Page 1 of 1

Why UVI is used, and not UV Flux

PostPosted: Wed Jul 24, 2019 7:31 am
by Claudiusx
I thought I would take the time to discuss to anyone curious as to why you would want to be measuring UVI (UV Index) as opposed to UV flux (mW uv cm2).

If you don't care to read this all, and just want the meat and potatoes, click here: Short and sweet version

Firstly, what is UVI?

UVI is simply a standard measurement of sunburn and D3 producing Ultraviolet light.
But doesn't all UV cause sunburn and D3 synthesis? The answer is no. Not even all wavelengths of UVB light do that. This is why it's important to focus on UVI, and not uv flux in our situation.

The wavelengths that are largely contributed to sunburn and d3 synthesis are mainly in the shorter wavelengths. When you are measuring UV flux, you are measuring a wide range of wavelengths, including those outside of the range of concern for us. This can lead to issues of assuming you have enough UV, but not, or visa versa, assuming you have enough, but providing dangerous amounts!

Luckily for us, we have the Solarmeters. The solarmeter 6.5 measures UVI directly, the best choice if you had to chose. The Solarmeter 6.2 measures Uv Flux.

The problem with the Solarmeter 6.2, is that it doesn't differentiate between the short wavelengths and the longer wavelengths. It is only the short wavelengths that largely contribute to D3 synthesis, and the sunburning effect. In essence, you can have a uv flux reading of over 400, and still not be providing the rays your dragon needs for proper D3 production. Additionally, you can have a UV flux of only 50, and be providing way too much! Here is an example from Dr. Baines on this exact scenario:

It was back in 2006-7 when we first noticed something weird... that although morning sunlight in Australia would easily give readings of 400 uW/cm2 and above, and never harmed anything, certain lamps gave readings of only 50 uW/cm2 and caused horrific photo-kerato-conjunctivitis, even skin "burns", shock and death.
Until we realised that if you measured the UV Index from these lamps, you were getting ratios of 10:1! i.e., 50 uW/cm2 was UVI 5! and when people were setting up the lamps to match their Solarmeter 6.2 "sun" readings of 300 - 400 uW/cm2, they were putting UVI 30 - 40 over their reptiles!
Then we got spectrometer analysis of these lamps, and found out what was happening. The lamps were emitting enormous amounts of really short-wavelength UVB, almost UVC... and very little long-wavelength UVB. The shorter the wavelength, the fiercer the rays...only very small amounts of borderline UVC will have the same effect as a very large amount of long-wavelength UVB.
Now, the UV Index meter only "sees" short-wavelength UVB, because those are the "sunburning" wavelengths, also the wavelengths that make vitamin D3. So it gives realistic readings for D3 synthesis, and if readings are high, then also, warns against burns and eye damage.

So in conclusion, if you don't have a solarmeter 6.5, (which is totally not a necessity mind you) then your best bet is to follow manufacture instructions for your bulb. Or, find a reliable site online such as which is Dr Baines site which gives you the UVI levels for your bulb at certain distances. If you are using Arcadia bulbs (which is highly recommended) you can find this information on their site here:


Short and Simple version

PostPosted: Wed Jul 24, 2019 7:32 am
by Claudiusx
D3 synthesis largely occurs only in the shorter wavelengths of the uv spectrum. UV flux is a measurement that doesn't differentiate between short or long wavelengths. This is why UVI is what is used by manufactures and leading experts in the herp field. UVI is a measurment of only the wavelengths that contribute to sunburning and D3 synthesis.


Re: Why UVI is used, and not UV Flux

PostPosted: Tue Jul 30, 2019 3:03 am
by CooperDragon
Moved by moderator from enclosures to advanced lighting/enclosures.