The transducer scans the sample line by line (in grids) on the XY plane and on request — in case of samples with bows or protruding elements — in the Z direction and then displays the electromagnetic pulses as pixels with specific gray values.
The information from the individual pixels is used to generate an image showing all the recorded signals. To begin with, the ultrasound reflection at the sample surface is displayed. If the sample is intact, the signal is reflected again a second time at the rear side of the sample. The time difference of arrival of the two signals from the upper and lower side of the sample yields information about its thickness. If the sample contains a defect, this interface—between sample and defect—will also cause sound reflection to occur.
Acoustic microscopy operates using frequencies that extend to the gigahertz range. As a general rule, the higher the frequency, the greater the achievable resolution and the lower the penetration depth of the sound waves.
Since the attenuation in the coupling media increases quadratically as the frequency rises, the lens must be brought as close as possible to the sample under investigation. This low working distance and the difficulty of creating transducer arrangements offering high local resolution require a type of scanning microscopy in which the sample is investigated pixel by pixel.
The ongoing further development of their high-resolution transducers enables Analytical Systems to improve the measurement accuracy. The ultrasound microscopes currently offer the highest frequency range up to a maximum of 1,000 MHz, delivering resolutions all the way down to 0.8 µm. Naturally, this depends on the material and the density of the sample to be investigated.
Depending on the operating frequency of the ultrasonic testing probe, the imaging resolutions shown below can be achieved: