This work is focused on the issue of non-measured points – one of the most important problems in surface texture measurements using optical methods. The fundamental aim of this research is to analyse errors of surface texture measurements caused by the presence of non-measured points. This study is divided into two parts. In the first part, circles with non-measured points were artificially created on peak portions of measured surfaces. In the second part – the results of measurement by a Talysurf CCI Lite interferometer were analysed. A measurement area of 3.3 × 3.3 mm contained 1024 × 1024 points. The measurements were performed with different intensity of light. Changes of parameters regarding the analysed errors depended on a surface type. The following parameters are susceptible to errors: skewness Ssk, areal material ratio Smr, as well as the following feature parameters: Spd, Sda, Sdv, Sha and Shv. Inaccuracies of measurement in valley parts of two-process textures led usually to larger errors of parameter computations compared with deviations in peak portions.

Non-measured points (NMPs) are one of vital problems in optical measurement. The number and location of NMPs affect the obtained surface texture parameters. Therefore, systematic studying of the NMP is meaningful in understanding the instrument performance and optimizing measurement strategies. This paper investigates the influence of measurement settings on the non-measured points ratio (NMPR) using structured illumination microscopy. It is found that using a low magnification lens, high exposure time, high dynamic range (HDR) lighting levels, and low vertical scanning interval may help reduce the NMPR. In addition, an improved approach is proposed to analyze the influence of NMP on areal surface texture parameters. The analysis indicates that the influence of NMP on some parameters cannot be ignored, especially for extreme height parameters and feature parameters.