Abstract: In the Newton's ring experiment, the diameter of the circular fringe is the fundamental data of measurement. In order to improve the accuracy of measurement, this article theoretically analyzes the reasons that cause systematic errors: firstly, the diameter does not pass through the center of the circular fringe (introducing different chord-center distances), and secondly, the two endpoints of the chord deviate from the position of the extreme light intensity. So in theory, a new method is proposed to completely eliminate the above errors in Newtons ring experiment: replacing the diameters with the chord length of all circular fringes with equal chord-center distance, and combining the extreme value of light intensity to locate the two endpoints of the chord. Based on this method, two new measurement arrangements are proposed for the Newtons ring experiment: in the case of using a reading microscope, a common chord for all concentric circular fringes can be added to the Newtons ring instrument as a reference line for measuring the chord length of each circular fringe, and the chord length can be used instead of the diameter to eliminate the first systematic error; in the case of image analysis method, the light intensity distribution on the common chord can be further obtained, and by locating the two endpoints of the chord through the extreme light intensity, the accurate length of the chord of all circular fringes can be obtained, thereby eliminating the above two types of systematic errors. Then, the image analysis method is applied to the newly designed Newtons ring device for measurement, proving that this method is effective and feasible.

Key words: wave optics, newtons ring, light intensity distribution, common chord, extreme value positioning