Objects are ‘seen’ by the action of visible light reflecting from them and into the eye or the camera lens. In October 1910, the RPS Journal published, for the first time photographs taken with ‘invisible rays’, radiation from other parts of the spectrum. (Finney 2010). In principle, what these images show are invisible aspects of their subjects. I have photographed in the near-infrared on film and written about it in the RPS Analogue Group journal (vol2, members subscription only). A problem is the modern ‘infra-red’ films are better described as ‘extended red sensitivity’ as they extend only a short distance into the infrared and are used with a dark red (R72) filter, rather than the totally opaque filter of the discontinued Kodak HIE film. Therefore, photographs taken with these films partly show ‘seen’ as well as unseen aspects of the subject.
Better results can be made with a converted digital camera. Digital sensors are sensitive further into the near-infrared than current films, and cameras typically have a built-in infrared-blocking filter. Several companies offer conversion services, removing the blocking filter and replacing it with some form of IR-passing filter, at a cost in the region of £300 to convert a typical DSLR. As a proof-of-concept exercise, I have bought an IR-converted point-and-shoot compact (£120 from ‘infraready’ trading on eBay), and am pleased with the results. The camera has an 850nm cut-off filter, which means that its range starts some way from where current ‘infrared’ films peter out, and is similar to that of HIE.
The basic shapes ‘seen’ by the camera are the same as we see by visible light, but there are some tonal shifts (in addition to the obvious two) as surface finishes respond differently. The two major effects are the darkening of skies (Rayleigh-scattered blue light has very little IR) and the lightening of foliage due to the Wood effect (internal reflections and scattering of IR in plant cells is similar to visible light in ice crystals, so foliage ‘sparkles’ under infrared (Finney 2010)).
To make this work as a response to the assignment would require some special aspect of the ‘unseen’, rather than simply photographing ordinary scenes and exploiting a cool look. IR is useful in identifying diseased plants, because healthy cells respond differently from diseased cells, but I am unlikely to find sufficient practical examples.
Our garden is regularly visited by foxes, but seeing them is rare (and having the opportunity to photograph them is rarer). However, they often leave evidence of their presence as they attack birds and scavenge dustbins, and visit the sheltered verandah on rainy nights.
A set of images of the debris they leave would say something about the (unseen) animals and their habits.
Finney, A. (2010). A Century of Infrared Photography. In: R. Reynolds, ed., Portfolio Two, 1st ed. Bath: Royal Photographic Society, pp.110-115.