Reflected infrared photography

Infrared radiation was discovered by William Herschel (Figure 2) in 1800. His description of the time read "It now being evident that there was a refraction of rays coming from the sun, which though not fit for vision, were yet highly invested with a power of occasioning heat, I proceeded to examine it as follows..." Herschel dubbed this 'new' radiation as the "thermometric spectrum." Herschel used a slit, prism and a series of thermometers to demonstrate the existence of radiation beyond red in the spectrum (Figure 3). Today the infrared spectrum is recognised as extending from about 700nm up to wavelengths of about 1mm where it overlaps with radiowaves. Photography is normally confined to the near infrared - 700 to 900 nm. A great deal of confusion continues to arise concerning infrared photography and the measurement of infrared energy in the form of heat. This confusion often leads to futile attempts to detect thermal patterns through the use of infrared photography in cases where the technique does not apply. Contrary to what many people believe, the infrared record in a photograph is not a measure of ambient temperature variation - it is a record of the amount of near infrared radiation reflected or transmitted by the subject. Thermal photography cannot be done with infrared sensitive film. Far infrared (approximately 2 -15mu) is usually regarded as "heat' and can only be recorded by converting the radiation into a visible form using an image converter tube, and this is the province of thermography. Hot objects, naturally enough, are usually emitting a good degree of near infrared - leading to the ability to capture images of hot irons, etc., on infrared film and also contributing to the confusion!

Figure 2. Infrared radiation was discovered by William Herschel.

Figure 3. Herschel used a slit, prism and a series of thermometers to demonstrate the existence of radiation beyond red in the spectrum.

The term 'photographic infrared' is used loosely to describe the region 700 -1350nm, the range over which silver halide emulsions can be sensitized; but most infrared emulsions are only sensitive up to about 900nm, which then becomes the practical upper limit. Infrared sensitive emulsions have been available since the 1930's, so reflected infrared photography is certainly not a new area of study. Indeed there has been a great deal of work done in the field. Gibson's standard work Photography by Infrared last published in 1978 contained over 1,800 references to applications of infrared photography and thermography. While it would be foolish to duplicate Gibson's efforts - and the reader is referred to his excellent text for this extensive bibliography - much has changed in the practical field. New films, filters and light sources are now used and this article attempts to bring this topic up to date and to present practical working methods.

Some of the earliest uses of the reflected technique continue to the present day. One of the first applications was in the field of document examination for criminal forgery; art museums also made early use of the technique to examine paintings, thereby revealing their authenticity and development. Medical investigators were quick to appreciate the value of the technique and even with CT, ultrasound, Doppler, NMR and PET scanning, there is still no other non-invasive method that can reveal the same information. The selective reflection of infrared by healthy and diseased plants coupled with its ability to penetrate haze held to early applications in the fields of agriculture and forestry - applications which continue to be in use today in the form of remote sensing from satellites. There are few photographic techniques that enjoy such widespread use in science, medicine and engineering. The ethereal nature of black-and-white infrared images, or the strange false colour representations of colour infrared, have excited pictorial photographers for years and amateur photographers continue the fascination. A recent web search by the authors on infrared photography led to over 72,000 citations!

In its simplest form the technique only requires sunlight, an infrared transmission filter over the camera lens and some infrared sensitive film in the camera to produce quite dramatic results. Chlorophyll reflects infrared almost totally so foliage reproduces nearly white and infrared is scattered less so blue skies appear nearly black (Figure 4); add to this the inherently high granularity of the infrared film and pictorialists have a wonderful time. The human eye and indeed that of many animals, is insensitive to infrared radiation which has led to the widespread use of infrared imaging for surveillance at night. In this circumstance the radiation source must be covered with the infrared transmission filter, not the camera's.

Figure 4. An example of a reflected infrared photograph taken in daylight. Note that foliage appears nearly white and the sky nearly black.

The reflected infrared technique works by using a source of infrared radiation to light the subject, and then filtering out all visible light by fitting an infrared transmission filter over the lens. A specially sensitized film then records the infrared reflected from the subject. Figure 5 is a diagrammatic representation of the basic technique for use in scientific and medical applications. Each component of the infrared imaging chain is considered on the following pages.

Figure 5. Diagram of the basic reflected infrared technique.

References:

• Gibson, H., 1978. Photography by Infrared. John Wiley. NY. USA.

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