| Info Sheets |
| Why do CCD based verifiers
require a factory recalibration ? |
The Sandilyam PC6000, PC6500 and PC7000 verifiers all use linear
CCD sensors to capture scan reflectance profiles of bar codes.
These CCD sensors work very much like digital cameras in the
manner in which they capture an image, but they only image a
single line scan across a bar code similar to the scan line
that you see with a hand held laser scanner.
The accurate measurement of reflectance across the full width
of the scan relies on the bar code symbol being imaged as if
the illumination were perfectly uniform and the sensor elements
equally sensitive at all points along the scan line. In practice
this is not necessarily the case, however, the electronics within
the head of these units applies a function that enables the
units to perform as if this were the case, so that when the
scan reflectance profile reaches the PC or Macintosh computer
the application can make this assumption and proceed with the
analysis. To achieve this the heads must be put through a factory
calibration process.
The accurate measurement of reflectance
across the full width of the scan relies on the bar code symbol
being imaged as if the illumination were perfectly uniform and
the sensor elements equally sensitive at all points along the
scan line. In practice this is not necessarily the case, however,
the electronics within the head of these units applies a function
that enables the units to perform as if this were the case,
so that when the scan reflectance profile reaches the PC or
Macintosh computer the application can make this assumption
and proceed with the analysis. To achieve this the heads must
be put through a factory calibration process.
After many thousands of hours of use the intensity of
the LEDs that illuminate the bar code symbol will gradually
change. If this gradual dimming is not completely uniform then
a factory recalibration would need to be performed once again
to get back to the situation of creating uniform scan reflectance
profiles from the scanning heads. Sandilyam recommend checking
the uniformity of the profile as least once a year and if necessary
the factory recalibration can be performed to correct any observed
changes from the ideal.
If a uniform profile is not being achieved then a bar code symbol
may start to get lower grades, particularly for modulation,
than it should really be getting but this may not always be
obvious to the user when simply trying to verify bar code symbols.
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