A method for determining a thickness of a layer of a fluorescent substance, wherein the fluorescent substance has a wavelength-dependent absorption coefficient (α) and a wavelength-dependent fluorescence (P em), wherein the absorption coefficient (α) has a non-zero value at least at a first or a second fluorescence wavelength, and the method comprising the steps of A) exciting the layer with an electromagnetic excitation radiation, B) detecting a first power of fluorescence radiation comprising a first fluorescence wavelength emitted by the layer, C) detecting a second power of fluorescence radiation comprising a second fluorescence wavelength emitted from the layer, wherein the first fluorescence wavelength and the second fluorescence wavelength are different from each other, and wherein the first power and the second power have non-zero values, D) forming a quotient from the first power and the second power, and E) deriving the thickness of the layer from the quotient, wherein the excitation radiation comprises an excitation wavelength, and wherein the excitation wavelength and the first and second fluorescence wavelengths are different from each other, characterised in that the fluorescent substance is a coating agent in the form of an oil droplet, wherein the oil drop is placed on a reflective background, wherein the detected first power and second power is based solely on the intrinsic fluorescence of the fluorescent substance, such that no additional dyes are added to the fluorescent substance, and wherein an optical path length of the fluorescence radiation is evaluated so that the thickness of the layer derived in step E) is independent of the orientation of the sample. the method comprising the steps of A) exciting the layer with an electromagnetic excitation radiation, B) detecting a first power of fluorescence radiation comprising a first fluorescence wavelength emitted by the layer, C) detecting a second power of fluorescence radiation comprising a second fluorescence wavelength emitted from the layer, wherein the first fluorescence wavelength and the second fluorescence wavelength are different from each other, and wherein the first power and the second power have non-zero values, D) forming a quotient from the first power and the second power, and E) deriving the thickness of the layer from the quotient, wherein the excitation radiation comprises an excitation wavelength, and wherein the excitation wavelength and the first and second fluorescence wavelengths are different from each other, A) exciting the layer with an electromagnetic excitation radiation, B) detecting a first power of fluorescence radiation comprising a first fluorescence wavelength emitted by the layer, C) detecting a second power of fluorescence radiation comprising a second fluorescence wavelength emitted from the layer, wherein the first fluorescence wavelength and the second fluorescence wavelength are different from each other, and wherein the first power and the second power have non-zero values, D) forming a quotient from the first power and the second power, and E) deriving the thickness of the layer from the quotient, wherein the excitation radiation comprises an excitation wavelength, and wherein the excitation wavelength and the first and second fluorescence wavelengths are different from each other, wherein the excitation radiation comprises an excitation wavelength, and wherein the excitation wavelength and the first and second fluorescence wavelengths are different from each other, characterised in that the fluorescent substance is a coating agent in the form of an oil droplet, wherein the oil drop is placed on a reflective background, wherein the detected first power and second power is based solely on the intrinsic fluorescence of the fluorescent substance, such that no additional dyes are added to the fluorescent substance, and wherein an optical path length of the fluorescence radiation is evaluated so that the thickness of the layer derived in step E) is independent of the orientation of the sample. the fluorescent substance is a coating agent in the form of an oil droplet, wherein the oil drop is placed on a reflective background, wherein the detected first power and second power is based solely on the intrinsic fluorescence of the fluorescent substance, such that no additional dyes are added to the fluorescent substance, and wherein an optical path length of the fluorescence radiation is evaluated so that the thickness of the layer derived in step E) is independent of the orientation of the sample.