Dr Gan Woon Siong and Ng Choon Huat

Acoustical Technologies (S) Pte Ltd, Innnovation Centre

Nanyang Technological University, Singapore

Scanning Acoustic Microscope (SAM) is a system in which high frequency sound waves are used to image microscopic objects. The novelty of this form of microscopy is that the contrast depends on the variation of the elastic properties of the specimen. This is fundamentally different to that used in an optical microscope and thus objects that appears uniform or opaque under optical microscopy show high contrast under SAM.

Although SAM is known to researchers for its ability to detect subsurface cracks and discontinuities in a material, it is sometimes more desirable to able to quantify the interaction between the acoustic waves and the specimen, and to measure the elastic constants with the spatial resolution that SAM provides.

The elastic constants are determined from the velocities of leaky waves that can be obtained from V (z) measurement. The V (z) curve, which is a record of the transducer output voltage V as a function of the distance z between the lens focus and the specimen surface, is unique to a material and referred to as the material signature.

In recent development, SAM is now able to produce 3D images instead of 3D intensity profiles. This capability allows researcher to rotate the images to show the defects at various angles. The theoretical basis is that the system point spread function is a 3D function. The image formation equation is the 3D convolution of the 3D point spread function and the objective function. The spatial distribution of the transducer output is the resulting 3D convolution of the 3D point spread function in the broader sense and the objective function. Besides this, the multiple reflection of the sound wave during measurement and the regular reflections caused by the existence of the non working model of the lenses also have important influences on the image formation. Hence, to produce good 3D images, software is developed to eliminate aberrations and ghost images that caused by multiple reflections. Marching Cube Algorithm is used for volume rendering for 3D images as it is representative of 3D data field and uses voxel unit to generate equal value surface technology.

Besides viewing the image in 3D, SAM allows researchers to slice the B and C scan images up to several hundred layers to reveal the condition of the specimen under investigation.