The scanners work similar to the photo-copying 
machine. However, rather than reproduce another hard- 
copy, the light intensity of the original document is 
converted and stored in a digital form, i.e., the raster 
imagery. 
There are three major indices for a scanner, 
» the geometric resolution, traditionally this is 
measured in dpi (dots per inch) or lpm (line per 
mm); 
the radiometric resolution, this is normally 
represented by the depth of each pixel, such as 4 
bits imagery gives 16 gray-scales, and 8 bits 
imagery gives 256 gray-scales; 
the physical dimension, that is, how large the 
original document can be. 
The common value for the desk-top document scanners 
are 300 dpi maximum geometric resolution, 16 or 256 
gray-scales, 8.5 x 14 inches (legal size) physical 
dimension. The specifications of three commercially 
available models are listed in Table 1. Converting to 
pixel size, 300 dpi provides 0.0849 mm resolution; while 
600 dpi provides 0.0425 mm. 
The slide scanners have relatively higher spatial 
resolution. Typically, the physical format is designed as 
35 mm by 35 mm. The related data of a commercial 
model is listed in Table 2. 
Converting to pixel size, the resolution is 0.0137 mm. 
Because the standard format of a 35 mm camera is 36 
mm x 24 mm, the slides can be digitized in either 
horizontal (landscape) or vertical (portrait) orientations. 
    
The resulting digital image, to the maximum spatial 
resolution, would have about 4.46 M pixels. For 8-bits 
pixel depth, the file size would be 4.46 Mbytes. 
THE MANUAL OPERATION 
An operation module is designed to mimic the 
original analogue system. The flow is the same as 
illustrated in Figure 4. The only difference is that 
instead of mounting photos onto slide projector, the 
image is loaded and displayed on the screen. Several 
functions are provided: 
image display, 
* image enhancement, 
* coordinate digitization. 
For VGA mode 18, the spatial resolution is 640x480 with 
16 colors. That means, on the screen, 640x480 pixels can 
be addressed and displayed as one of 16 colors. 
Apparently, this is quite limited. Higher resolution 
devices are increasingly becoming available. For 
example, AcerPower Ultra-VGA from Acer provides 
1024x768 pixels with 256 colors (for IMB video 
memory). Although there is massive progress in the 
graphic display devices, the designed system uses VGA as 
the base platform due to its generality. The program 
automatically determines the type of the video display 
and activates the proper procedure. The design principle 
is to display the full image in a one-to-one ratio on the 
screen at the same time. 
THE SEMI-AUTOMATED SCHEME 
A semi-automated (computer-aided) module is also 
designed. This module is based on the mono-plotter 
concept (Shih & Derenyi, 1992). With image matching 
technique, the operator only has to digitize the feature 
Table 1: The Specifications of Three Document Scanners 
maximum 
maximum 
et 
X X X 
  
maximum 
Converting to pixel size, 300 dpi provides 0.0849 mm resolution; while 600 dpi provides 
0.0425 mm. 
Table 2. The Specifications of a Slide Scanner 
maximum