CHARACTERIZING THE PERFORMANCE OF A CONFOCAL ACOUSTIC MICROSCOPE

The transfer functions of confocal optical imaging systems are applied to equivalent ultrasonic imaging systems. The ideal complex amplitude line-scan signals across a step edge with various degrees of defocus, excluding attenuation and aberrations, are generated. The scalar magn itudes of the resulting ideal complex image profiles are differentiate d to obtain the unique theoretical impulse response functions (IRFs) a nd a Fourier transform is applied to the data to obtain the correspond ing transfer functions (TFs). The experimental single-frequency and br oadband image profiles for a step edge are differentiated and a Fourie r transform is applied to obtain TFs. The effects of attenuation and a berrations are investigated and the experimental data are compared wit h the theoretical data. The relationship between broadband and single- frequency acoustic images is described. A practical model for the dete ction limits for 2D cracks obtained from the response for a pair of st ep edges is proposed. The numerical calculations are compared with exp erimental data. The strong contrast and small limit of detection usual ly seen in the imaging of surface cracks are associated with the excit ation of surface acoustic waves (SAWs). In this study, a similar sensi tivity to detect cracks is achieved by the defocus effect of pure long itudinal waves, which can be clearly explained by theory.