Image Processing Reference
In-Depth Information
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Task of Im
aging and Role of Image Sensors
Various kinds of instruments such as digital still cameras (DSCs), camera phones, and
camcorders allow us to enjoy personal images. Moreover, broadcasting cameras, which pro-
vide high-definition images, are indispensable to the television industry. Not only are there
cameras for personal enjoyment, but there are also cameras for other applications such as for
automobiles and security systems, and endoscopes for medical use. Cameras are not only
used for visible light, but they are also used for thermography, which visualizes thermal
distribution by infrared imaging; there are also cameras for ultraviolet and x-ray imaging.
In addition, there are cameras that capture very-high-speed phenomena and cameras that
obtain highly accurate color information. Furthermore, there are cameras whose images are
used not by the human eye but by machines, such as cameras for automated driving and
machine vision, which judge information obtained from images. As just described, various
kinds of cameras are utilized in a very wide range of fields.
Why are there so many kinds of imaging systems as typified by cameras? The reason is
to obtain images with adequate image quality for the purpose of each imaging system. In
each imaging system, the role of each image sensor is to pick up image information of high
enough quality for that system.
In this chapter, the factors that determine image information are confirmed. Then, the
structure of image sensor output and image information are set out. Unless explicitly
stated otherwise, the explanations are based on “almost all image sensors” (see Section
1.2.3). Concerning the terms used in this topic,
image information
is used for image infor-
mation in a broader sense,
optical image information
is used for information contained in
optical images, and
image signal
is used for image sensor output obtained from optical
images.
1.1 Factors Constructing Image Information
What is image information made up of? For the sake of simplicity, let us initially focus
on a monochrome still picture. There is a concentration distribution of black and white in
two-dimensional space in a monochrome still picture. The concentration indicates the light
intensity, which is brighter at lower concentrations. That is, the concentration is the light
intensity distribution at each position in two-dimensional space. Therefore, an image is
constructed using the information on space (position) and the intensity of the light at that
position.
Let us now consider color still images. Since information on the wavelength of light must
be added, color image information is formed by light intensity, space, and wavelength.
Moreover, in the case of color moving images, time information when light reaches the
image should be added. Thus, the image information is composed of four factors: light
intensity, space (position), wavelength, and time.
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