B Infrared Cameras -- Do they use a Lens to Focus?

[LightningInAJar]

TL;DR Summary

Do IR cameras need a lens?

Can infrared cameras focus IR radiation as a visible light camera focuses visible light? I think IR doesn't pass through glass well? What are these germanium lens about? I don't think my Flir IR camera has a lens.

 

[Baluncore]

https://en.wikipedia.org/wiki/Germanium#Optics
https://en.wikipedia.org/wiki/Zinc_sulfide#Optical_material
https://en.wikipedia.org/wiki/Infrared_photography#Focusing_infrared

 

[Melbourne Guy]

@LightningInAJar, your question brought back a FLIR project and the fun of minimum resolution for crack detection, but Teledyne FLIR has this 'selecting a lens' primer that notes the physical configuration for the IR aspect which may help with your enquiry.

 

[Vanadium 50]

https://en.wikipedia.org/wiki/Germanium#Optics
https://en.wikipedia.org/wiki/Zinc_sulfide#Optical_material
https://en.wikipedia.org/wiki/Infrared_photography#Focusing_infrared

It's even simpler than that. lnPut "Infrared camera" into Wikipedia , search for "lens" and out pops the answer.

Why the OP didn't bother, I don't know. I guess it's easier to have us do the work for him.

 

[LightningInAJar]

I don't understand all that stuff. Can IR be focused? Or are its properties too different?

 

[berkeman]

I don't understand all that stuff.

All what stuff? Can you please make it a habit to quote the post that you are commenting on? And indicate that you have actually read any links suggested to you by saying what you do and do not understand in each? Lordy.

 

[Baluncore]

IR can be focussed, just like other light, but a lens for IR imaging must transmit IR light. Most materials used for visible light block the far IR. That is why the lens for IR must be made from specially selected materials.

Since the refractive index, and so the focal length of a lens, is a function of wavelength, the focal point for IR will be in a slightly different place to that for visible light.

 

[russ_watters]

I don't understand all that stuff. Can IR be focused? Or are its properties too different?

Ir is light. What's the problem?

My Seek Thermal camera has a lens and manual focus.

 

[sophiecentaur]

I don't understand all that stuff. Can IR be focused? Or are its properties too different?

It all depends on which wavelength of IR you are interested in. In the 'old days' of film cameras, many of them had an alternative focus / distance setting so that you could use IR film 'blind'. So, in principle, it's fine - especially when you realize that many solid state sensors are so sensitive to IR that they have an IR filter stuck on the front of them. A bit more loss in the lens shouldn't affect results too much.
Saying you "don't understand all that stuff" is a shame. Most PF members use the forum to increase their understanding. If you want to do a successful technical project then. you may need to take on a significant amount of new stuff. It's quite rare to 'just get things to work' without some knowledge and you don't need to know the whole lot.

 

[LightningInAJar]

IR can be focussed, just like other light, but a lens for IR imaging must transmit IR light. Most materials used for visible light block the far IR. That is why the lens for IR must be made from specially selected materials.

Since the refractive index, and so the focal length of a lens, is a function of wavelength, the focal point for IR will be in a slightly different place to that for visible light.

Thank you. This is what I wanted to understand. Wasn't sure if IR had different properties from visible light or if any material could focus as optical glass can, but outside the visible spectrum.

 

[sophiecentaur]

if any material could focus as optical glass can, but outside the visible spectrum.

If you want a good focussing action over a wide bandwidth then you could do worse than using a paraboloid reflector with a metallic surface. The biggest optical and IR telescopes all use this method and so do radio telescopes, of course. Despite passing through most materials, X Rays can also be focussed with a metallic reflector - the only difference is that reflection only works at very oblique angles. An X Ray scope works this way (a highly 'offset' reflector)

A massive advantage of a reflector is that the geometry works exactly the same way for all wavelengths - no chromatic aberration to bother about.