Rockwell Automation 2755 Atomscan Bar Code Scanner - Manual (Page 21)

Chapter 3
Designing the System
3–10
Raster Height
A raster scanner is used when the bar code symbol is of poor quality:
•
symbols printed on a dot matrix printer
•
bars and spaces not uniform
•
bars have voids or spaces have spots
The raster scanner puts lines through ten locations in the symbol, which
increases the probability of finding a location that can be reliably decoded.
The Raster Height is the distance between the raster scan lines farthest apart
at a given distance from the scanner.
Distance from the Scanner Raster Height
inches centimeters inches millimeters
1
2
3
4
5
6
7
8
9
10
2.5
5.1
7.6
10.2
12.7
15.2
17.8
20.3
22.9
25.4
.06
.09
.13
.16
.20
.24
.27
.30
.34
.37 .15
.24
.33
.42
.50
.59
.68
.77
.86
.95
For good quality bar code symbol and a correctly aimed bar code scanner, at
least 5 scans must pass through an entire symbol to assure that the symbol is
decoded. With fewer scans there will be more “No Read” counts, even when
the symbol is good. If the symbol is of poor quality, more scans per symbol
may be required.
Ladder orientation gives a different number of scans than picket fence
orientation, depending on symbol height, symbol width, and scan width.
Calculating the Number of
Scans Per Symbol