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Old Mar 3rd 2019, 05:39 AM   #1
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Fresnel lens properties

Hi,

Can anyone tell me the transmission properties of a plastic fresnel lens, that I got from a microfiche viewer?

I'm particularly interested in: Will it pass UV light?

If so, when focused to half, will it be double strength, so double heat, light and UV?

Camerart.
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Old Mar 4th 2019, 02:06 AM   #2
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First .... what is the lens made of ??? glass or acrylic ...

My guess is glass , different glasses have different transmition properties

Suppose you have a 10 cm x 10 cm lens and you focus the sun to a 1 cm x 1 cm size ... the area has been reduces by 100 times so you will get 100 times the light intensity ...minus loses from scattering from the lens , and absorption by the lens... perhaps 15%

Last edited by oz93666; Mar 4th 2019 at 02:15 AM.
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Old Mar 4th 2019, 06:00 AM   #3
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Hi O,

The lens is made of Plastic.

C
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Old Mar 4th 2019, 06:17 PM   #4
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Originally Posted by Camerart View Post
Hi O,

The lens is made of Plastic.

C
That would most likely be acrylic , also called perspex ...

Put "transmission wavelengths of acrylic " in google search and you get charts like this ....



A search also shows ..."Unlike glass, pure acrylic (PMMA) does not filter ultraviolet (UV) light. Acrylic can transmit UV light down to 300 nm. Standard acrylic, like e.g. Plexiglas, includes an additive to make the acrylic absorb UV light"

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Last edited by oz93666; Mar 4th 2019 at 06:26 PM.
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Old Mar 5th 2019, 03:01 AM   #5
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Hi O,

Excellent, thanks for your research.

Just to let you know why I ask. I have a tortoise that needs the warm seasons extending, which I do now by keeping her indoors, with a 150W reflector lamp heating her, till she's over 30Deg. I wanted to make a frame with the fesnel lens, outside, where she can get double-ish the light, heat and UV, from the low sun at the moment. (I'll be careful about bright days)

Cheers, C.
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Old Mar 5th 2019, 09:07 PM   #6
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Originally Posted by Camerart View Post
Hi O,

Excellent, thanks for your research.

Just to let you know why I ask. I have a tortoise that needs the warm seasons extending, which I do now by keeping her indoors, with a 150W reflector lamp heating her, till she's over 30Deg. I wanted to make a frame with the fesnel lens, outside, where she can get double-ish the light, heat and UV, from the low sun at the moment. (I'll be careful about bright days)
Cheers, C.
Ahh I see .... I would suggest a lens will not help .... A lens focuses heat into one small area , it does not increase the amount of energy entering the system .. It will get no hotter with a lens than with a glass roof , and anyway it's probably at night that the temperature drops...

If I were doing this I would just make a very well insulated den for the creature , expanded polystyrene , straw , or similar then his own body heat will not escape , and there's no danger of over cooking , as in sunlight ...

He has evolved to live outside , so just a little help is all that should be needed.
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