Polaroid SprintScan SS35, Repair Manual

Page: 155 / 187

Slide Scanner Repair Manual Diagnostics and Troubleshooting
155
Uncorrected Dark Uniformity Test
The dark portion of the test is based on the offset uniformity profile of the slide scanner. The
dark performance of the CCD is tightly specified and tested due to the limited correction range
of the SprintScan 35ES. If the uncorrected signal (Figure 5-20) exceeds the scanner’s
correction range, streaks will develop in dark signal.
The Mean Uncorrected Dark Level is the mean response of the CCD in the dark, as
measured in digital numbers in terms of the LSB’s of the A/D converter. Thus the result is on
a 10-bit scale for the SS35ES and a 12-bit scale for the SS35 Plus. In either case, it is
checked to be no more than a specified amount above the uncorrected calibration target. For
the SS35ES, that target is 16 digital numbers (dn) and for the SS35 Plus, it is 68dn.
Note: The CCD signal has received analog offset adjustment to properly place the
sensor’s dark reference pixels.
The Uncorrected Dark Standard Deviation checks the standard deviation of the uncorrected
dark profile response of a scan of the black calibration target. The stand-alone test also
reports the uncorrected dark profile minimum maximum-minimum to fill out the statistical
analysis. The min and max-min data is reported only and is not tested against constraints.
Failure of either of these tests is generally caused by:
• Light leakage (flare) on the back of the black calibration portion of the film carriage.
• Poor placement of the light tunnel.
• A bad CCD.
Figure 5-20. Uncorrected dark uniformity profile
Bad Pixel Flag
The Bad Pixel Flag count is reported for information only to track the number of pixels
that the internal calibration algorithm’s have flagged for replacement. It is the metric of the
general condition of the CCD’s only. The presence or absence of this count will generally
change from scan to scan and should not affect the quality of the output to the customer.