Traditional topography measurements for rough objects are usually based on point-contact scan. The way of pointcontact scan is usually not a high-efficient way to obtain data. Hence, in practice, only a few traces are measured which result in limited data. In order to obtain topography of sufficient amount of data, the demand for rapid noncontact measurement is increasing. In this work, we propose a dual wavelength digital holography (DWDH) measurement system with a synthetic wavelength of 51µm . We apply the DWDH system for 3D measurement of rougher surfaces and objects with a complex and small-size structure to further explore the measurement capability of DWDH. The proposed system enables the possibility to obtain the entire topography with millions of points of a rough surface in a single noncontact measurement. Firstly, we measure a cylinder workpiece of 10mm radius with a large roughness of Ra = 2.6µm which causes severe noise to measurement. The measured radius is 9.8099mm, which indicates a good accuracy. The mean standard deviation of all measured points is 1.08µm, which indicates a good repeatability and good anti-noise capability of the DWDH system. Then the proposed system is applied to measure the tooth surface of a gear of 1mm module and 18 teeth which has a complex and small-size structure. The measured surface shape agrees with the ideal gear tooth surface shape well. And the mean standard deviation of all measured points is 0.94µm , which also verifies its good repeatability. Our work presents the possibility of broader DWDH applications in industrial topography measurements.

In single-wavelength digital holography (DH), the phase wrapping phenomenon limits the total object depth that can be measured due to the requirement for well-resolved phase fringes. To address this limitation, dual-wavelength DH is proposed, enabling measurement of much deeper objects. In single-wavelength DH, because the object depth is limited, the depth of focus (DOF) of DH’s optical system at a reconstruction distance is sufficient to cover the object depth. To date, many autofocusing algorithms have been proposed to obtain a correct reconstruction distance. However, in dual-wavelength DH, because the object depth is extended, the DOF at a reconstruction distance cannot cover the extended object depth. The extended object depth can span multiple DOFs, causing partially out of focus object depth. Therefore, in dual-wavelength DH, relying solely on autofocusing algorithms for a single distance is insufficient. But extended autofocusing algorithms, which can autofocus objects through multiple DOFs, are demanded. However, there are no such extended autofocusing algorithms in dual-wavelength DH. Therefore, we propose an extended autofocusing algorithm for dual-wavelength DH based on a correlation coefficient. The proposed algorithm is able to focus the whole object depth when the depth spans multiple DOFs. Through theoretical analysis, simulations, and experiments, the necessity and effectiveness of the proposed algorithm are verified.

Dual-wavelength digital holography (DH) extends the depth range of measurement, but it faces an issue that it can't focus the whole extended depth range. In single-wavelength DH, the depth of focus of a correct reconstruction distance is usually sufficient to cover the whole depth range. Therefore, in single-wavelength DH, autofocusing algorithms which can obtain a correct reconstruction distance have been proposed and can solve the issue of focusing the whole depth range. But in dual-wavelength DH and even multi-wavelength DH, the extended depth range of the measurement often spans multiple depths of the focus of the optical system, which means that the depth of focus of a single reconstruction distance is not capable to cover the whole extended depth range, making part of depth out of focus. Therefore, in dual-wavelength DH, only autofocusing is not enough, but extended focusing is demanded. However, we lack extended focusing approaches in dual-wavelength DH. Therefore, we propose an extended focusing approach based on correlation coefficient (CC) to focus the whole extended depth range in dual-wavelength DH. The proposed approach includes three steps: (1) dividing the extended depth range by the depth of focus of the system; (2) using the CC-based autofocusing approach to find the correct reconstruction distance of each divided region and reconstructing this region; (3) concatenating reconstructed image of each region and constituting the whole-depth-focused image. Through simulation experiments and practical experiments, the effectiveness and necessity of the proposed approach are verified.