Freeform measurements with tactile and optical sensors on a cylinder coordinate measuring instrument

Recently, a new high accuracy cylinder coordinate measuring machine was presented for the measurement of high precision aspheric lenses. The high accuracy of the instrument is based on an internal compensation system, i.e. any nonsystematic movement of the axis is measured and these data are used to correct the measurement data. For the measurement the sample is clamped on a rotational axis and with two linear axis and a point sensor linear and circular scans can be collected. Thus a grid of data points over the surface of the sample describes the topography. Important is that the system can be equipped with both a tactile and a new type of optical point sensor. As it is desirable to measure optical components without contact the optical sensor is usually favoured. However, the optical sensor has other limitations, e.g. the slope of the surface, than the tactile sensor, thus the application of both sensors increases the range applications. Performing the same measurement task with both sensors allows to compare the performance of the two sensors. In principle, the system is not limited to rotational symmetric samples, i.e. with the combined movement of the rotational and the two linear axis also freeforms can be measured. Considering the measuring strategy freeforms may be categorized in off-axis aspheres, cylinder and toroidal shapes and others. Applying the cylinder coordinate instrument to measure off-axis aspheres, two different approaches can be described. One approach for off-axis aspheres is to center the aspheric sample on the rotational axis. This can be easily performed with help of the automatic centering and tilting table on top of the rotational axis included in the system. However, if the sample only consist of an outer segment of an asphere it will not be possible to center the sample. The measuring process itself is in principle the same as for rotational symmetric objects. One difference is that for the outer circles the probe does not detect a surface on a part of its circular trace. The other approach is to consider the off-axis asphere as a freeform. Without any centering the three-dimensional data are collected by turning the sample and following the surface with the probe by moving the linear axis. A following analysing process will fit the measured data into the nominal shape or relates the data to the mounting device or reference marks which can be measured too. Measurements with the tactile and optical sensor applying both approaches are presented. The results reveal the performance of this system and the two sensor types to measure offaxis aspheres. For the measurement of cylinders and toroidal shapes it is also useful to center the samples in order to be able to collect two linear profiles describing the two base contours (line and circle for the cylinder) independent of the 3D Data. The 3D-topography is than measured considering a freeform by a set of polar traces. For e.g. rectangular outer shapes these circles may also be interrupted. Results of the measurement of cylinders again for the tactile probe in comparison with the optical probe are presented and discussed. These investigations demonstrate the capability of the system in a contact and non-contact mode for variety of applications in the field of free-form metrology.