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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004
The use of LEDs in the DSW allows especially for a much
more efficient implementation of sequential color capture
than has been used in the past. It is faster. Previous models
all employed a rotating colour wheel with red, green and blue
optical filters to make the colour separation while the scanner
stood stationary at a given location in the film. This capture
process was slow since it was a mechanical device that
required at least 1 second to move between filters. Since the
switching time between colour LEDs depends on electronic
constants instead of mechanical ones, there is significant
room for improvement. The change times have now been
brought down in the new model to less than 100
milliseconds, or 10 times as fast. We are reaching the point
where three band colour captures in the new model are as
fast as black and white captures in the DSW600. Of course,
the other salient benefit is elimination of mechanical
susceptibility to failure. The new source is illustrated in
figure 2.
3.3 The sensor
The third of the key technologies improved in the new model
is the sensor itself. Faster and larger sensors that are also
lower in noise translate directly into speed and quality
improvements. The DSW uses area sensors, created or
influenced by the digital photographic camera industry. The
current developments in this field have emphasized 35 mm
film format equivalency, of which state of the art is about 11
megapixels, causing a 9 um sensor pitch. To accommodate
high speed photography, the readout rates have been greatly
improved over the cameras of five years ago. The standard
computer PCI bus is now becoming the bottleneck and we
look forward to the next generation PCI XPRESS bus. The
Redlake ES11000 1l-megapixel camera tested for the
DSW700 is illustrated in figure 3.
Figure 3. Redlake ES11000 1 I-megapixel sensor
This speed advance is partly due to the use of multiple
readout points on the chip, or taps, which can multiplex
several slower pixel streams into a single high-speed stream.
Also the individual tap speed can be higher while still
preserving a clean 12-bit readout owing to the advanced state
of the art in CCDs. The result is that composite readout
speeds of 60-320 MB/sec are available. Since higher speed
still commands a higher price, we choose the best
compromise for our product’s price/performance at any given
time and can readjust the mixture when prices decline.
As a result of all these technologies, noise in the system has
declined to where average capture variations are below |
percent, both short- and long-term. This creates a cleaner,
more consistent scan and allows research efforts to be
redirected away from perfecting raw capture towards higher
level image processing, such as dodging or automatic light
balancing in roll-film scanning.
3.4 Other improvements
Ever since the DSW200, the heavy, carefully designed and
manufactured cross-slide system, with its band drives and
heavy motors, has enabled scanners to be calibrated to «2 um
rmse on each axis. Nevertheless, improvements in encoder
technology have emerged over the years and the latest
generation of non-contact heads on the 0.5 um encoders
fitted to the DSW reduce backlash. In addition, with
consideration for the larger sensor requiring fewer stage
movements, the stage may be run at a slightly slower speed.
The benefits for the stage should be greater accuracy along
with less mechanical wear, while still achieving a
significantly increased scan production rate.
3.5 Software developments
The launch of the DSW700 is accompanied by a new release
of the SCAN, Image Utilities and FastDODGE software
suite. The main enhancements, of course, have been
necessitated by the changed hardware, for example new
software functions to control the LED light source and the
Redlake ES11000 11-megapixel sensors chosen for the initial
series of the new model. At the same time, improvements
useful to all scanner customers have been added, for example
the availability of the JPEG2000 format on output.
4 DSW700 PERFORMANCE
As noted earlier, the big gain from the LED light source and
11-megapixel sensor is in speed. The published
specifications for the DSW700 are expected to be less than 2
minutes for a black and white scan at 12.5 pm and about 3
minutes for colour. The latter implies that a 500-foot roll can
be scanned in around 24 hours at 12.5 um, a most significant
milestone.
As a result of the encoder upgrade described in section 3.4
above, the DSW700 stage should usually calibrate to <I pum
rmse on each axis, a performance superior to most
comparators or analytical plotters, though the published
specifications is expected to state a more conservative 1.5
um precision.
On the radiometric side, the major improvement lies in the
reduction of Newton rings, the severity of which has been
dramatically reduced. But the combination of the LED light
source and the high-performance sensor has reduced capture
noise, speeded up capture time and improved tonal
sensitivity. These all have the very practical benefits of
making scan set-ups and project times much faster and easier
than before.
5 DSW700 UPGRADES
The arrival of the DSW700 opens up a wide range of upgrade
possibilities for existing customers. DSW300, DSW500 and
DSW600 models may all be updated to DSW700 status. All
