There are two principal configurations for imaging a three-dimensional (3D) object:

• ▪ Localized imaging, which is described in this chapter.

• ▪ Tomographic imaging, which is described in Chapter 4.

There are two configurations for localized probing of a 3D object:

• ▪ Axial sectioning, i.e., dividing the object into a set of axial lines parallel to the z axis in the depth direction, as shown in Fig. 3.0-1(a), and imaging each of these axial lines as a one-dimensional (1D) object – a function of z. Each 1D image is called an A-scan. A set of A-scans may be used to form a planar 2D image, e.g., in the x–z plane, called a B-scan, as shown in Fig. 3.0-1(b). A set of parallel B-scans may be used to form the entire 3D image.

• ▪ Lateral sectioning, i.e., dividing the object into lateral (transverse) planar slices, as shown in Fig. 3.0-1(c), and imaging each of these slices as a two-dimensional (2D) object, i.e., a function of x and y. Each 2D image is called a C-scan and the 3D image takes the form of a stack, called the z-stack.

In any of these configurations, the resolution of scanning and sectioning in the lateral and axial directions is dictated by the ability to localize the probe beam and/or the sensor beam, which is limited principally by the wavelength; the shorter the wavelength, the easier it is to localize.