Need help focusing IR using a fresnel lens

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    Fresnel Ir Lens
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Discussion Overview

The discussion revolves around the challenge of focusing infrared (IR) light onto a small rectangular area using a Fresnel lens. Participants share their experiences and suggestions regarding methods to accurately find the focal point of the lens.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant seeks assistance in focusing IR light with a multiple Fresnel lens and expresses difficulty in locating the exact focal point.
  • Another participant inquires about the specific wavelength range of the IR light being used.
  • A suggestion is made to use a visible light source to locate the focal point, with the assumption that it will be similar for IR light.
  • A participant mentions using a laser pointer at 600nm but finds it too focused to discern differences and asks for alternative light sources.
  • Another participant recommends using sunlight or a ceiling lamp as potential light sources, providing a formula related to focusing distance.
  • A cautionary note is added regarding the use of sunlight, emphasizing safety when focusing it onto surfaces.

Areas of Agreement / Disagreement

Participants generally agree on the need for a visible light source to find the focal point, but there are varying opinions on the best methods and sources to use. The discussion remains unresolved regarding the optimal approach for focusing IR light specifically.

Contextual Notes

Participants have not established specific parameters for the IR wavelength or the characteristics of the Fresnel lens, which may affect the accuracy of their suggestions. There are also unresolved aspects regarding the effectiveness of different light sources in this context.

Gianna
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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 =)
 
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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.
 
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?
 
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.
 
Note:
BE VERY CAREFUL if using the sun as a light source. Only focus the sun onto non-flamable surfaces!
 

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