# How does the UV get through bulb glass?



## DoctorCosmonaut (Feb 5, 2011)

So how does UVB get through the glass surrounding a UVB bulb if it can't easily/at all get through windows, etc?


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## PeanutbuttER (Feb 5, 2011)

Wow, I honestly have no idea. I have never even considered that. Maybe the glass on a bulb is more pure or thinner? I know that they specially coat the bulbs to male them produce useable uv.


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## Tom (Feb 5, 2011)

I'm not sure about your question, but here are two related points:
1. Ever get a sunburn through the window on a long car trip? I have. How can you get a sunburn if the glass filters out all the UV?
2. How come the UV can't get to your tortoise if the bulb is more than 12 or 18" from your tortoise? UV light travels millions of miles from the sun, penetrates all the layers of the atmosphere, all the dust, smoke, smog, fog, etc... and is still powerful enough to make D3 in our tortoises skin. Its still so strong after that that it bounces into the shade. A tortoise in the shade outside still gets enough UV to get the job done. So how come the UV generated by a bulb can't travel a few extra inches? Does it just stop and hover there at 18"?


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## DoctorCosmonaut (Feb 5, 2011)

Tom said:


> I'm not sure about your question, but here are two related points:
> 1. Ever get a sunburn through the window on a long car trip? I have. How can you get a sunburn if the glass filters out all the UV?
> 2. How come the UV can't get to your tortoise if the bulb is more than 12 or 18" from your tortoise? UV light travels millions of miles from the sun, penetrates all the layers of the atmosphere, all the dust, smoke, smog, fog, etc... and is still powerful enough to make D3 in our tortoises skin. Its still so strong after that that it bounces into the shade. A tortoise in the shade outside still gets enough UV to get the job done. So how come the UV generated by a bulb can't travel a few extra inches? Does it just stop and hover there at 18"?



Then why not put your tort in a greenhouse or near the window and never worry about UVB? When I've read about glass greenhouses UVA goes through glass, but not UVB (or not measurable amounts?).


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## Balboa (Feb 5, 2011)

Glass doesn't block ALL the UV, just something like 99% or whatever, (I forget off the top of my head)

Supposedly, they use special glass in UV lamps that transmits more uv (I know that's the case for blacklights) hence the higher cost, however I know no way to confirm that they are actually using that glass.

In the case of a standard fluorescent most of the light energy is initially created as UV and is converted by the phosphors to visible light. Some UV always gets past the phosphors. I sometimes wonder if they don't just skimp on the phosphors with reptile lamps so more slips past. Certainly cheaper that way.

The suns rays are very directional. Each individual "Ray" is parallel to the next, no dispersion.

A lamps rays are diffuse, spreading out as they leave the lamp.

So the 12 inch requirement comes from that being the point at which the rays have spread out enough where not enough of them hit the reptile at once.

Any of that help guys?


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