Chromatic focus variation for surface metrology

Optical metrology plays a vital role in a wide range of research and inspection areas in the industry. The focus variation instrument is a popularly adopted method for additively manufactured parts because it can measure steep surfaces with very high slopes and irregularities. The contrast/sharpness between the neighbouring pixels is used to determine the image focus position, and hence the surface height is obtained. In traditional focus variation instruments, a piezoelectric actuator (PZT) is used for mechanical scanning in image capturing. The slow mechanical movement is a limiting factor for fast measurement in modern advanced manufacturing applications. Another issue associated with the traditional focus variation instruments is the nonlinear hysteresis motion of PZTs and their physical size. To overcome the above limitations, we propose a chromatic focus variation method to axially shift the focus along the optical axis without any mechanical movement using a wavelength scanning mechanism combined with a chromatic objective lens. This work presents initial measurement results obtained using the proposed system and its comparisons with the commercial Alicona focus variation system.