I found this in a UV Meter Group...as per Fran Baines:
"I'll try to answer as best I can.
All measurements need to be from the lamp surface to the lizard's skin. If the distance was measured from the substrate, then (as you point out), if a lamp had to be placed 30cm away, a large lizard could touch the lamp with his head. Clearly this would not be safe and his head would get far too much UVB.
To explain why the distances I've mentioned in different posts seem to be "all over the place", and why the T5 lamps can be so much further away than the T8 lamps, I need to fill in some background info.
Working with the UK zoos association BIAZA, I am developing a UV guide for zoos which simplifies UVB requirements for all species into 4 categories or "zones", and suggests suitable UV gradients within these zones.
Unfortunately none of the UK zoos involved in this project keep green iguanas, so I have not been sent any recommendations for these, yet. However, I am about to write to someone who has many years of expertise with this species to contribute some data.
No-one has yet sent any recommendations for tegus, either red or black-and-white, so I'm a bit in the dark about their basking needs, too.
In the meantime I am going to assume that green iguanas, at least, have lighting requirements somewhat similar to a Panther Chameleon - in other words, an arboreal, sub-tropical/tropical rainforest dweller which basks in full morning sunlight then moves into shade for the middle of the day.
This species has been classified as Zone 3 - an "open or partial sun-basker which actively thermoregulates".
I think tegus live in slightly more open, savanna-type habitat so I will guess that they are also Zone 3, but possibly experience even more sun during a typical day.
Basking species can be offered UVB gradients in 2 ways; the "shade method", which is typically the use of long T8 or T12 UVB tubes stretching right across an enclosure - in which a low level of background UVB (like UVB in the shade on a sunny day) is provided across a large area; or the "sunbeam" method in which much higher UVB - like direct mid-morning sunlight - is provided, but restricted almost entirely to the basking area.
Therefore, we are suggesting (based on limited data for similar species, with UVB levels measured out in the wild) the following options for Zone 3 species:
Shade Method: UV Index range 1.0 - 2.6 in a large area of their enclosure, so they experience this for a large part of their day.
OR
Sunbeam Method: UV Index range 2.9 - 7.4 in just the basking area, with a gradient, likely down to zero, as they move away from the basking zone. So they experience higher UVB, but for a shorter time, each day.
The ranges are very vague. This is not a mistake. We simply have no data for what levels are optimum; or whether in terms of health and vitamin D production, a full day at lower UVB levels is as good as shorter periods of higher UVB. What we do know, however, is that species vary in the maximum levels which they tolerate, in the wild.
The ranges are considerably lower for "occasional baskers" like Chinese Water Dragons ("zone 2")and much, much lower for crepuscular and shade dwelling, non-basking species like leopard geckos("zone 1"). And higher for "mid-day baskers" like uromastyx ("Zone 4").
So what is really important is the GRADIENT - the reptile must be able to choose to move in and out of the UVB - and the MAXIMUM, which we have chosen as 7.4 for zone 3 species since that is the highest UV index in which a "open sun basker" was recorded exposing itself to, in full sunlight in the wild, in the study we used.
As a rough guide, the range UVI 3 - 7 is typical of full tropical sunlight from about 8:30am (UVI 3) to 10:00am (UVI 7)- which seems appropriate from anecdotal observations of the times reptiles are seen out and in sunlight, and measurements I've made myself, too.
The traditional T8 tubes, even the best ZooMed ReptiSun 10.0 or Arcadia D3+ WITH a reflector fitted, are hard pressed to create UVI 2 at greater distances than about 25 - 30cm above the lizard's back. This is why they are really only suitable for the "Shade Method" and even so, must be very close to the lizard's back to provide that level.
But many generations of reptiles have lived good lives under T8 lamps with no reflectors, hung a lot further than 25-30cm away. So I'm not criticizing....
My point is, I suspect that natural sunlight as experienced by wild reptiles always provides a lot more than the bare minimum reptiles need, and if we imagine a reptile's vitamin D levels as fuel in a tank, in the wild it is almost always a full tank.
I also suspect that reptiles can run very well on half-full tanks for a long time, and may be perfectly healthy, although more susceptible to illness. And that this is probably the condition of many reptiles in captivity today (and humans too); perfectly healthy but more susceptible to illness than their outdoor wild cousins.
If the "sunbeam" method is employed, there should be the opportunity for the much higher UVB to "fill up the tank" while the reptile basks, instead of the "trickle-feed" all day, with the "shade" method. How well this works has, sadly, never been satisfactorily tested to my knowledge.
But to do this, until the ZooMed and Arcadia T5s came out, you needed a good quality UVB mercury vapour lamp or UVB metal halide. Mercury vapour lamps such as the original MegaRay have been extremely successful used in this way, as BobMac says. The disadvantages are that the beam can be very narrow - so only a small part of a large lizard receives the "sun" level of UV - and the visible light is of poor quality, owing to a very spiky "mercury" spectrum with few colours in it.
When the T5 tubes came out, and I first tested one in an aluminium reflector, I was shocked by the amount of UVB (and visible light) this was producing. The UVB was easily as much as a good mercury vapour lamp! I am concerned when I see people replacing T8s with T5s at the same distance, and far more so if they fit the T5 with a reflector. We are so used to "tubes" being "weak" that we just don't get it....
