PHOTOACOUSTIC METHOD WITH A MEASURING LIGHT HAVING A PREDETERMINED WAVELENGTH RANGE FOR THE DETERMINATION OF PROPERTIES OF AN INHOMOGENEOUS SAMPLE

摘要

The invention relates to a photoacoustic method comprising a measurement light having a predefined wavelength range for determining properties of an inhomogeneous sample, the sample having a mean absorption length μ from the range of 1 to 100 micrometers for the predefined wavelength range, the method comprising the following steps: a) radiating at least one measurement light pulse having a predefined pulse duration and a predefined intensity onto a measurement area of the area F with √Fμ in the surface of the inhomogeneous sample; b) detecting at least one pressure transient at the measurement area, the pressure transient resulting from the absorption of the at least one measurement light pulse in the inhomogeneous sample with production of a pressure wave propagating to the measurement area; c) calculating a value for the energy density absorbed by the sample during the pulse duration from the curve of the at least one pressure transient at the start and at the end of the at least one measurement light pulse; characterized by d) repetition of steps a) to c) for different angles of incidence of the measurement light with respect to the normal of the measurement area, the energy density values determined in c) each being indicated with the angle of incidence; e) modeling the inhomogeneous sample as a stack of layers, each layer being assigned at least a layer thickness and an absorption coefficient, at least one absorption coefficient of a layer being a fitting parameter; f) performing a fitting procedure for the fitting parameters of the stack of layers, the division of the energy density values indicated with the angle of incidence into contributions of the individual layers being varied by variation of the fitting parameters until a predefined consistency criterion is met; g) reading out the fitted fitting parameters as values at least for the depth-resolved absorption coefficient of the inhomogeneous sample.