Infrared Photography - Digital Cameras

Digital Cameras

Digital camera sensors are inherently sensitive to infrared light, which would interfere with the normal photography by confusing the autofocus calculations or softening the image (because infrared light is focused differently from visible light), or oversaturating 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 the 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 centre of the image as their coatings are optimised for visible light and not for IR.

An alternative method of DSLR infrared photography is to remove the infrared blocker in front of the sensor 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.

Since the Bayer filters in most digital cameras absorb a significant fraction of the infrared light, these cameras are sometimes not very sensitive as infrared cameras and can sometimes produce false colors in the images. An alternative approach is to use a Foveon X3 sensor, which does not have absorptive filters on it; the Sigma SD10 DSLR has a removable IR blocking filter and dust protector, which can be simply omitted or replaced by a deep red or complete visible light blocking filter. The Sigma SD14 has an IR/UV blocking filter that can be removed/installed without tools. The result is a very sensitive digital IR camera.

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 artefacts 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.

Another common technique with near infrared filters is to swap blue and red channels in software (e.g. photoshop) which retains much of the characterisic 'white foilage' while rendering skys a glorious blue.

Several Sony cameras have the so-called Night Shot facility, which physically moves the blocking filter away from the light path, which makes the cameras very sensitive to infrared light. Soon after its development, this facility was 'restricted' by Sony to make it difficult for people to take photos that saw through clothing. To do this the iris is opened fully and exposure duration is limited to long times of more than 1/30 second or so. It is possible to shoot infrared but neutral density filters must be used to reduce the camera's sensitivity and the long exposure times mean that care must be taken to avoid camera-shake artifacts.

Fuji have produced digital cameras for use in forensic criminology and medicine which have no infrared blocking filter. The first camera, designated the S3 PRO UVIR, also had extended ultraviolet sensitivity (digital sensors are usually less sensitive to UV than to IR). Optimum UV sensitivity requires special lenses, but ordinary lenses usually work well for IR. In 2007, FujiFilm introduced a new version of this camera, based on the Nikon D200/ FujiFilm S5 called the IS Pro, also able to take Nikon lenses. Fuji had earlier introduced a non-SLR infrared camera, the IS-1, a modified version of the FujiFilm FinePix S9100. Unlike the S3 PRO UVIR, the IS-1 does not offer UV sensitivity. FujiFilm restricts the sale of these cameras to professional users with their EULA specifically prohibiting "unethical photographic conduct".

Phase One digital camera backs can be ordered in an infrared modified form.

Remote sensing and thermographic cameras are sensitive to longer wavelengths of infrared. They may be multispectral and use a variety of technologies which may not resemble common camera or filter designs. Cameras sensitive to far infrared including those used in infrared astronomy often require cooling, since all objects at room temperature (including the camera body, optics, and the detector itself) are glowing all the time at these wavelengths (see thermal radiation). Lower priced thermographic digital cameras extend the spectral range less far into infrared. These cameras are generally used for building inspection or preventative maintenance but can be used for artistic pursuits as well, such as this image of a cup of coffee.