Science & application of photography in the infrared & ultraviolet – Bernard Foot

Bernard stressed that his talk would mainly cover the use of near-infrared (NIR), but also to some extent mid IR (MIR) & far IR (FIR); on the UV side, the talk would focus on Near UV (the UVA and UVB bands).

He covered briefly the history of conventional films, where the first films were sensitive to only blue and near UV light, and dyes were subsequently used to extend sensitivity to green (orthochromatic), red (panchromatic), NIR & MIR. Modern digital camera sensors are sensitive to UV & IR, but manufacturers incorporate filters to eliminate this. However specialist companies can remove these to provide a full spectrum camera. One must use an IR- or UV-transmitting filter in front of the lens to remove unwanted frequencies. Modern lens technology is not suitable for UV: specialist UV lenses are available but very expensive, but some older, simpler lenses work well and can be purchased relatively cheaply.

The illuminating light source has to be selected appropriately: daylight is a good all-round source, tungsten filament bulbs are good for IR, & compact fluorescent lamp or LED for UV. There are also “black” lights which provide only invisible IR or UV light.

Bernard described how the different reflectance & absorption of IR and UV by various materials could be used: detection of stains and physical abuse in criminal forensics and the detection of skin ailments. The strong reflectance of IR by chlorophyll led Kodak to introduce a false-colour IR film in the 1960s to help the US Air Force differentiate camouflaged military equipment from foliage; this technology can also be used for the detection of pollution, diseased plant life, and deforestation, and in archaeology.

IR can penetrate certain materials, making it useful to photograph through haze (but not through water-based fog and mist), to penetrate inks which have obscured information or been used to make forgeries, and to see what is below the top layer of a painting. A live demonstration was given to confirm reports that IR camera could see through certain types of clothing.

There was then a description of UV fluorescence, and how it can be used in diagnosing illnesses, detecting stains caused by human secretions in criminal forensics, detecting forged banknotes, and finding cracks in metal components.

The final part of the talk looked at recording the EM emissions from objects (as opposed to light reflected from them). Conventional IR photography can be used to record objects at temperatures down to about 250 C, but for lower temperatures thermal imaging technology must be used. The applications for this include finding people, detecting problems in buildings and industrial plant, and diagnosis of medical conditions. Thermal imaging cameras typically cost multiple thousands of pounds – but now an app for a smart phone is available at £200, including a low-resolution plug-in thermal camera.