The substrate temperature is one of the key parameters, which determines the optical properties of deposited dielectric coatings. Depending on the employed material and application, the accurate knowledge of the substrate temperature is crucial for the system performance. Currently, the heating process inside the evacuated deposition plant is usually performed by radiation sources. The temperature of the substrates is governed be their emission and absorption characteristics and in minor degree, by heat conduction to the substrate holders. Pyrometric methods and temperature sensors are commonly used to measure the temperature close to the substrates. In any case, the measured temperature can deviate extremely from the real substrate temperature. In particular, if materials with a high transmittance in the IR range are used, the heating process can be controlled only with a large error by the conventional methods. In this contribution, an adapted method using optical characterisation of the substrate is presented to determine precisely the substrate temperature. In the described method, the optical transmittance of the components is measured and the absorption characteristic in the VUV and visible spectral range is used to evaluate the current substrate temperature.