Thermoforming generally describes the shaping of a thermoplastic semifinished part above its softening temperature to a three dimensional part. The sup-step of heating the sheet can be seen as the very crucial process step. In most thermoforming machines, the sheet is heated by means of infrared radiation inside a radiation furnace. Depending on the thickness and the absorption characteristics of the sheet material as well as the working temperature of the radiators, different temperature profiles across the sheet thickness will occur. The knowledge of this temperature distribution is important to control the heating step since the sheet temperature directly influences process stability and part quality like sagging, burnings, orientations and wall thickness distribution of the part or accurate reproduction of surface details. In this paper a new approach by combining three different infrared sensors for determining the temperature distribution across the sheet thickness at any time of the heating phase will be presented. At first, the different amounts of radiation will be calculated in order to separate reflected and transmitted radiation from the actual measuring signals. At second the sheet will be divided into a certain number of layers and the amount of emitted radiation of each layer will be calculated and compared to measured signals in order to finally determine the real temperature distribution across sheet thickness. Calculated and measured results will be shown using polypropylene as sheet material.