Method for determining the thickness of a layer in a sample stack of at least two layers of strained silicon, silicon-germanium, germanium, gallium-arsenide, indium-phosphide, indium-arsenide, indium-gallium-arsenide, mercury-telluride, III-V and II-VI ternary and quaternary semiconductor alloys, other oxides and nitrides, photo resists, thin metal layers, glass, quartz and plastic materials or other materials used in flat products of the semiconductor- and solar cell industry with an assembly comprising a light source for illuminating a stack of layers and a detector (800, 810) for detecting light reflected by the stack of layers in defined first and second wavelength ranges, the method comprising the steps of (a) obtaining a plurality of calibration curves (400, 410, 420, 430, 440, 450) by the calibrating steps of providing more than two reference stacks of layers, where each layer of the reference stacks has a known thickness, the same material as the sample stack and the layers occur in the same order as in the sample stack and wherein the thicknesses of the layers are varied around their nominal values; illuminating the reference stacks with light from the light source; and detecting the intensity of light reflected by the reference stacks resulting from a wave reflection at interfaces between two materials which is described by Fresnel formulas with the detector in a first wavelength range; thereby providing a first series of calibration curves in the first wavelength range; (b) obtaining a second plurality of calibration curves in the same way as the first series of calibration curves for a further wavelength range which is different to the first wavelength range; (c) illuminating the sample stack of layers with light from the light source; (d) detecting the intensity of the light reflected by the sample stack of layers with the detector (800, 810) in the first and the further wavelength ranges (340, 350); and (e) determining the thickness of the layer in the sample stack of layers from the intensity detected by the detector (800, 810) by means of the first and second series of calibration curves. (a) obtaining a plurality of calibration curves (400, 410, 420, 430, 440, 450) by the calibrating steps of providing more than two reference stacks of layers, where each layer of the reference stacks has a known thickness, the same material as the sample stack and the layers occur in the same order as in the sample stack and wherein the thicknesses of the layers are varied around their nominal values; illuminating the reference stacks with light from the light source; and detecting the intensity of light reflected by the reference stacks resulting from a wave reflection at interfaces between two materials which is described by Fresnel formulas with the detector in a first wavelength range; thereby providing a first series of calibration curves in the first wavelength range; providing more than two reference stacks of layers, where each layer of the reference stacks has a known thickness, the same material as the sample stack and the layers occur in the same order as in the sample stack and wherein the thicknesses of the layers are varied around their nominal values; illuminating the reference stacks with light from the light source; and detecting the intensity of light reflected by the reference stacks resulting from a wave reflection at interfaces between two materials which is described by Fresnel formulas with the detector in a first wavelength range; (b) obtaining a second plurality of calibration curves in the same way as the first series of calibration curves for a further wavelength range which is different to the first wavelength range; (c) illuminating the sample stack of layers with light from the light source; (d) detecting the intensity of the light reflected by the sample stack of layers with the detector (800, 810) in the first and the further wavelength ranges (340, 350); and (e) determining the thickness of the layer in the sample stack of layers from the intensity detected by the detector (800, 810) by means of the first and second series of calibration curves.