Calibration of horizontally-placed planarity standards

Fused silica plates are used in most metrology laboratories as planarity standard. Usually, these standards are calibrated in upright position, to minimize bending effects due to gravity. On the other hand, for particular applications or very large flats, it would be useful to know the absolute surface error when they are supported in horizontal position, i.e., with their axis vertical. Here we report a study regarding this setup for planarity calibration, comparing two different experimental methods with respect to predictions of finite elements analysis.