Robust measurement of surface topography for additive manufacturing using imaging confocal microscopy

Abstract

Additive manufacturing (AM) has brought a new level of innovation to the production of metal parts, allowing for greater design freedom and on-demand fabrication capabilities. However, the measurement of the resulting surface texture has increased difficulties. Whilst optical techniques provide fast measurements, the high surface irregularities, the large range of scales of the features, and the high variations in surface reflectivity largely compromise performance. In practice, conventional systems have an insufficient field-of-view (FOV) or produce an excessive amount of measurement artifacts. We present an optical system optimized to measure AM parts, featuring a sufficiently wide field-of-view to capture the surface scales of interest, and a high numerical aperture (NA). We integrated a commercially available objective (SHR Plan Apo 2×, Nikon) with a custom-designed tube lens in a measuring head, achieving a magnification of 4.2× and an effective NA of 0.26. The system implements the confocal-like HiLo technique, and we experimentally demonstrate successful characterisation of metal AM parts. Furthermore, we report and discuss the design of a 6×/0.45 NA microscope objective, with great promise for increased performance. Additionally, we explore high dynamic range (HDR) approaches to tackle the issue of the high reflectivity variation. Multiple low dynamic range images taken at a range of illumination intensities are combined to reconstruct a single topography map. In all, the combination of the optical system, capable of capturing a large scale of spatial features, with the HDR capability offers a versatile solution that can be used in a variety of scientific and industrial contexts.