Method for determining the flatness of a material strip

A method for determining flatness of a material strip, the material strip predefining a longitudinal direction, comprising:

• recording measured values at a plurality of measurement points by at least two radiation sources and a plurality of detectors, the measurement points being disposed transverse to the longitudinal direction and spaced apart from each other in the material of the strip and being sensed by at least two detectors, each detector detects radiation at various solid angles,
• moving the material strip in the longitudinal direction relative to the radiation sources and the detectors, and recording rows of measured values substantially encompassing all measurement points at each of several given intervals,
• calculating a slope of the material strip for each recorded measurement point from the measured values of detector pairs,
• calculating a wavelength and a phase of slope changes for successive rows of measured values at a known relative velocity in the longitudinal direction,
• calculating at least one extremum and the respective associated closest-row of extreme measured values from the wavelength and the phase,
• calculating a transverse contour by summing the slope values of the rows of extreme measured values;
• and then determining the amplitude of the transverse contour, and
• calculating an elongation of the material strip from the wavelength and the amplitude of the transverse contour.