Characterising 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 magnitudes of the resulting ideal complex image profiles are differentiated to obtain the unique theoretical impulse response functions (IRFs) and a Fourier transform is applied to the data to obtain the corresponding transfer functions (TFs). The experimental single-frequency and broadband image profiles for a step edge are differentiated and a Fourier transform is applied to obtain TFs. The effects of attenuation and aberrations are investigated and the experimental data are compared with the theoretical data. The relationship between broadband and single-frequency acoustic images is described. A practical model for the detection limits for 2D cracks obtained from the response for a pair of step edges is proposed. The numerical calculations are compared with experimental data. The strong contrast and small limit of detection usually seen in the imaging of surface cracks are associated with the excitation of surface acoustic waves (SAWs). In this study, a similar sensitivity to detect cracks is achieved by the defocus effect of pure longitudinal waves, which can be clearly explained by theory.<>