Shape measurement method of optical components using background shifted shadowgraph

A novel, non-contact, simple, rapid, and quantitative method for measuring the shape of optical components is proposed. Mass-produced optical components manufactured by injection molding processes may have individual differences in refractive index distribution due to shape errors caused by sink marks and warping, as well as residual stresses. Therefore, quality control through measurement is needed for Cyber-Physical Production Systems. In this study, a slight shift of the background pattern was added to the conventional background-oriented Schlieren method, which has the advantage of being able to measure the deflection of light rays caused by existence of an object with a different refractive index than its surroundings using a simple optical system. This approach eliminates the need for a reference material whose refractive index matches that of a measurement object, and enables quantitative measurement of a physical quantity related to the spatial second derivative of the refractive index of the object, which is visualized only qualitatively by shadowgraphy. This method, named background shifted shadowgraph (BSS), accurately measures surface curvatures with radii of approximately 50–100 mm for uniform refractive index objects, with less than 5% error ratio confirmed experimentally. Due to its simplicity, speed, and quantitative capabilities, the BSS method holds strong potential for application in automated inline inspection systems.