A novel and rapid method for the evaluation of the stress-optical constants, with high accuracy, and furthermore, of the principal stresses in plane-stress fields, is developed. A monochromatic source is used. A collimated light beam incident at the tip of a crack, which exists at the edge of a thin plate under plane-stress conditions, forms a caustic projected on a screen placed at either side of the plate. Loading of the specimen will increase the size of the constraint zone and consequently the size of the caustic until it becomes measurable with conventional tools and with high accuracy. The ratio of the size of the two caustics, one obtained with the transmitted-shadow method and the other obtained with the reflected-shadow method, will give the stress-optical constants directly, for optically isotropic and anisotropic materials, with the use of a chart which is also developed by the authors.