A method and apparatus for measuring strain on a surface of a substrate utilizes a substrate surface coated with at least one coating layer. The coating layer provides both luminescence and photoelasticity. The coating layer is illuminated with excitation light, wherein longer wavelength light is emitted having a polarization dependent upon stress or strain in the coating. At least one characteristic of the emitted light is measured, and strain (if present) on the substrate is determined from the measured characteristic.