The method involves arranging homogeneously designed calibration bodies (1) in an optical path (2), where absorption of the bodies is distinctive such that absorption capacity of one of the bodies is smaller and absorption capacity of another body is higher than absorption capacity of a plate-shaped material to be measured. Intensity of radiation for detector elements (5) in a detector cell (4) is measured, and the absorption capacities of the bodies are assigned to respective characteristics of the bodies, where the absorption capacities are interpolated between the characteristics. The calibration bodies are made from glass, plastic or metal.
