Over the following years, the spectrum of optical distance sensors in particular was expanded. The laser distance sensors mentioned above were supplemented by chromatic focus sensors for measuring surface contours and flatness on reflective surfaces. That sensor determines the distance to the work piece surface based on the different focal planes of the colors that make up white light, so it is often simplifying referred to as a white-light sensor. Area sensors that use the focus variation principle, such as the Werth 3D Patch, or confocal sensors, such as the Nano Focus Probe (NFP) capture surface topographies with high point density to determine roughness, shape, and other geometric properties. With both measurement principles, one 3D measurement point is determined for each pixel in the camera. To do so, the sensor is moved perpendicular to the work piece surface while the contrast (for focus variation sensors) or image brightness (for confocal sensors) is evaluated to determine the distance to the work piece surface. Line sensors are used to rapidly measure relatively large areas of the work piece surface. The Laser Line Probe (LLP) and now, since 2017, the Chromatic Focus Line Sensor (CFL) is available.  The CFL combines high measuring speed with high accuracy in an unprecedented manner. The various aforementioned measurement principles differ in potential accuracy and primarily in their dependence on the contrast, reflective behavior and inclination angle of the work piece surface. Specifications using uniform scales and certified calibrations allow objective comparison of the performance of various coordinate measuring machines, ensuring traceable measurements.