Optical form measurements with a scanning point sensor in null configuration

We present measurements of the form of optical surfaces using a scanning chromatic confocal point sensor. The motion of the coordinate measuring machine (CMM) is controlled to accomplish a fixed distance between the sensor and the specimen at each measurement point. The results show that this null sensor configuration enables measurements with a few ten nanometres repeatability up to a maximum acceptable surface slope of 30° limited by the sensor. Individually measured line profiles can be merged together to yield a 3D topography. The system does not require any prior knowledge of the surface form in contrast to many interferometric procedures. It can be applied to various specimen geometries by adapting the parameters of the movement system. The measuring system was tested using calibrated spherical surfaces. In a first application, we investigated recently developed asphere and freeform transfer standards. The system has the capability for measurements in the sub-µm range and will be used to test and characterize asphere and freeform standards in future.