A Caliper for measuring Photographs, X-Rays and Drawings

Abstract

THERE is at present no instrument on the market suitable for measuring distances in photographs, X-rays or drawings both accurately and in considerable numbers. Our particular problem was to measure various dimensions of the human figure on 12 in. × 10 in. pictures of subjects photographed in the nude for studies of body-build. The measurements had to be accurate to 0.1 mm., and a large number of them had to be recorded. In this technique, known as photogrammetric anthropometry, needle-pointed calipers are used, so as to locate the precise point at which the edge of the body shades into the white background of the picture. Some form of enlargement of the scale of measurement other than a vernier, which is awkward to read and thus gives rise to too many reading errors, was clearly desirable. We first used calipers based on a scissors principle, with a magnification of scale of about × 5 due to the needle-pointed arms being shorter than the arms carrying the reading scale¹. However, these proved unsatisfactory in practice due to the scale being on the chord rather than the arc of the opening circle, and because adjustment to remedy this varied from one instrument to another and was not entirely reliable.