Fast 3D form measurement using a tunable lens profiler based on imaging with LED illumination

Extended-Aperture Shape Measurements Using Spatially Partially Coherent Illumination (ExASPICE)

We have recently demonstrated that the 3D shape of micro-parts can be measured using LED illumination based on speckle contrast evaluation in the recently developed SPICE profilometry (shape measurements based on imaging with spatially partially coherent illumination). The main advantage of SPICE is its improved robustness and measurement speed compared to confocal or white light interferometry. The limited spatial coherence of the LED illumination is used for depth discrimination. An electrically tunable lens in a 4f-configuration is used for fast depth scanning without mechanically moving parts. The approach is efficient, takes less than a second to capture required images, is eye-safe and offers a depth of focus of a few millimeters. However, SPICE's main limitation is its assumption of a small illumination aperture. Such a small illumination aperture affects the axial scan resolution, which dominates the measurement uncertainty. In this paper, we propose a novel method to overcome the aperture angle limitation of SPICE by illuminating the object from different directions with several independent LED sources. This approach reduces the full width at half maximum of the contrast envelope to one-eighth, resulting in a twofold improvement in measurement accuracy. As a proof of concept, shape measurements of various metal objects are presented.

Colorful 3D Reconstruction and an Extended Depth of Field for a Monocular Biological Microscope Using an Electrically Tunable Lens

This paper presents a monocular biological microscope with colorful 3D reconstruction and an extended depth of field using an electrically tunable lens. It is based on a 4f optical system with an electrically tunable lens at the confocal plane. Rapid and extensive depth scanning while maintaining consistent magnification without mechanical movement is achieved. We propose an improved Laplacian operator that considers pixels in diagonal directions to provide enhanced fusion effects and obtain more details of the object. Accurate 3D reconstruction is achieved using the shape-from-focus method by tuning the focal power of the electrically tunable lens. We validate the proposed method by performing experiments on biological samples. The 3D reconstructed images obtained from the biological samples match the actual shrimp larvae and bee antenna samples. Two standard gauge blocks are used to evaluate the 3D reconstruction performance of the proposed method. The experimental results show that the extended depth of fields are 120 µm, 240 µm, and 1440 µm for shrimp larvae, bee tentacle samples, and gauge blocks, respectively. The maximum absolute errors are -39.9 μm and -30.6 μm for the first and second gauge blocks, which indicates 3D reconstruction deviations are 0.78% and 1.52%, respectively. Since the procedure does not require any custom hardware, it can be used to transform a biological microscope into one that effectively extends the depth of field and achieves highly accurate 3D reconstruction results, as long as the requirements are met. Such a microscope presents a broad range of applications, such as biological detection and microbiological diagnosis, where colorful 3D reconstruction and an extended depth of field are critical.

Flash-profilometry: fullfield lensless acquisition of spectral holograms for coherence scanning profilometry

Flash-profilometry is a novel measurement approach based on the fullfield lensless acquisition of spectral holograms. It is based on spectral sampling of the mutual coherence function and the subsequent calculation of its propagation along the optical axis several times the depth-of-field. Numerical propagation of the entire coherence function, rather than solely the complex amplitude, allows to digitally reproduce a complete scanning white-light interferometric (WLI) measurement. Hence, the corresponding 3D surface profiling system presented here achieves precision in the low nanometer range along an axial measurement range of several hundred micrometers. Due to the lensless setup, it is compact, immune against dispersion effects and lightweight. Additionally, because of the spectral sampling approach, it is faster than conventional coherence scanning WLI and robust against mechanical distortions, such as vibrations and rigid body movements. Flash-profilometry is therefore suitable for a wide range of applications, such as surface metrology, optical inspection, and material science and appears to be particularly suitable for a direct integration into production environments.