CHAPTER I

 INTRODUCTION

 Human vision is a remarkable combination of high resolution

sensors and a powerful processing machine. This combination permits

understanding of the world through sensing and interpretation of

visual images. The faculty of vision is so natural and common that

few pause to think how marvelous it is to acquire such clear and

precise information about objects simply by virtue of the luminous

signals that enter the eyes. Without consciousness of the complicated

process, objects are recognized by the characteristic qualities of the

radiations they emit. With the help of memory and previous

experience, the sources of these manifestations are perceived. This

process is known as sight, perception or understanding.

 Images and photographs have long been used to identify and locate

objects. By photographing an area, perhaps from afar, a scene could

be given detailed study. This study might disclose the presence of

objects of interest and determine their spatial location. Images from

satellites show weather, agriculture, geology and global actions.

Special images may contain additional scientific information including

object spectral characteristics, velocity, temperature, and the like.

 The traditional medium of these images has been photographic film.

 It is capable of high resolution and is sensitive to visible and near-

 visible wavelengths. Unfortunately, film based methods are slow due

 to exposure, processing, and analysis time. This time lag is not a