It is important to realize that we are immersed in a sea of electromagnetic radiation but our eyes are only capable of seeing the wavelengths of the visible spectrum. However, the fact that we cannot see radiation of other wavelengths does not mean that such radiation does not exist. Just as a blind person cannot see the visible spectrum, so normally sighted people are blind to the presence of radio waves and infrared radiation. DSLRs are designed for photographing subjects in the "normal" light of the visible spectrum that the human eye is equipped to detect.
The ability of the human eye to detect electromagnetic radiation falls off sharply for wavelengths longer than 600nm. At a wavelength of 700nm a normal human eye is effectively blind. However, the sensitivity of silicon sensors in DSLRs extends to about 1100nm, which is well within the range described as infrared. This is potentially a problem because DSLR sensors record much more than the radiation in the visible spectrum seen by a photographer. All DSLRs are therefore equipped with an infrared filter immediately in front of the sensor to remove the infrared element from images.
Despite the presence of an infrared filter, it is still possible to capture infrared images with a normal digital camera - with some reservations. (However, bear in mind that many so-called digital infrared images are actually the product of various sophisticated processing techniques and therefore not true representations of infrared.) A principal problem with capturing infrared radiation in an image using a digital cameras is of course the presence of its infrared blocking filter which is put in place to enhance the visible-light performance of the sensor. The result is that the sensitivity of a sensor to the infrared wavelengths is much reduced - perhaps by 10 or 12 EV. Infrared images may still be captured with a suitable opaque infrared (visible light blocking) filter attached to the lens, but long exposures are usually required.
Another, potentially much more satisfactory, approach is to purchase a digital camera not equipped with an infrared-blocking filter. A few such models are available. It is also possible to have the infrared blocking filter removed from a standard DSLR, so allowing the camera to record wavelengths in both the visible and infrared spectra. An infrared blocking filter must then be attached to the lens to restore the "normal" visible light function of the camera. If an opaque infrared (visible light blocking) filter is attached to the lens, the same camera can be used to records infrared images.
A third approach is to have the infrared blocking filter normally fitted in front of a silicon sensor replaced by a visible spectrum filter, so limiting the use of a camera to the infrared spectrum. Both Nikon and Canon may be willing to undertake such modifications, and other specialized companies may be found by searching the internet. The advantage of this latter approach is that infrared work can be undertaken with fully functioning autofocus and autoexposure, and normal viewfinder function. The disadvantage is that the work is highly specialized, and expensive, and may be impossible to reverse. Any warranty on the camera is also likely to be invalidated.
The Hoya R72 opaque filter is perhaps the best available general purpose infrared (visible light blocking) filter. Such filters are easy to fit to DSLRs as their lenses are threaded for the purpose. With a digital compact camera it may be necessary to obtain an appropriate lens adaptor tube. The camera should then be attached to a stable tripod so that slow shutter speeds can be used.
Exposure determination is not straightforward and inevitably invites the use of bracketing. The automatic exposure systems in DSLRs are based upon a separate array of sensors which may have a somewhat different sensitivity to infrared wavelengths. Digital compact cameras normally use the image sensor to measure exposure, so the two camera types are quite different in this respect. Test shots may be necessary to establish exposure errors with DSLRs. Once a shot has been captured, be sure to examine the red channel histogram. Almost all the data from which the image is composed is stored in the red channel, so it is vital to ensure that the histogram does not hit the upper end of its range. If the data is limited in this way, reduce the exposure in 1EV stops until the information is all within range.