Digital cameras are very sensitive to IR light, so much so that manufacturers place an IR filter (hot mirror) in front of the sensor to screen IR light while allowing visible light to pass. It is only visible light that eventually passes onto the sensor to create our photographic image. However, even in this condition, you can still photograph an IR image by placing a filter on your lens that prevents visible light from entering through your lens, and thus leaving only the IR wavelengths to hit the sensor. A common Infrared filter is the Hoya Infrared R72 . It only allows light with wavelengths longer than 720 nm to pass through it. However, as I mentioned, there is an internal filter (the hot mirror) just before your sensor that prevents IR light from entering. This means that with the R72 filter in front of your lens, you will now have to wait much longer for an exposure and therefore need a tripod (i.e. a shot that could have been taken hand held at 1/250 sec. can now take up to a few seconds). There are no exact rules to this as different subjects reflect IR light differently and various weather conditions contribute to the light variable as well.
Also, since the R72 Filter prevents visible light from passing through, you will not be able to compose or light meter after the filter is placed. You would have to do that before placing the filter on the lens.
As I have already mentioned, various objects reflect IR light differently. Foliage will be much brighter and the type of day, sunny or overcast will also affect your light exposure. Add to that the fact that digital cameras have an internal light meter sensor that is designed to only “see” visible light. This means that getting the right exposure will require some trial and error. This is the beauty of shooting digital, you can correct your exposure on the spot!
More on Infrared Photography, customizing your white balance, exposure compensation issues and heck, even Long Exposure Infrared Photography coming soon, keep posted!