Focusing IR with a multiple Fresnel lens

[Gianna]

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 =)

 

[Redbelly98]

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.

 

[Gianna]

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?

 

[Redbelly98]

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.

 

[Redbelly98]

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