Natural lighting & the UV Index

[Kirkie]

I received the following via a Yahoo group. Its by Frances Baines of UV Guide and was in response to a query about Redfoot lighting. I think it shows how the thinking is moving away from the provision of doses of shortwave UVB (your UW/CM2) to an idea that if you replicate full spectrum sunlight the reptile will be ok, vitamin D3 wise.

It may also put Alans mind to rest about his dodgy Solarglo's.

Throw away your Solarmeter 6.2, get a 6.5.

As you know, your red-footed tortoises are a forest species, so they would probably not expose themselves to midday sun; but bask in morning shafts of sunlight coming through the trees. So you're absolutely right, 200 uW/cm2 might be fine if it was sunlight that you were offering them. 200uW/cm2 of sunlight (measured on a 6.2 Solarmeter)is equivalent to about UVI 3.5 (measured on a Solarmeter 6.5)- that's about what you'd get in full sunlight in the tropics at about 8.30am.

However, mercury vapour lamps have totally different spectra, which usually have a higher proportion of UVB in the shorter, more photoreactive wavelengths (more "powerful" in terms of their effect on skin).

So the reading on a Solarmeter 6.2 (which measures the whole of the UVB region, not just the shorter wavelengths)from sunlight can't be compared to the reading from a mercury vapour lamp. It's a bit more complicated. I use a combination of a 6.2 and a 6.5 meter and work out a formula for converting the readings to UV Index units.
What brand of mercury vapour lamp do you have? It makes a great deal of difference.... If it's a fairly well known brand that I've measured recently, I can give you a rough idea of what the Solarmeter reading would be, to make the "strength" of the UVB roughly the same...*
To get about UVI 3.5 with one of the new MegaRay Solar Raptor 100-watt bulbs, you'd want to aim for readings of about 140 uW/cm2. 200uW/cm2 from one I measured last week would give a UV Index of 4.9. That's fine, but it's more like full tropical sun a bit later in the morning, maybe around 9.30am.

If you were using a ZooMed Powersun 100W, however, you'd want to aim for readings of only about 40 uW/cm2. These have a far higher proportion of their output in the shorter wavelengths. 200uW/cm2 from a new 100W Powersun I measured last August would give a UV Index reading of UVI 18.5. That's higher than the strongest tropical sun at sea level on the equator.

[Alan1]

Hmm, so a nice tan in 5 minutes from a Powersun

[Kirkie]

I'm using a Repti Glo 10% UVB tube with my hatchlings. Its a 2 foot in a four foot table so it has a nice tapering UVB effect. It reads 90 UW/CM2 and a UV Index of 4.5. I like the thought that they are wandering through lower level UVB all day whilst getting "full tropical sun" light levels. I'm gonna add a reptisun 2.0 when the controller turns up.

[Alan1]

No matter what length the waves are, short or long or somewhere in between, the huge differences between the same make and strength of bulb is not good enough really. I wonder why there can be such a massive difference. Two Megarays I had, one at about 130uW/CM2 and the other at only 70. Solarglos, my one at 50uW/CM2 and Sarah's at 140. My two Arcadias, one at 182uW/CM2 and the other at 143. I don't know what it is about the 'hand made' process that can give such variances. Someone not doing their job properly maybe?

[Kirkie]

I'm starting to umderstand the varience between manufacturers with each "knowing" the correct amounts of UVB exposure required. Within a single range of bulbs I keep reading reference to a layer of coating that controls the amount of UVB leaving the bulb. This must be a very thin layer to allow light through and so microscopic differences in the depth of the layer of coating are going to affect the output.*If its done "by hand" even more so.

[Alan1]

Some bulbs come with "optimum UVB levels" written on their box. With the Arcadias it states on the box that the data is provided by uvguide.co.uk and reproduced with permission and is as follows..

Typical

Luminous Flux (lm) 1900
UV-A intensity @ 30cm 1000uW/cm2
UV-B intensity @ 30cm 175uW/cm2
Colour Temperature 4200
Colour Rendering 50

with 175 being typical (but not guaranteed of course) I reckon they would replace a bulb that was producing well below that, for example 100uW/cm2. As I say my 2 were 182uW/CM2 and 143/uW/CM2.

These are the best bulbs I've had to date for light, not too much heat, not blowing and for slow deterioration of UVB levels, plus frances baines said she was very impressed with them during testing, that was before they were on the market. I only hope they don't start trying to improve that particular bulb and end up with the Megaray problems.

Everyone wants something better and no doubt that will come but in the meantime I'll let others do the testing out