Measurement of wedge and decentre of an aspheric optic using a multiwavelength interferometer
Abstract
This paper investigates the feasibility of measuring wedge and decentre of an aspheric optic using a multiwavelength interferometer in a single, unified setup where surface form, wedge, and decentre are measured in succession. Wedge and decentre errors of an aspherical lens were measured on a multiwavelength interferometer using a method called through-the-lens (TTL) measurement. This measurement principle is based on the absolute measurement capability of multiwavelength interferometry (MWLI) where high density 2D and 3D shape information (surface form) and geometric parameters are utilized to measure decentre error (coaxiality) and any inclination of the surfaces with respect to the optical axis. To validate the results from this measurement approach, wedge and decentre of this aspherical lens was also measured using a sophisticated fixture comprising of three spheres that forms a reference ring. The lens was placed in a customized adapter plate which was mounted within the reference ring. Surface form of one side of the lens and its positioning in relation to this reference ring was measured. Thereafter, surface form of the other side of the lens was measured with respect to the reference ring by flipping the fixture upside-down. Due to absolute measurement capability of the interferometer, the results from both sides of the lens were correlated to determine wedge and decentre errors of the two surfaces. It was observed that there is a good agreement between the two methods used for evaluating wedge and decentre errors.