Digital holography as a tool for high-speed high-precision 3D-measurements for industrial applications

Digital multi-wavelength holography is an emerging technology for very precise and fast 3D measurement. Here, we present a novel digital holographic system that uses a 65-Megapixel camera to achieve high resolution measurements on an 18 × 14 mm² field of view resulting in a lateral sampling of ~2 μm in x- and y-direction. Using three single frequency lasers for illumination in a temporal phase shifting scheme, we achieve data acquisition times below 150 ms for full 65- Megapixel 3D-measurements. The choice of the three lasers enables an unambiguous axial measurement range of 400 μm. On a calibrated height standard with a 20 μm step repeatability of <0.01 μm (1 standard deviation) is demonstrated. More challenging and of high interest for industrial applications are measurement samples that consist of surfaces with varying surface roughness, reflectivity or material. These kinds of samples require a sensor with a high dynamic range and pose several geometrical optical challenges: Light from differently reflecting or scattering surfaces travels through the optical system on different paths. Without compensation, this results in small, yet non-neglectable errors in the measured height values. We have applied approaches well described for single-point interferometers to the full-field imaging system used in the presented optical setup. Without a-priori knowledge about surface quality of the sample, we can compensate for these errors. Thus, the presented digital holographic sensor is able to achieve repeatability of ~0.1 μm (1 standard deviation) for height features consisting of rough and specular surfaces.