Small-angle deflectometry with coherent and incoherent illumination

Deflectometry systems can be divided into large-angle deflectometry and small-angle deflectometry. Large-angle deflectometry systems can measure complex mirror geometries with reproducibilities of a few ten nanometres. Small-angle deflectometry systems measure the form of slightly curved surfaces, e.g. for synchrotrons or free-electron laser mirrors, with nanometre or even sub-nm accuracies. The height resolution of deflectrometric systems depends on the angle sensitivity of the sensor. Theoretically, there is no limit to this sensitivity, but in practice there are limitations due to the discretisation of the image sensor, pixel size, readout noise as well as the size of the image spot and the total image size. Coherent illumination, e.g. by lasers, offers the possibility of high radiance which enables short integration times. The disadvantage is that coherent illumination leads to speckle noise. Incoherent illumination such as LED illumination avoids speckle noise, but enables only a low radiance for apertures smaller than 1 mm. Small-angle deflectometric flatness measurements with coherent and incoherent light sources with sub-mm lateral resolution are performed and compared.