The spectra of the T5 HO tubes from ZooMed and Arcadia are NO DIFFERENT to their T8 tubes. All that is different is the output. There is a lot more UVB; but it's all in the same proportions that have proven safe and reasonably sun-like for years. (I am only speaking of those two brands. Others may vary. Caveat emptor.) I will address BobMac's concerns regarding spectra in another post.
But what either a ZooMed ReptiSun 10.0 or Arcadia D3+ 12% T5, fitted with a reflector, can do, is provide either the "shade method" at a greater distance above the reptile - and I'm talking about approximately 50 - 60 cm above the reptile's back for about UVI 2.0, - or they can provide the "sunbeam method" at closer distances, for example, at 40cm, one lamp I tested in one reflector gave me UVI 3.5 at 40cm and UVI 7.0 at 25cm.
(These distances are very approximate. Every set-up will give slightly different results. Just compare the same tube in 4 different reflectors, in the report in the Files!... and individual tubes vary, too..)
So if I had a species like a bearded dragon, near the top of zone 3 in terms of sun-loving, I'd set up the T5 for the "sunbeam" method; I'd make sure he couldn't get nearer to that lamp than about 25cm if he really stood up tall; and had a gradient from that maximum downwards, across his basking zone - the tube would be as close as possible to the basking lamp without risking damaging the fixture from the heat.
If I was setting up a large enclosure and I wanted the T5 tubes screwed firmly to the ceiling, at some distance from the reptile, then I would use the T5s in the "shade method" and seek to cover as large a part of the enclosure as possible with a UV Index of between 1 and 2.5. If the lizard could occasionally stretch up and get a bit closer then I would not be concerned, unless he got closer than 25cm; because I know that the UV Index of 7.0 is still perfectly safe for a Zone 3 species; it's as if he got into a sunbeam. I just haven't chosen to offer his UVB mainly in that way.
But if the T5s screwed to the ceiling would be within the basking zone, and the lizard would only be within range when basking, well... he isn't going to be there for very long, each day. So "shade method" won't work. You'd need "sunbeam method", and that requires stronger UVB.... but the tube on the ceiling may be too far away to get the level you need. One solution would be to add a second tube. That's what I was trying to suggest in the post you mentioned. A diagram would help, but not easy to create or add to a list like this.
What I actually said in that post was:
"If Cyan only spends a short amount of time in the top basking zone, i.e., only when he's actually basking, you could consider buying a double fixture, i.e., twin tubes mounted close together, to double the UV to about UVI 3 - 4 at 40-50cm. That would be like "morning sunlight" for basking in. But he wouldn't want that much, all day long."
When I actually looked at the results from the tubes I realised I misjudged the effect. I corrected this in my next post; I put "you'd expect a UVI of 4 - 5 in the zone about 50cm beneath the two tubes."
Does any of this make sense?
You know, I reckon we all way over-think this UV thing; we try to make the measurements far too precise. If a lizard stands in a morning sunbeam, maybe his back gets UVI 6. If he moves three inches to the side, a branch provides a little dappled shade and the UVI is maybe only 4. If he turns round, that part of his back comes right out of the sun into his own shadow and maybe gets only UVI 1. All of this happens in a few seconds. Then a cloud goes across the sun and the UV drops all around, to 1.5.....
Likewise there is a UV gradient beneath a reptile lamp. If a lizard stands in the middle of the beam, maybe his back gets UVI 6. If he moves three inches to the side.... (you get my drift?)
As long as the spectrum is safe, there's an adequate gradient, the lizard can't get too close and can move into shade, then if the temperatures are right (so he doesn't have to hog the basking zone (too cool) or avoid it (too hot)) then that lizard should be able to regulate his UVB exposure to suit his needs.
You've got a very lucky iguana and tegu, that their owner goes to so much trouble to care for them properly!
Good luck with your new build!
To address a couple of your other points:
> 4) Do the longer, higher wattage lights give off more UVB over greater distances? Or are they just longer?
In general, the longer tubes do give higher readings than the shorter ones, I have to say this is my experience comparing T8 18watt 24" tubes with T8 25watt 30" tubes. I think most of the difference is probably due to the meter sensor "seeing" more tube, since the thickness of the phosphor and glass seems the same, so you'd expect the output per inch to be the same. I haven't compared different lengths of T5s as I have not been donated any except for 24watt, 22-inch ones.
Maybe someone who's tested T5s of different lengths, could post some readings? Todd Goode?
>while the iguana basks pretty much all day.
Is his ambient (air) temperature warm enough? I don't think they bask all day in the wild...
> 5) I plan on putting an extra white light for visibility. I was planning on buying the "double" tube controllers from Arcadia as well, but they are HO t5 controllers and thus the white light tube would also be high wattage HO type, such as the 6500K Zoo med "Ultra sun". I am worried that this may be FAR too bright?
Not at all. Even the brightest daylight T5-HO tubes are dismally gloomy compared to real sunlight.
I tested a nice daylight hydroponics 6500K T5-HO recently. As close as 30cm with reflector fitted, I got 5,100 lux; at 50cm I only got 2,270 lux. I haven't tested a T5 ZooMed UltraSun but I expect it will be similar.
Compared to sunlight (around 100,000 lux) that's not very bright at all. But indoors I think the combination will look very nice, both you and the lizards will probably like it!
Best wishes,
Frances"
If I read this right Jimmys back closet point should be a little over 9 inches (just like I have him now)...so if I want to put the light fixture up higher his highest point needs to be raised as well...which I can do with either putting something under his rock/hide (like extra tile) or getting him a taller one. I am thinking raising his original would be best so not to stress him out...
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