Automatic selection of focal lengths in a Depth From Defocus measurement system based on liquid lenses

A simple and compact Depth-From-Defocus (DFD) setup, using telecentric illumination and liquid-lens based camera observation, was shown to perform well for 3D shape acquisition over extended measuring range. A further step to ameliorate the system performance is described in this paper. We focused on finding an algorithm to speed up the calibration step of the method, that automatically determines the minimum number of focal lengths to be used in the calibration and measurement procedure. As a result, the calibration is significantly shortened (up to 80% with respect to the original procedure), and the need to manually (and to some extent arbitrarily) select the focal length pairs is overcome. Measurement errors down to 0.73 mm over the measurement depth range of 130 mm, corresponding to 0.55% of the depth range are achieved, in total agreement with the original system.