OPTICAL FOOT pressure measuring systems (pedobarographs) are based on a technique developed by Chodera (CHODERA, 1957), and have been used to assist in the assessment of a wide range of foot abnormalities (BETTS et al., 1991). In these systems, a white rubber, plastic or photographic paper foil is placed on top of a glass plate. The upper surface of the foil is covered with light proof material to eliminate background illumination (FRANKS et al., 1986). When the glass plate is illuminated at the edges, it acts as a light 'pipe' and total internal reflections occur at the glass/air boundaries. If pressure is applied to the foil, it is brought into close contact with the glass and so total internal reflection no longer occurs. Light rays refract out of the glass and are scattered in all directions from the white foil. At a microscopic level, the foil has a rough surface and the applied pressure increases the area in contact with the glass. An image of the pressure pattern is seen if the plate is viewed from underneath, and the intensity at any point is related to applied pressure. Many different designs of optical foot pressure system have been used, together with a wide variety of types of foil. For the systems to be accepted for routine clinical use, it is essential to be able to compare results obtained at different centres (LORD et al., 1986). The major disadvantage of these optical systems has been the difficulty of calibrating the data obtained. The Sheffield optical pedobarograph incorporated the calibration techniques described below, and was steadily improved during the 1980s. No further development has been carried out in Sheffield, because it was assumed that the use of optical pedobarographs would give way to systems based on arrays of discrete transducers. In fact, this has not happened, and the Sheffield system is still in use at a number of centres. It is considered to be more stable and reliable than systems based on discrete transducers, and it has higher spatial resolution. Rapid developments in video and PC technology in the 1990s would now make it possible to reproduce the Sheffield pedobarograph very effectively with modern components. It therefore seems appropriate to provide guidelines on calibration techniques based on experience from Sheffield.