A method for determining a layer thickness of a sample during processes causing a shrinking or growing of said layer thickness, the method comprising: a) providing the sample inside a processing chamber , wherein the sample comprises a semiconducting wafer and wherein the sample is a AlGaN layer or a p GaN layer; b) etching or depositing the sample in the processing chamber , wherein the etching is atomic layer etching and the depositing is atomic layer deposition; c) wherein said the etching or depositing is performed in a cyclic mode, d) wherein during the cyclic etching or cyclic depositing of the sample , the following steps e) to i) are carried out: e) irradiating optical radiation onto the sample , the optical radiation comprising light of a first wavelength (λ1) and light of a second wavelength (λ2) the first wavelength (λ1) being different from the second wavelength (λ2); f) measuring a first reflection signal from the sample with respect to the first wavelength (λ1); g) measuring a second reflection signal from the sample with respect to the second wavelength (λ2); h) calculating a thickness of the sample based on the first reflection signal and the second reflection signal; i) each of the first wavelength (λ1) and the second wavelength (λ2) is selected from a range of 300 nm to 370 nm, wherein j) a look-up table is used for calculating a thickness of the sample , the look-up table including correlation data of the thickness of the sample and respective measured reflectance signals at the first wavelength (λ1), wherein the used look-up table is generated by averaging the measured reflectance data with an averaging time that is identical to cycle time. a) providing the sample inside a processing chamber , wherein the sample comprises a semiconducting wafer and wherein the sample is a AlGaN layer or a p GaN layer; b) etching or depositing the sample in the processing chamber , wherein the etching is atomic layer etching and the depositing is atomic layer deposition; c) wherein said the etching or depositing is performed in a cyclic mode, d) wherein during the cyclic etching or cyclic depositing of the sample , the following steps e) to i) are carried out: e) irradiating optical radiation onto the sample , the optical radiation comprising light of a first wavelength (λ1) and light of a second wavelength (λ2) the first wavelength (λ1) being different from the second wavelength (λ2); f) measuring a first reflection signal from the sample with respect to the first wavelength (λ1); g) measuring a second reflection signal from the sample with respect to the second wavelength (λ2); h) calculating a thickness of the sample based on the first reflection signal and the second reflection signal; i) each of the first wavelength (λ1) and the second wavelength (λ2) is selected from a range of 300 nm to 370 nm, wherein j) a look-up table is used for calculating a thickness of the sample , the look-up table including correlation data of the thickness of the sample and respective measured reflectance signals at the first wavelength (λ1), e) irradiating optical radiation onto the sample , the optical radiation comprising light of a first wavelength (λ1) and light of a second wavelength (λ2) the first wavelength (λ1) being different from the second wavelength (λ2); f) measuring a first reflection signal from the sample with respect to the first wavelength (λ1); g) measuring a second reflection signal from the sample with respect to the second wavelength (λ2); h) calculating a thickness of the sample based on the first reflection signal and the second reflection signal; i) each of the first wavelength (λ1) and the second wavelength (λ2) is selected from a range of 300 nm to 370 nm, wherein j) a look-up table is used for calculating a thickness of the sample , the look-up table including correlation data of the thickness of the sample and respective measured reflectance signals at the first wavelength (λ1), <EOS>