Need help focusing IR using a fresnel lens

  • Thread starter Gianna
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  • #1
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Hello everyone,

I want to focus IR on a small rectangular area using a Fresnel lens. I tried trial and error to find the focal point of the lens but can't seem to be able to get the exact point. The frensnel lens is a multiple fresnel lens. Can anyone please help?

Thanks =)
 

Answers and Replies

  • #2
Redbelly98
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Welcome to PF.

What wavelength (range) of IR?

Try using a visible source to find the focal point. It will not be very different for IR.
 
  • #3
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I tried using a laser pointer to do that. It was about 600nm. But it was too focused to let me see any difference. Can u suggest any other light source?
 
  • #4
Redbelly98
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You can take it outside and use the sun. Or inside, if the sun is visible through a window.

Less accurate, but maybe reasonably accurate, is to use a ceiling lamp or other lamp, at a distance at least 10 focal lengths away from the lens.

1/f = 1/(focusing distance) + 1/(lamp distance)

Hope that helps.
 
  • #5
Redbelly98
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Note:
BE VERY CAREFUL if using the sun as a light source. Only focus the sun onto non-flamable surfaces!
 

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