The process by which an image is formed and interpreted can be conceptualized as a chain of physical events, i.e., the imaging chain, that starts with the light source and ends with viewing the displayed image. The principal links of the imaging chain are the radiometry, the camera, the processing, the display, and the interpretation of the image (Fig. 2.1). The imaging chain begins with the radiometry of the electromagnetic energy that will create the image. This energy may originate from the sun, a light bulb, or the object itself. The electromagnetic energy is then captured by the camera system with optics to form the image and a sensor to convert the captured electromagnetic radiation into a digital image. The image is then processed to enhance the quality and the utility of the image. Finally, the image is displayed and interpreted by the viewer.
Each link in the imaging chain and the interaction between the links play a vital role in the final quality of the image, which is only as good as the weakest link. Figure 2.2 shows examples of images that have a dominant weak link in the imaging chain as well as one that balances the weak links so that no single weak link dominates the resulting quality. The dominant weak links shown in Fig. 2.2 are (a) poor optics, (b) motion blur, (c) sensor noise, (d) overexposure, (e) low contrast, and (f) processing that oversharpened the image. Figure 2.1 The principal links of an imaging chain.
Mathematical models that describe the physics of the image formation have been developed to help us understand how each link impacts the formation of the final image. These models are essential for identifying the weak links as well as understanding the interaction between the links and the imaging system as a whole. The mathematical models are typically categorized into the key elements of the imaging chain, namely radiometry, the camera (optics and sensor), the motion associated with the camera, the processing, the display, and the interpretation (Fig. 2.3). The camera models are generally divided into the optics (the part of the camera that shapes the light into an image) and the digital sensor (the part of the camera the converts the image formed by the optics into a digital image).