Theoretical aspects and practical applications of moiré topography

When a surface is observed at a distance through a parallel line grating which is illuminated by a source of light laterally displaced to the viewing axis, the grating is superimposed on its shadow and this gives rise to a moiré fringe pattern. The fringes represent cross-sections of the surface parallel to the grating plane. The principle of the formation of the contour map has been studied assuming a grating represented by a Fourier series, a source of light represented by a 3D intensity distribution and a viewing aperture of finite dimensions. The experimental system used for illumination and observation of the moiré fringes consists of an equispaced parallel-lined glass grating and two small light sources positioned symmetrically about the imaging device. The arrangement constitutes a 3D imaging system with an accuracy of the order of 1 mm. The technique has been applied in radiotherapy problems such as the design of contour compensators and the measurement of body contours. Moiré patterns have been analysed with the aid of a PDP-8 computer, and have been used to calculate surface areas and volumes such as in breast studies and superficial tumour measurements.