PHOTOACOUSTICS METHOD WITH A MEASURING LIGHT COMPRISING A PREDETERMINED WAVELENGTH RANGE FOR DETERMINING THE PROPERTIES OF AN INHOMOGENEOUS SAMPLE

摘要

The invention relates to a photoacoustic method with a measuring light having a predetermined wavelength range for determining properties of an inhomogeneous sample, wherein the sample has a mean absorption length μ from the interval 1-100 micrometers for the predetermined wavelength range, comprising the steps:a) irradiating at least one measuring light pulse having a predetermined pulse duration and predetermined intensity onto a measuring surface of the surface F with F ¼ of the surface of the inhomogeneous sample;b) detecting at least one pressure transient at the measuring surface, wherein the pressure transient is produced by absorption of the at least one measuring light pulse in the inhomogeneous sample to generate a pressure wave propagating to the measuring surface;c) calculating a value for the energy density absorbed by the sample during the pulse duration from the course of the at least one pressure transient at the beginning and at the end of the at least one measurement light pulse;marked byd) repeating steps a) to c) for different angles of incidence of the measurement light relative to the normal of the measurement surface, wherein the energy density values determined in c) are respectively indexed with the angle of incidence;e) modeling the inhomogeneous sample as a layer stack, wherein each layer is assigned at least one layer thickness and an absorption coefficient, wherein at least one absorption coefficient of a layer is a fit parameter;f) performing a fit procedure for the fit parameters of the layer stack, wherein the distribution of the energy density values indexed to the angle of incidence on contributions of the individual layers is varied by varying the fit parameters until a predetermined consistency criterion is fulfilled;g) reading the fitted fit parameters as values at least for the depth resolved absorption coefficient of the inhomogeneous sample.