A method for determining the thickness of a layer in a sample stack of at least two layers with an assembly comprising a light source for illuminating a stack of layers and a detector for detecting light reflected by the stack of layers in a defined first wavelength range, the method comprises a first step of obtaining a calibration curve by the calibrating steps of providing two or more 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; illuminating the reference stacks with light from the light source; and detecting the intensity of light reflected by the reference stacks with the detector in the first wavelength range. Further steps of the method comprise illuminating the sample stack of layers with light from the light source; detecting the intensity of the light reflected by the sample stack of layers with the detector in the first wavelength range; and determining the thickness of the layer comprised in the sample stack of layers from the intensity detected by the detector by means of the calibration curve. The method is characterized in that further calibration curves are obtained of reference stacks, where the thickness of another, different layer is known also, thereby providing a first series of calibration curves in the first wavelength range; a second plurality of calibration curves is obtained in the same way as the first series of calibration curves for a further wavelength range; and the thickness of the layer of the sample stack is determined from the intensity detected by the detector by means of the first and second series of calibration curves.