Fresnel lens properties

Jan 2019
6
0
Uk
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.
 
Apr 2017
528
130
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:
Apr 2017
528
130
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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Jan 2019
6
0
Uk
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.
 
Apr 2017
528
130
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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Jan 2019
6
0
Uk
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.
Hi O,

I'm assuming that if the lens is focused slightly smaller than it's full size, then the focusedl area will be slightly warmer, is this correct?

At night she goes into an insulated box, with a frost protection underfloor heater.

These tortoises came from a warmer climate, and can be in danger if the warm seasons aren't extended. She has just come ut of hibernation, and didn't eat for a week, until I added a 150W reflector bulb, where she 'sunbathes' for a coule of hours till she's over 30DEg, then she eats.

I was hoping to use my fresnel lens instead of the bulb. Once they are hot, they move away from the heat.

C.