Posting this because some people seem confused about IR.
I'd just like to point out here that infrared is a really wide chunk of spectrum. It goes from 0.74µm to 300 µm (according to Wikipedia), which is a factor of about 400. By comparison, the visual spectrum has a width of about a factor of 2, and yet our biology still gives us four different sensors crammed within that little sliver. Imagine if you had a rainbow with red, orange, yellow, green, blue, indigo, violet, and 50 more colors which were just as distinct from each other, and you get an appreciation of how big IR is.
Near infrared is the stuff used by artistic IR photographers (film and digital), remote controls, and night vision cameras. Regular CCDs designed for visible light also pick up near IR, so most cameras have a built-in filter which blocks it out. This is what you remove to turn your camera into an IR camera, and it also accounts for why some things just show up wrong in some digital pictures. Maybe you've seen a fireplace with purple flames -- that's because the IR filter isn't getting rid of all the near IR.
Near infrared is really just light that you can't see. Most things don't emit near infrared unless they're very hot (like fire) or designed to do so (like LEDs).
Far infrared is used in thermal imaging. It is a completely different beast, with much longer wavelengths.
1) Thermal infrared is low-resolution. The resolution of an imaging system is limited by the aperture of the lens relative to the wavelength of light that you are using. A proficient photographer with a nice prime lens and a tripod can come close to the resolution limit of visible light with no trouble, but the (much) longer wavelength of far infrared means that the relative size of a lens is much smaller. You simply cannot get a sharp picture of thermal infrared due to the increased diffraction.
2) Thermal infrared requires different sensors. Your standard CCD simply does not pick it up, you need something like HeCdTe.
3) Thermal infrared requires different lenses. Far infrared does not pass through glass like it does through air, you need to use a different material to make the lens, or just use a mirror.
4) Thermal imaging is more sensitive to thermal noise. That's why you cool the sensors. All sensors have thermal noise, even visible-light sensors. You will get cleaner pictures by refrigerating your visible-light CCD too, but it's a much bigger deal in far IR.
5) Atmosphere absorbs IR. Well, not all of IR. But the atmosphere is fairly opaque in some IR bands. If you build a sensor at 6-7µm you might as well submerge the thing in pea soup, the atmosphere is that opaque.
So [near] IR photography and [far IR] thermal photography are completely different beasts.
I'd just like to point out here that infrared is a really wide chunk of spectrum. It goes from 0.74µm to 300 µm (according to Wikipedia), which is a factor of about 400. By comparison, the visual spectrum has a width of about a factor of 2, and yet our biology still gives us four different sensors crammed within that little sliver. Imagine if you had a rainbow with red, orange, yellow, green, blue, indigo, violet, and 50 more colors which were just as distinct from each other, and you get an appreciation of how big IR is.
Near infrared is the stuff used by artistic IR photographers (film and digital), remote controls, and night vision cameras. Regular CCDs designed for visible light also pick up near IR, so most cameras have a built-in filter which blocks it out. This is what you remove to turn your camera into an IR camera, and it also accounts for why some things just show up wrong in some digital pictures. Maybe you've seen a fireplace with purple flames -- that's because the IR filter isn't getting rid of all the near IR.
Near infrared is really just light that you can't see. Most things don't emit near infrared unless they're very hot (like fire) or designed to do so (like LEDs).
Far infrared is used in thermal imaging. It is a completely different beast, with much longer wavelengths.
1) Thermal infrared is low-resolution. The resolution of an imaging system is limited by the aperture of the lens relative to the wavelength of light that you are using. A proficient photographer with a nice prime lens and a tripod can come close to the resolution limit of visible light with no trouble, but the (much) longer wavelength of far infrared means that the relative size of a lens is much smaller. You simply cannot get a sharp picture of thermal infrared due to the increased diffraction.
2) Thermal infrared requires different sensors. Your standard CCD simply does not pick it up, you need something like HeCdTe.
3) Thermal infrared requires different lenses. Far infrared does not pass through glass like it does through air, you need to use a different material to make the lens, or just use a mirror.
4) Thermal imaging is more sensitive to thermal noise. That's why you cool the sensors. All sensors have thermal noise, even visible-light sensors. You will get cleaner pictures by refrigerating your visible-light CCD too, but it's a much bigger deal in far IR.
5) Atmosphere absorbs IR. Well, not all of IR. But the atmosphere is fairly opaque in some IR bands. If you build a sensor at 6-7µm you might as well submerge the thing in pea soup, the atmosphere is that opaque.
So [near] IR photography and [far IR] thermal photography are completely different beasts.