6.5 model can also be used to compare the output between different types and brands of UV bulbs, as well as between UV bulbs and the sun.
If you want to buy only one meter, choose the 6.5 model (UVI) or the reptile labeled 6.5R version of it.
Reptile keepers have used the Solarmeter 6.2 UVB meter for years to track the decrease of output from UV bulbs over time. In other words, it alerts you to change the bulb when the UVB output has fallen off too much.
Zoo Med's Digital Ultraviolet Radiometer (ST-6) is the same meter as Solar Light Company's Solarmeter 6.2, just with a different cover coloring and Zoo Med's label on it. In short, ZM ST-6 meter = Solarmeter 6.2.
The newest version, Solarmeter 6.2R (R = reptile), is also the same meter as the 6.2, but it has a reptile label on the cover. In short, Solarmeter 6.2 = 6.2R.
When measuring the UVB emission of an indoor reptile bulb with the Solarmeter 6.2 (6.2R), it's common to hold the meter 12" away from the bulb. This is probably the most common reading distance among hobbyist because it allows for data comparison (identical bulbs only w. 6.2). However, to measure the amount of UVB your tortoise is actually exposed to, you'll need to hold the UV meter sensor at the level of your tortoise's head or limbs where the skin is exposed.
Solarmeter 6.5 UV index (UVI) meter is utilized by tortoise keepers to measure the UVI of reptile UVB bulbs. This meter is now the standard for measuring reptile UV bulb output.
Zoo Med's Digital UV Index Radiometer (ST-7) is the Zoo Med labeled version of Solar Light Company's Solarmeter 6.5. In short, ZM ST-7 meter = Solarmeter 6.5.
The newest version, Solarmeter 6.5R (R = reptile), is also the same meter as Solarmeter 6.5, but it has the reptile specific Ferguson zones chart on the cover. In short, 6.5 = 6.5R.
Update 2020: Lugarti now also sells Solar Light Company's Solarmeter 6.5R under their label as the Lugarti Reptile UV Index Meter. It has the Ferguson zone chart on the front as well.
Ferguson zones of 1-4 allow you to check if the UV output of your indoor UVB bulb is appropriate for your species. Most tortoises are listed in the zone 3 with a maximum UVI 2.9-7.4 in the basking area. Scroll down for info.
Indoor UV lighting for tortoises should provide a UVB gradient (just like we provide a temperature gradient) from highest recommended UVB level down to zone 1 level. This allows tortoises to regulate their UVB exposure. Always provide a shaded area in addition to the UV zone. Blasting reptiles with high levels of UV all day long with no hiding places can cause eye problems, skin damage, and other illnesses.
UV index (UVI) rating with numbers from low 1 to extreme 11+ is used with humans. However, for reptiles, we now have the Ferguson zones! These recommendations are based on UV readings taken of basking reptiles in the wild by Dr Gary Ferguson and his team.
Ferguson system consists of four zones, from 1 to 4. The UVI zones 2 and 3 are of special interest to turtle and tortoise keepers and can be used as a guideline to set up indoor UV lighting. Use the Solarmeter 6.5 (6.5R) to measure the UVI of your reptile UVB bulbs and adjust the bulb height up or down to get the correct reading for your species.
The four Ferguson zones with the average and maximum recommended UVI exposures:
Related page: UVB readings