A note about UV fall-off

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Juvie Member
Hi all,

I have been following the forum for quite some time and was under the impression that I had things pretty well sorted in terms of light, heat, and UV. My smaller dragon is in a 50 gal tank with a Reptisun 10.0 tube mounted under the screen with no plastic cover and a reflector. My big guy has an Arcadia 12% desert tube, again with a reflector and nothing obstructing the bulb. Sounds good, but...

Using points from a credit card, I purchased a Solarmeter 6.5R because my bigger dragon has been "off" since brumating, and I was worried that maybe something still wasn't right with his husbandry. Turns out, this was a good move, because I hadn't taken into account the weakening (attenuation) of UV with distance. I know from the forum about the bulbs going bad over time & have been diligent that way, but this was a shock.

I have a bit of a science background and know that radiation like UV light drops off as the square of the distance, but we're talking inches here, so I was shocked to see how much loss I was getting. It turns out that even with a fancy bulb, I had it mounted too high, and he was getting some UV, but not the recommended dose/day. With distances corrected, I'm hoping his appetite returns, though I also had a fecal and exam done at the vet to rule out other causes & am waiting for results.

I guess my point here is that you can have all the right gear and still manage to use it improperly, and that even a few inches can make a significant difference in output at the target. The UV meter is expensive, but if you are having issues with your dragon's health and can rule out other things, it might be worth buying or borrowing a meter to 2x check your setup, along with checking out the manufacturer specs of the bulb you're using.


BD.org Sicko
Staff member
You're correct. That's why when we recommend bulbs we usually also recommend the distance they should be from the basking surface. They are nothing like the sun at all, and their uv output is greatly affected by distance. If you look at uvguideuk you'll see many charts where they have tested different bulbs to show a visual representation of the output of different bulbs.



Juvie Member
Original Poster
claudiusx":1r90ys0d said:

Thanks Brandon! That link timed out for me, but I can always poke around on Google for something similar. It just never occurred to me that there'd be such a steep slope drop-off given the distances involved! Not that we need another check-off box on our beardie "things to worry about" list, but hopefully someone else will find this information useful when they're 1st investigating lighting.


BD.org Sicko
Staff member
I'm glad you were able to get a UV Meter. They are a great item to have. As mentioned, we advise general distances based on test results from similar bulbs but if you can test the output of the bulb directly, that's ideal. The output does tend to drop off quickly as you move away from or to the sides of the bulb so it's good to test your setup out and make sure you're offering plenty of options. I like to have the shade areas between 0-.5 UVI with areas around the basking spot (still pretty warm) around 1-3UVI while the center of the basking zone is 3-6 UVI or so. I've had good responses from my dragons when providing those levels. The suggested distances try to replicate those levels as closely as possible given limited information about each bulb.
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