A non-storage type image pick-up tube line scans a sample for picking up the fringes. A circuit responds to the maximum and minimum values of the output from the image pick-up tube generating a signal corresponding to the brightness of the fringes, signals corresponding to the positions of the bright and/or dark spots of the fringes, and signals corresponding to the spacings between a reference point and the positions of the bright and/or dark spots of the fringes.