Infrared radiation can't be seen with the naked eye, and its strength varies with the temperature of the subject. It's defined as those wavelengths beyond the deepest reds of the visible spectrum at 700 nanometres (billionths of a metre) through to the wavelength of microwaves--around 100,00 nanometres. However, most infrared-sensitive films cover a much smaller span--usually between 700 and 1,200 nanometres.
Special infrared-sensitive films didn't become widely available until the 1930s, and while they allowed images to be recorded in the dark, emulsions were slow, thereby limiting their application to static scenes. Military and scientific use of infrared images has driven the technical progress in this field of photography, and as early as the First World War, infrared-sensitive plates were used by the USA for aerial reconnaissance.
Infrared imaging is one aspect of modern photography that still finds favour with film usage, although it's perfectly possible to make infrared images digitally. The use of special lens filters is essential to the process, for both digital and film infrared image capture.
USING INFRARED FILM
Most people are familiar with the characteristics of a black and white or colour infrared print. The use of black and white infrared film remains popular with landscape photographers because of the way it renders a clear sky as a very dark, almost black area. Against such a sky, clouds stand out even more, and foliage of deciduous trees can look reminiscent of a scene after a heavy snowfall. This is because the chlorophyll in foliage is transparent to the infrared wavelengths so won't block its reflectance. By contrast, coniferous trees appear dark and water becomes black.
With colour infrared, the effect can be more surreal as the colours depicted aren't the 'natural' hues as reflected by visible daylight. Instead, the colour intensity will be determined by the amount of infrared radiation reflected by the subject. Somewhat appropriately, the greater the level of infrared reflected, the redder the subject will appear on film: the coolest subject areas will be recorded as blue, the warmest as red.
To achieve the maximum effect, it isn't enough just to load your camera with infrared film (and remember always to load and unload this film in complete darkness)--a special filter also needs to be fitted to the lens of the camera.This is because all infrared films are sensitive to daylight as well as infrared, so if no filter is used, the effect of infrared will be largely obscured by the visible light image rendered on the film. In most applications, a plain red filter is sufficient to reduce the amount of visible light reaching the film plane, without rendering the scene through the viewfinder completely dark--as is the case when using a pure infrared-transmitting filter.
Older lenses used to have an infrared focusing mark on the focus ring, but this is absent on most modern autofocus lenses. The effect of minimising the amount of visible light entering the lens when using an infrared filter means that focusing is best done manually and as an approximation, with the aperture stopped down to maximise depth of field. Such a technique means exposure times can be long, even though most infrared films have fast ISO ratings, so make sure to use a tripod.
Digital sensors are receptive to more than just visible light waves, so manufacturers place special blocking filters on the sensor to keep infrared light from interfering with the normal daylight received for photography. The different wavelengths of infrared can also interfere with the camera's distance calculations when autofocus is in use. Digital cameras can be modified for infrared use by having the blocker removed and then using a red or infrared filter to further enhance the effect, but this is a complicated and expensive procedure for anyone without a specific or professional interest. …