Image Formation and Digitization

Abstract

Image acquisition is the first step of digital image processing and is often not properly taken into account. However, quantitative analysis of any images requires a good understanding of the image formation process. Only with a profound knowledge of all the steps involved in image acquisition, is it possible to interpret the contents of an image correctly. The steps necessary for an object in the three-dimensional world to become a digital image in the memory of a computer are as follows:

• Becoming visible. An object becomes visible by the interaction with light or, more generally, electromagnetic radiation. The four basic types of interaction are reflection, refraction, absorption, and scattering. These effects depend on the optical properties of the material from which the object is made and on its surface structure. The light collected by a camera system is determined by these optical properties as well as by the illumination, i. e., position and nature of the light or, more generally, radiation sources.

• Projection. An optical system collects the light rays reflected from the objects and projects the three-dimensional world onto a two-dimensional image plane.

• Digitization. The continuous image on the image plane must be converted into image points on a discrete grid. Furthermore, the intensity at each point must be represented by a suitable finite number of gray values (Quantization).