a method is provided for calibrating a sensor used for surface weight measurement or thickness measurement on a material sheet via transmission measurement and / or reflection measurement . the method may include : providing a two - dimensional calibration sample with a known surface weight ; detection of the transmission values and / or the reflection values of the calibration sample via the sensor on a plurality of different positions two - dimensionally or areally distributed across the surface of the calibration sample ; and the detection of the calibration value for the sensor via calculation of average from the plurality detected transmission values and / or reflection values and the composition of the average transmission and reflection values with the known surface weight of the calibration sample . also provided is a method for measurement of layer thickness and / or surface weight of material sheets using the calibration method , as well as an apparatus for layer thickness measurements and / or surface weight measurement .