THE SPATIAL AND PRESSURE RESOLUTION OF FUJI PRESSURE-SENSITIVE FILM

Due to its ease of application, Fuji prescale pressure-sensitive film is currently one of the more popular methods, within the biomechanics community, for assessing contact areas and pressures within articulati ng joints-in addition to its use in industry. This material produces a stain on the application of pressure due to the rupture of microscopi c bubbles releasing a liquid which, in turn, causes patches of color t o be formed; a greater pressure produces a darker stain. These stains are often converted into digital images and manipulated to produce fal se-color pressure-maps, an approach which is beyond the simple methods of analysis suggested by the manufacturer. Due to the granular nature of Fuji film stains, the two user-defined variables which will determ ine the accuracy of any pressure-map are: (a) the size of the sample-a rea used to capture data from the original stain during the digitizati on process and (b) the number of pressure-intervals identified on each map; the chosen values should match the spatial and pressure resoluti ons of the film. Despite the importance of these factors, the literatu re presents a bewildering array of values, particularly for the number of pressure-intervals, with no validation of those chosen; consequent ly, little guidance is provided for other potential users of Fuji film . This paper discusses the relationship between sample-area and pressu re-interval and introduces a method for examining their effect on the resulting pressure-maps. The results obtained using 'Super Low' grade Fuji film suggest that the authors of some previously reported methods may have been overambitious in their choice of sample-area and pressu re-intervals. Finally, a series of suggested values of sample-area siz e and pressure-intervals are provided.