Microgeometric measurements of plastic lenses and metallic inserts by optical profilometry

This study focuses on measuring the micro-geometry parameters of plastic lenses and metallic inserts by optical profilometry. The motivation is to seek a correlation between these parameters and the optical performance of the moulded lenses, to improve the injection-moulding fabrication process. A specific issue met in the measurement was the planar orientations. This is important to localise and compare the matching samples but challenging because inserts and lenses exhibit nearly perfect central symmetry and lack usable datum features. The plastic lenses have two spherical cap interfaces, one concave and one convex. To accommodate measurements of either surface, a special holder was designed and 3D printed. The lenses exhibit two diametrically opposed small keys, which are exploited for the planar orientation. Identifying the reference system for the lens is crucial for defining the measurement and comparing micro-geometric with macro-geometric measurements. Here we discuss the method developed to set the reference system in spite of the spherical symmetry, to enable hybrid metrology techniques and measure the lenses both optically and with a contacting coordinate measuring machine. Profilometry measurements were performed on a set of 12 lenses mould from 4 different insert pairs, using a 20x objective in coherent scanning interferometry (CSI) mode. The morphological parameters examined include Power Spectral Density (PSD), roughness, and texture parameters according to ISO 21920 and ISO 25178, respectively.