Rockwell Automation 2755 Atomscan Bar Code Scanner - Manual (Page 14)

Chapter 3
Designing the System
3–3
Bar Code Symbol Length and Height
When measuring a symbol, orientation must be ignored. Its height is
measured from one end of a bar to the other, and its length is always the
distance from one end of the symbol to the other, including the “Quiet
Zones”. A Quiet Zone is the empty space before or after the bars, and is
usually equal to 10 times the Narrow Element Width.
Figure 3.1
Parts of a symbol
Quiet Zones
Symbol Length = L
Symbol
Height = H
Quiet
Zones
L
H
Symbol Quality
A bar code reader cannot reliably read a symbol of poor quality. We strongly
advise testing proposed bar code symbol samples to ANSI Standard
X3.182-1990, “Bar Code Print Quality Guideline”.
•
Low-cost verifiers that can test this standard are available from several
companies.
•
Symbol samples can be submitted to an independent symbology testing
company.
The ANSI guideline specified six parametric tests plus two pass/fail tests to
determine the printed symbol quality. The tests result in an overall letter
grade of A, B, C, D, or FAIL assigned to the symbol.
•
Grade A printed symbols: any scanner should be able to read them.
•
Grade B symbols: many scanners can read them, including AtomScan
scanners.
•
Grade C symbols may appear to decode successfully, but in production
the performance may drop substantially. It is usually not worth taking a
chance on symbols lower than Grade B.