Infra Red Photography

To understand Infra-red photography you need first to understand light. The human eye can only see a very small band of light waves called the visible spectrum.

Infrared radiation (IR), or infrared light, is a type of radiant energy that's invisible to human eyes as the waves of infrared radiation spectrum are longer than those of visible light, but we can feel as heat. The human eye can see spectrum wavelengths from about 400nm-700nm (from purple to red); infrared is the light beyond 700nm.

Many scientific uses of photography require the recording of the near UV and IR regions, using conventional camera lenses, bounded in the UV by spectral transmittance of the elements and in the IR by the available dye sensitisation of silver halide materials.

The visual focus of a normal lens is not correct for UV or IR. In general the focal length increases for these regions). Two remedies for this is either to extend the lens further by about 0.3 per cent of its marked focal length or to use a bellows extension by about

Many old lenses in focusing mounts have a sup­plementary index mark, usually in red, which is for IR work. The lens is focused visually and the distance setting transferred to the secondary index. This index may also serve for the UV focus, but modern lenses using high refractive index glass pass very little beyond 400 nm.

If we take photographs using infrared-equipped film or cameras, we experience a world that looks very different from that we are accustomed to seeing. Colours, textures, leaves and plants, human skin, and all other manner of objects can reflect the IR spectrum in unique and interesting ways, ones that cannot be mimicked with software tools. Vegetation appears white or near white. Skin takes on a very milky, smooth texture, although veins close to the skin surface can be accentuated and take on a rather ghoulish appearance. Eyes can appear a bit ghostly with the irises registering very dark tones and the whites of the eye taking on a greyish hue. Black clothing can appear grey or white depending on the fabric. The IR spectrum can pass through sunglasses that, to the eye, appear extremely dark or mirror-like (see image below). Blue skies take on a much more dramatic appearance as well.

Photographers started experimenting with IR in early part of the 20th century. During WWI, IR photography proved extremely valuable, as images using the IR spectrum were not affected as much by atmospheric haze as normal photos. IR images were also able to show stark distinctions between vegetation and buildings, better identifying potential enemy targets such as camouflaged munitions factories and other key sites. Rivers, streams, lakes, and other waterways were depicted in a very dark hue, making them much more obvious.

During the 1960s, IR photography saw a number of photographic uses, as some of the leading musicians of the day, such as the Grateful Dead and Jimmy Hendrix, popularised its use via their psychedelic album covers. With the advent of the digital camera in the late 1990s, both regular and IR photography were about to change substantially. In addition to professional and amateur photographers, law enforcement officials rely on IR photography to detect forensic evidence not discerned through normal eyesight.

I have used a full spectrum camera to take the images on this site. That is a converted camera where the filter that excludes light outside of the visible spectrum has been removed from the sensor. Of course this would mean all spectrum including visible colours would be recorded if it were not for the fact that I add filters called IR pass filters to control what spectrum I wish to record and what I wish to block. Filters are called block filters or band pass filters. Block filters block certain frequencies and pass all other frequencies and band pass filters (BP) pass certain frequencies and block the other frequencies. You maybe wondering why we need two types of filter in this way, the main reason is that some photographers, for example Astro photographers need to pass some spectrums but specifically block others.

Reflected IR light produces a fascinating array of surreal effects. With the right filters Infrared photography produces some very distinct effects which make them aesthetically pleasing.

Common Filters

The orange line in the chart shows the band pass filter in a normal camera which passes the visible spectrum band but blocks all, but a small part of the non viable spectrum above 700nm. As a result of this if you use an IR filter on unmodified camera you will need very long exposure times and even then will only capture a small amount of IR spectrum, as most of the spectrum above 700nm will be blocked by this internal filter in the camera.

Once a camera has been modified to remove this non-visible spectrum band pass filter, a new filter needs to be added to block everything but the spectrum you require and to pass the spectrum you do require.

The chart also shows the effect of some the IR band pass filters used on this site. Note that they block most of the visible spectrum below the stated value but generally pass all the spectrum above the stated value, although some I use also block the higher IR bands. Some filters allow some of the visible spectrum and this allows for what is called "false colour" during processing as this requires a mix of the IR spectrum with some of the visible spectrum. It all depends upon what you wish to achieve.

• 550nm Inspired by "false colour" IR film, as the 550nm lets in the most visible light of all IR usable band pass filters creating deep blue skies and blood crimson reds.
• 642nm This lets some visible light in, producing the most vibrant colours when processing for "false colours". Leaves are golden yellow, and skies are bright blue. B+W contrast is lower than the deeper IR filters.
• 720nm This is the tried and true classic IR filter. It allows some limited visible light for "false colour", and good contrast for black and white.
• 850nm This filter is good for a dedicated black and white IR. The camera and will produce bright whites and pronounced darks. With a custom white balance in camera, the picture is close to pure B+W without any processing.

• 950nm This filter is good for extreme contrast black and white, all visible light is blocked.

I used a 642 "BP" short for "Bandpass" for the images HERE. The filter gives a 200nm spectral window from 642nm to 842nm. It blocks the longer infrared.

As can be seen from the images the BP creates very deep red and gives all light up to 842nm. You get images which are perfectly suited for channel-changeing, and which will show an incredible level of colours and great contrast do to the blocking of the longer IR: A great set of data to start creative image processing!

• I used a 742nm for the images HERE. It removes almost all visible light.

The filter cuts off the part of the spectrum where Chlorophyll looks green and shows its high reflectively in the near infrared. If trees are photographed in spring and summer under blue skies you get stunning images with white trees and clouds in front of a near black background.
Darkens the background during twilight.