When a lens is placed on the so-called reference device, the interference fringes of the “Newton's rings” are seen from top to bottom.

The sub-micron dimensional gap between surfaces creates interference fringes of light, and the different overlapping effects of the reflected light waves create different fringes, which are called Newton's rings. In order to check whether the curvature of a finely honed lens meets the specifications, the curvature quality of the lens can be judged by placing the lens on the lens called the original instrument and observing the Newton's bad, i.e., the interference fringes, that appear from above.