Laux, PatrickPatrickLauxBertz, AlexanderAlexanderBertzCarl, DanielDanielCarlReichelt, StephanStephanReichelt2025-07-032025-07-032025https://publica.fraunhofer.de/handle/publica/48910710.1117/1.OE.64.7.074101Surface roughness is a critical quality parameter in metal sheet processing, yet the current measurement methods are limited and just capable of assessing areas of up to a few square millimeters in-line. A fast measurement of larger areas can be achieved using the spectral speckle correlation (SSC), where the speckle pattern of the surface is captured at two slightly different wavelengths. It was shown that by correlating these two speckle fields, the surface roughness can be determined: the rougher the surface, the lower the correlation. This allows contact-free measurements, larger working distances, and the identification of local roughness variations within a single measurement, aligning with the areal integrating category of ISO 25178 part 6 standards. Previously, the roughness was evaluated at the image center or optical axis, yielding a single roughness value for the entire image. We demonstrate the SSC technique’s ability to measure the areal roughness parameter Sq of roughness standards ranging from 0.81 to 2.07 μm, with deviations of less than 5% compared with reference values obtained using a white light interferometer. The work includes the derivation of suitable sub-image sizes for spatial resolution, connection to existing standards, and the reduction of surface structure influences through defocused imaging and high-pass filtering. Experimental results show the method’s potential for surface measurements of areas with a 15 mm diameter.enSpectral speckle correlationRoughness measurementLarge surfacesOptical metrologyLaser measurementSurface roughnessjournal article