Abstract
The method of caustics has been used for many years for studying deformations around crack tips in metals and other materials, but a systematic and easy-to-understand description of the principles and practice of photographing high quality caustics images has not been made available to potential practitioners of the method. This paper describes how sharp, high contrast caustic images can be obtained using simple equipment and procedures based on well-known principles of geometrical optics. The critical parameters are the light source diameter, which must be small, and its distance from the reflecting surface, which must be large; and high resolution, short depth of field and high initial magnification of the photographic system. Monochromatic filters or laser light sources can improve caustic images if the lens used is not highly corrected for chromatic errors. If collimated (parallel) light is desired to avoid making corrections for the divergence of the illumination, long focal length collimator lenses are necessary to minimize the light source diameter contribution to the caustic ring thickness.
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