In INFRARED PHOTOGRAPHY the image sensor is sensitive to infrared light. The part of the spectrum used is referred to as near-infrared to distinguish it from far-infrared, which is the domain of thermal imaging. Wavelengths used for photography range from about 700 nm to about 900 nm. Usually an "infrared filter" is used; this lets infrared (IR) light pass through to the camera, but blocks all or most of the visible light spectrum (the filter thus looks black or deep red).

When these filters are used together with infrared-sensitive film or sensors, very interesting "in-camera effects" can be obtained; false-color or black-and-white images with a dreamlike or sometimes lurid appearance known as the "Wood Effect," an effect mainly caused by foliage (such as tree leaves and grass) strongly reflecting in the same way visible light is reflected from snow. There is a small contribution from chlorophyll fluorescence, but this is extremely small and is not the real cause of the brightness seen in infrared photographs. The effect is named after the infrared photography pioneer Robert W. Wood, and not after the material wood, which does not glow under infrared.

The other attributes of infrared photographs include very dark skies and penetration of atmospheric haze, caused by reduced Rayleigh scattering and Mie scattering, respectively, compared to visible light. The dark skies, in turn, result in less infrared light in shadows and dark reflections of those skies from water, and clouds will stand out strongly. These wavelengths also penetrate a few millimeters into skin and give a milky look to portraits, although eyes often look black.

Digital camera sensors are inherently sensitive to infrared light, which would interfere with the normal photography by confusing the auto focus calculations or softening the image (because infrared light is focused differently than visible light), or over saturating the red channel. Also, some clothing is transparent in the infrared, leading to unintended (at least to the manufacturer) uses of video cameras. Thus, to improve image quality and protect privacy, many digital cameras employ infrared blockers. Depending on your subject matter, infrared photography may not be practical with these cameras because the exposure times become overly long, often in the range of 30 seconds, creating noise and motion blur in the final image. However, for some subject matter the long exposure does not matter or the motion blur effects actually add to the image. Some lenses will also show a 'hot spot' in the center of the image as their coatings are optimized for visible light and not for IR.

An alternative method of digital SLR infrared photography is to remove the infrared blocker in front of the CCD and replace it with a filter that removes visible light. This filter is behind the mirror, so the camera can be used normally - handheld, normal shutter speeds, normal composition through the viewfinder, and focus, all work like a normal camera. Metering works but is not always accurate because of the difference between visible and infrared reflection. When the IR blocker is removed, many lenses which did display a hotspot cease to do so, and become perfectly usable for infrared photography. Additionally, because the red, green and blue micro-filters remain and have transmissions not only in their respective color but also in the infrared, enhanced infrared color may be recorded.

While it is common to use a filter that blocks almost all visible light, the wavelength sensitivity of a digital camera without internal infrared blocking is such that a variety of artistic results can be obtained with more conventional filtration. For example, a very dark neutral density filter can be used (such as the Hoya ND400) which passes a very small amount of visible light compared to the near-infrared it allows through. Wider filtration permits an SLR viewfinder to be used and also passes more varied color information to the sensor without necessarily reducing the Wood effect. Wider filtration is however likely to reduce other infrared artifacts such as haze penetration and darkened skies. This technique mirrors the methods used by infrared film photographers where black and white infrared film was often used with a deep red filter rather than a visually opaque one.
(Source Wikepedia)

All my infrared photo’s were taken with digital camera’s were the mirror in front of the sensor was replaced with one that lets infrared light through. I used several camera’s of Nikon and Canon with a diversity of different infrared versions. Most of them were taken with “Full IR mirrors”.