Abstract: Relative retardation resulting from passage of polarized white light through a stressed transparent material is measured automatically, by dividing the emerging polychromatic light into plurality of component beams, each containing one wavelength (or color) only, transforming the light intensity carried by each component beam into an electrical signal, and using these electrical signals to measure the relative retardation, proportional to the stress in the transparent material.Means are provided to compare the spectral distribution of the light intensity measured at several wavelengths to the calculated distribution using an assumed value of retardation. Iterative (repetitious) calculations are performed until the measured distribution and calculated, using assumed retardation, agree. The retardation yielding agreement with the measured distribution is the measured retardation reflecting the stress in the material.