PURPOSE: To determine a strain distribution state, by uniformly illuminating a surface to be measured to catch double refraction (strain) by a video camera and performing predetermined operational processing by an image processing apparatus.

CONSTITUTION: The light of a light source 1 is reflected by an oval mirror 2 to uniformly illuminate a specimen 8 through a lens 3, a turn-back mirror 4, a convex lens 5, a diffusion plate 6 and a polarizer 7. The light passing through the specimen 8 is condensed to a video camera 12 through the Nicol detector 9 and the wide angle lens 11 both of which cross the polarizer 7 at a right angle. The image signal output thereof is inputted not only to a CRT 19 but also to an image processing apparatus 14. An image signal is digitized by the apparatus 14 to be inputted to a microcomputer 15 and a CPU 16 calculates the size and code of the double refraction of the entire surface of the specimen 8 as the function of an angle of rotation when the detector 9 is rotated to store the same in a memory 17 and a variable density image wherein the angle of rotation is set to a variable inclusive of the size and code of double refraction is displayed on a display 19.