The choice of illumination can often mean the difference between success and failure of an imaging system. Inline illumination is often a popular choice when space is limited, because the inline illumination geometry is compact and effective. But it’s a mistake to choose inline illumination only because of the package size. You’ve got to consider your entire imaging process—primarily the characteristics of the object under test. Where a diffuse axial source projects light from many different directions at once, in-line illumination is a very directional source of light. Each type of illumination reveals different types of information about the object under test. Specifically, inline illumination is ideal for inspecting specular objects such as PCBs, semiconductor wafers, and CCDs.
Although diffuse axial illumination and inline illumination share some features, inline illumination geometry is defined by some specific characteristics. Inline illumination—also called coaxial illumination—is most often defined by light being introduced into the optical path (typically at the system stop). This is usually accomplished through a port on the side of the lens assembly through which an LED or fiber optic light guide is coupled to a beamsplitter.