By means of an interferometric line sensor system, the form of a specimen can be measured by stitching several
overlapping circular subapertures to form one 3D topography. This concept is very flexible and can be adapted to many
different specimen geometries. The sensor is based on a Michelson interferometer configuration that consists of a rapidly
oscillating reference mirror in combination with a high-speed line-scan camera. Due to the overlapping areas, movement
errors of the scan axes can be corrected.
In order to automatically adjust the line sensor in such a way that it is perpendicular to the measurement surface at a
fixed working distance, a white-light interferometer was included in the line-based form-measuring system. By means of
a fast white-light scan, the optimum angle of the sensor (with respect to the surface of the specimen) is determined in
advance, before scanning the specimen using the line-based sinusoidal phase shifting interferometer. This produces
accurate measurement results and makes it possible to also measure non-rotational specimens.
In this paper, the setup of the line-based form-measuring system is introduced and the measurement strategy of the
sensor adjustment using an additional white-light interferometer is presented. Furthermore, the traceability chain of the
system and the main error influences are discussed. Examples of form measurement results are shown.
The measurement of optical flats, e. g. synchrotron or XFEL mirrors, with single nanometer topography uncertainty is still challenging. At PTB, we apply for this task small-angle deflectometry in which the angle between the direction of the beam sent to the surface and the beam detected is small. Conventional deflectometric systems measure the surface angle with autocollimators whose light beam also represents the straightness reference. An advanced flatness metrology system was recently implemented at PTB that separates the straightness reference task from the angle detection task. We call it ‘Exact Autocollimation Deflectometric Scanning’ because the specimen is slightly tilted in such a way that at every scanning position the specimen is ‘exactly’ perpendicular to the reference light beam directed by a pentaprism to the surface under test. The tilt angle of the surface is then measured with an additional autocollimator. The advantage of the EADS method is that the two tasks (straightness reference and measurement of surface slope) are separated and each of these can be optimized independently. The idea presented in this paper is to replace this additional autocollimator by one or more electro-mechanical tiltmeters, which are typically faster and have a higher resolution than highly accurate commercially available autocollimators. We investigate the point stability and the linearity of a highly accurate electronic tiltmeter. The pros and cons of using tiltmeters in flatness metrology are discussed.
There is an increasing demand for accurate form measurements of large optics with dimensions of up to one meter. For example, reference flats of Fizeau interferometers have to be known in the nanometer range. Synchrotrons or XFEL mirrors are often more curved, but nevertheless have to be manufactured and measured at a comparable or even better level of accuracy. Different approaches like deflectometry or interferometry in combination with stitching methods are typically used for these purposes, but obtaining the low uncertainty levels needed is still a considerable challenge. In this paper, measuring concepts and systems used at PTB for these purposes will be presented. For flatness measurements, a so-called Deflectometric Flatness Reference (DFR) system and a Fizeau interferometer were used. Slightly curved surfaces can be measured with the Traceable Multi Sensor (TMS) method, and CMMs with point or line sensors were also available. We will also present the current measurement capabilities and some measurement examples of form measurements of optical surfaces at PTB. The different setups and their pros and cons will be discussed. Future developments in the field of large-optics measurement will also be shown.