300,00
RM1, speed 320 |
/ diaphragm 11
HP Scanjet IICX
250,00
200,00
150,00 |
grey value
100,00
50,00 |
o wn e wn x= ~ N © e oo x eo
= 2 2°98 9.0 - «€ v OO F^. ©
© © Oc o oc o - - - - - -
0,04
|
|
| 0,00
|
| optical density
Figure 9: relation gray value - optical density for the HP
Scanjet IICX compared with the RM1
The general limitation of CCD-lines and arrays with the
color and the density range are only allowing a limited
quality of generated orthophotos. By this reason in some
sensitive projects, the photos have been scanned twice -
at first with the RM1 for the image correlation and then
with a drum scanner for getting the optimal color
information.
5. RESOLUTION, MTF
The Rastermaster has a pixel size of 12ume12um. If this
shall be compared with the photographic resolution, by
theory it has to be multiplied with the Kell-factor (2ev2 -
2.8) for getting the minimal size of a linepair which can be
identified as a linepair and not as homogeneous gray. In
practice the relation between the size of a linepair and the
pixel is more close to 2.0. With this relation the size of the
linepair is 24um corresponding to 42 Ip/mm. It is not a
matter of course that this will be reached in practice by
the scanner. Some desktop publishing scanners are
existing, where this relation is totally missed. For
checking a test pattern (Siemens star) was scanned.
Figure 10: scanned test pattern
Based on the test pattern a resolution of 40 - 42 Ip/mm is
available with the Rastermaster. This corresponds to the
pixel size of 12um, but also to the usual resolution of
aerial photos. That means, a higher resolution is not
required. The resolution includes only the information
about the radiometric quality for a high image frequency.
The loss of contrast for lower image frequency can be
described with the modulation transfer function (MTF).
0,8 +
0,7 +
0,8 t
0,5 + contrast
0,4 +
0,3 -
Q.2 +
o1 1 resolution [Ip/mm] —
| Eu sue |; n b
i : EEA
0 ; T | T l7 = I T T T IT
1 10 20 30 37 41 45
Figure 11: modulation transfer function of the RM1
6. Operational Problems
The RM is controlled by a PC Pentium 90. This PC is
only used for the scanning, further processing of the
digital images usually happens on workstations, that
means the data have to be transferred. The data transfer
takes not much less time than the scanning and during
this time the scanner is blocked. By this reason the PC
was equipped in total with 8Gb disc space to store all
images of a day. The data transfer then can be made by
batch process over night. In addition a CD-writer is
connected to the PC to allow also the output on CD.
The scanning of one image including the handling takes
approximately 30 minutes, this is sufficient under the
conditions of a University, but we hope to reduce it with a
new light source.
Once in a group of digital images a shift of a scan-line by
1 up to 2 pixel could be detected. This may be caused by
abrading material from the friction drive of the photo
carrier. After cleaning the wheel, the problem has not
been seen again. A loss of reference has been seen also
in digital images scanned with other scanner types and
also in analytical plotters. By this reason the scanned
images should be checked not only for the correct
radiometric representation, but also for a loss of
reference. The abrading of the friction drive made it
necessary to replace the friction drive after one year of
operation. The material has now been changed from
aluminum to stainless steel, so the problem may be
solved.
As mentioned before it is necessary to cover the area
around the photos on the photo carrier by cardboard to
avoid an over-saturation of the sensor. Especially the
area around the fiducial marks may cause problems if the
settings are not optimized.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
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