Method for calibrating a pyrometer adapted to be situated outside a processing chamber and to intercept and measure thermal radiation from a sample positioned inside the processing chamber, the radiation travelling through a window before reaching the pyrometer, wherein the pyrometer has a first pyrometer channel working at a first effective wavelength (λ 1) and a second pyrometer channel working at a second effective wavelength (λ 2), the first wavelength and the second wavelength being different from each other, whereby the effective wavelength is defined as the maximum of the convolution of the detector's spectral sensitivity and the Planck's curve of the thermal radiation, averaged over a certain range of interest, the method comprising: a) pre-calibrating the first pyrometer channel by means of a calibration sphere with a cold LED light source; b) providing a sample inside the processing chamber, wherein the sample comprises a semiconducting wafer, c) heating the sample to a first temperature, the first temperature scanning over the complete temperature range of the pyrometer, d) measuring a first signal from the sample for different temperatures within the scanned temperature range and assigning each value of the first temperature to the corresponding temperature measured by the first pyrometer channel, wherein a bandwidth of the first pyrometer channel is smaller than 30 nm, e) measuring a second signal from the sample by the second pyrometer channel, wherein a bandwidth of the second pyrometer channel is greater than 200nm, f) calculating a corrected value of the second temperature (T2 corr) measured by the second pyrometer channel based on the first temperature measured by the first pyrometer channel, thereby calibrating the pyrometer, wherein the pyrometer further comprises an additional modulated light source and a filter for selecting the first wavelength λ 1, and wherein the method further comprises: g) calculating the reflectance R of the sample, wherein R= α 0 · D 0 is based on a measured signal D 0 at a clean window and a calibration factor α 0, h) calculating a corrected calibration factor α c based on the reflectance R and a measured signal D c received through the window including a coating, i) calculating a transmittance Tw 0 through a clean window and the transmittance Tw c through the window affected by the coating from the calibration factor α 0 and the corrected calibration factor α c, and j) calculating the real intensity of thermal radiation I p 0 at the first temperature T1 by using the reduced intensity of thermal radiation I p c, the transmittance Tw 0 through a clean window and the transmittance Tw c through the window affected by the coating, according to the formula I p 0 = (T w 0 /T w c) · I p c. a) pre-calibrating the first pyrometer channel by means of a calibration sphere with a cold LED light source; b) providing a sample inside the processing chamber, wherein the sample comprises a semiconducting wafer, c) heating the sample to a first temperature, the first temperature scanning over the complete temperature range of the pyrometer, d) measuring a first signal from the sample for different temperatures within the scanned temperature range and assigning each value of the first temperature to the corresponding temperature measured by the first pyrometer channel, wherein a bandwidth of the first pyrometer channel is smaller than 30 nm, e) measuring a second signal from the sample by the second pyrometer channel, wherein a bandwidth of the second pyrometer channel is greater than 200nm, f) calculating a corrected value of the second temperature (T2 corr) measured by the second pyrometer channel based on the first temperature measured by the first pyrometer channel, thereby calibrating the pyrometer, wherein the pyrometer further comprises an additional modulated light source and a filter for selecting the first wavelength λ 1, and wherein the method further comprises: g) calculating the reflectance R of the sample, wherein R= α 0 · D 0 is based on a measured signal D 0 at a clean window and a calibration factor α 0, h) calculating a corrected calibration factor α c based on the reflectance R and a measured signal D c received through the window including a coating, i) calculating a transmittance Tw 0 through a clean window and the transmittance Tw c through the window affected by the coating from the calibration factor α 0 and the corrected calibration factor α c, and j) calculating the real intensity of thermal radiation I p 0 at the first temperature T1 by using the reduced intensity of thermal radiation I p c, the transmittance Tw 0 through a clean window and the transmittance Tw c through the window affected by the coating, according to the formula I p 0 = (T w 0 /T w c) · I p c.