On non-uniform illumination in a lens polariscope and remedies

Controlled density methods in photoelasticity are influenced by variations in the uniformity of the maximum light intensity reaching the photographic film. Such nonuniform illumination is caused by nonuniform transmission of light by model and polariscope elements, diffraction and obstruction of light by the edges of the elements and camera shutter, and internal reflections of light from lenses and covering glasses. Effects appear as a peripheral zone of generally reduced intensity and a bright central spot. The peripherical zone may be avoided by the use of reduced size of field, while remedies for the bright spot may include a suitably chosen distance between first field lens and model, a small camera aperture and a translucent screen between light source and first field lens.