Refocus criterion in dual-wavelength digital holographic interferometry for accurate shape measurement

One classical application of digital holographic interferometry is the measurement of shape from multi-wavelength recordings. One crucial step in the process is the refocusing to the correct image plane, where failure in finding the correct focus may lead to dimensional errors and lower accuracy. The use of sufficient metrics is therefore required. This paper proposes a new approach utilizing the linear relationship between in-plane speckle motion and defocus as a response to an induced phase gradient. Two simple metrics, based on the cross-correlation properties of the reconstructed speckle images, are proposed and evaluated utilizing both simulations and experiments. It is shown that both metrics perform well and manages to estimate the true reconstruction distance.