the invention relates to a structure characterising device comprising means which are used for generating a first pump radiation and a second probe radiation and for transmitting different wavelength radiation , means for producing a time offset between said first pump and second probe radiation on the structure by means of detecting means of said second beam after the reflection or transmission thereof to said structure in such a way that an analysis signal is generated , means for processing said signal and identifying an area corresponding to the signal jump , for determining the jump amplitude according to different wavelengths , for comparing said amplitude with a theoretical amplitude variation pattern according to the wavelengths and for determining , for the wavelength characteristic for said theoretical pattern , a characteristic value associated to the structure thickness and to the radiation propagation velocity in said structure .