8.3 Image Input Systems

Image input systems are defined in this section as analog to digital (A/D) converters of analog images. The image input system provides digital data which are the converted tone or color of a film or photograph. In the case of a color image, the components of the three primary colors (Red, Green and Blue) are digitized by using three color filters.

The function of an image input system depends on the following factors.
(1) Film size: allowable maximum size
(2) Spatial resolution: pixel size or dot per inch (DPI)
(3) Gray scale : number of bits or bytes
(4) Speed: speed of A/D conversion and data
(5) Environment : dark room or illumination
(6) Accuracy : allowable error of coordinates
(7) Type of image : transparency or reflective

Table 8.3.1 shows the comparison between five image input devices with respect to spatial resolution, density resolution, positioning accuracy etc. Figure 8.3.1 shows the typical mechanism of the five image input systems.

a. Mechanical scanner:
An image placed around a drum is scanned using the rotation of the drum and a shift of a light source. Though the speed of scanning is not very high, it is widely used because the spatial resolution and density resolution are very high. Recently laser beams have been used as the light source which enables a faster speed.
b. Electronic image tube:
Electronic image tube such as a TV camera is used for A/D conversion of an image. However, the spatial resolution, density resolution and positioning accuracy are low. The advantages are its low price and ease of use.
c. CCD camera:
The electronic image tube is now being replaced by CCD cameras with higher spatial resolution and positioning accuracy. These systems are compact and lightweight
d. Linear array CCD camera:
A linear array CCD with very high resolution, for example 409 pixels per line is drived mechanically to enable line, scanning on a flat table. The spatial resolution, density resolution and positioning accuracy are very high, so that desk top scanners are becoming popular.
e. Flying spot:
An illuminated spot on a CRT is projected onto a film, at a given coordinate, with high speed. The density of the film can be digitized regularly as well as randomly depending on the input coordinates. The disadvantage is that a dark room is required.


Copyright © 1996 Japan Association of Remote Sensing All rights reserved