Parametric characterization of ground surfaces with laser speckles.

With well-known speckle measurement techniques, the root mean square height as well as the autocorrelation length of isotropic surfaces can be determined quickly and over a large area of interest. Beyond that, the present article studies the speckle-based measurement of anisotropic surfaces. For this purpose, a measurement setup and evaluation algorithm are presented that enable the characterization of unidirectionally anisotropic surfaces machined by grinding. As a result, four measurands are obtained from one speckle image: the machining direction, the autocorrelation length perpendicular to the machining direction, as well as two root mean square roughness parameters parallel and perpendicular to the machining direction. The first two measurands are obtained from a two-dimensional fast Fourier transform of the diffraction pattern resulting from the unidirectional tool marks and the latter two by a bidirectional evaluation of the speckle contrast. In addition to measurements on physical reference samples, a spatial light modulator is used to create a large number of surface topographies with known model parameters in order to quantify the measurement uncertainty.

Noise reduction in high-resolution speckle displacement measurements through ensemble averaging.

To measure surface displacement on micro samples, a non-invasive method with both a low displacement measurement uncertainty below 100 nm and high spatial resolution of around 20 µm is required. In digital image speckle correlation, both requirements can be fulfilled individually but not simultaneously. To lower the displacement measurement uncertainty without deteriorating the spatial resolution, an ensemble averaging technique over multiple uncorrelated speckle patterns is presented. To generate and reproduce different speckle patterns, two concepts for the respective modulation of laser light illumination are investigated: a low-cost concept with a rotating glass diffuser, as well as a faster concept using a digital micromirror device combined with a stationary diffuser with a maximum pattern rate of 17.9 kHz. Both setups lead to a measurement uncertainty reduction by one order of magnitude over a wide range of spatial resolutions. As a result, displacements in the micrometer range are measured with a measurement uncertainty of 40 nm and spatial resolution of 20 µm.