A process and mask for use in the process making it possible optically to measure precisely and rapidly sizes in elevation. A group of fringes of regular light, in uniform translation at constant velocity, is projected on the object to be measured and the image of the projected fringes is observed. The measurement of the phase difference of the reflected or diffused light signal at two points of the masked image plane is proportional to a variation in the relative height of the two corresponding object points, thereby allowing comparison to a control piece. The process makes it possible to measure absolute or relative sizes in elevation at two critical points of pieces in a production line. It also makes it possible to determine the profile of any piece.