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‘The Box’ – a skiagraphic approach to recording shape and size

This article was first published in The British Journal of Photography, 15 September 1988, under the title ‘‘Using the ‘photogram’ to investigate back pain’’ (Swann & Farmer, 1988) and is reproduced here by kind permission of the present owners of that journal, Apptitude Media, Ltd. Some passages have been expanded for greater clarification. The apparatus described here was devised as a solution to a problem posed by a consultant orthopaedic surgeon. As part of a comprehensive investigation into back pain, a considerable number of lumbar vertebrae excavated from 1st and 4th Century A.D. British archaeological sites, together with some 60 twentieth-century spines, mainly of Indian origin, were acquired for study. A quick and simple means of recording accurately their canal diameters was required. Physical measurement using calipers had proved impractical due to the irregularity of the canal walls. Conventional negative/positive photographic procedures were initially suggested, but rejected immediately due to the dimensional inaccuracies inherent in the process. Moreover, the mass of subject detail provided by a half-tone rendering was deemed superfluous and perhaps even a hindrance to the survey, an accurate and measureable record of the mean minimum size and shape of the vertebral canal in each bone being the sole requirement. Consideration of alternative methods led to the adoption of the skiagraphic technique. This provides a reversed silhouette image of an opaque object when it is placed in contact with a piece of light-sensitive photographic material while it is exposed to light. This technique was the basis of Fox Talbot’s early experiments with silver-sensitised paper (‘photogenic drawing’) and has been fundamental to numerous ‘creative’ photographic darkroom techniques since, the results often being called ‘photograms’. The technique in its crude form presents a major shortcoming, of particular significance in relation to this study. Whilst the outlines of flat opaque objects can be rendered with reasonable accuracy, a subject of irregular shape, and especially one which has an uneven base surface, permits light to ‘creep’ beneath those parts of the base which are not in contact with the sensitive material. This ‘creeping’ induces localised areas of unsharpness and indistinct outlines. The chief cause of this effect is the illuminant being used which, since it is usually the beam from a photographic enlarger or even simply the room light itself, is large and diffuse. The smaller and more direct the light source, the sharper the resultant image will be. It was postulated, therefore, that by using a point light source, combined with a means of forming parallel rays, it should be possible to produce a sharp image of a three-dimensional object which, subject to the dimensional stability of the sensitive material employed, would be an exact same-size silhouette of the subject and therefore capable of accurate measurement. (Figure 1). It was therefore reasoned that a pinhole placed in front of an intense source of illumination to form a point source, and used in conjunction with a plano-convex lens (Williams, 1977) to create parallel rays, would produce an image possessing the dimensional accuracy required for the project. The main advantages of the pinhole system are as follows: