exact frequency, amplitude and attenuation as well as the
surface spacing and susceptibility to reflection. These
interference fringes become rings around a surface point
disruption, like dust, that causes the distance gap to falloff
gradually and radially. The rings will also have a rainbow
coloured appearance, in a colour scan, due the exact
film/glass distance selecting the frequency of light at which
the reflections are different by a half or full multiple of that
frequency or colour. Sometimes the film is simply allowed to
curl slightly owing to the pressure plate not fully holding it
flat. In this case, long fringe patterns are formed along the
curl direction. All this must happen within the depth of field
of the scanner optical system to be seen, which is the
justification for a slight out of focus adjustment that some
graphics arts scanners use to reduce these rings. One dark
and light fringe spacing represents a one-quarter wavelength
change of distance between the two surfaces. The centre
point of contact is always dark, since contact produces
destructive interference, and the farther ring amplitudes
normally decrease with distance from the centre.
We have been exploring ways to reduce significantly the
formation of ring patterns by controlling one of the
contributing physical factors. We chose the reflection
coefficient of the surfaces. By suitably reducing the amount
of reflection between the glass pressure plates, we found we
can generally reduce the ring amplitude to below visibility.
This can also be incorporated in an upgrade for an older
scanner.
33 Illumination system
The illumination system for a scanner not only plays a role in
Newton rings, but also directly influences spatial image
quality, colour image separation and scan speed. Image
quality influences measurement accuracy as well, so
indirectly the effect of the lighting system percolates down
the entire imaging chain for the production
photogrammetrist. Image quality is a function of many
factors, here the emphasis is on sharpness, tonal gradation
and range, and colour accuracy. Sharpness, while desirable
for image quality, must be considered carefully, since it
usually correlates also with a higher perception of unwanted
artifacts in the film, such as scratches and dust. This often
forces a tradeoff between extreme sharpness on one side and
too many artifacts to remove or slightly less sharpness and a
lower number of visible imperfections on the other. For
strong detail to be captured, then either compromising or
tedious methods will be needed to remove the non-film
blemishes from the image. Using an automatic fill-in method
can distort the image, since trying to recreate what lies
underneath a dust particle must be artificially done by
extrapolation. Manual editing methods like those in
Photoshop are labour-intensive for large jobs. Too much
detail normally arises from a too strongly collimated light
source, which creates strong shadows around everything,
including dust and scratches. Another negative of this
detailed light is loss of scanning tonal range. The light
creates high contrast in the image by packing a smaller
density range within the available scanner grey levels.
Experience has shown us that a 12-bit sensor range is
optimally spread out within a 3.0 optical film density range
for mapping applications. High col-umniation reduces the
useable density range, which makes the system overly
sensitive in tonal work. High contrast light is useful in photo
interpretation: ideally a scanner would also support this
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004
application. To be successful in photogrammetry, however, it
must have light favorable to the traditional mapping
applications such as orthophotos. In general, diffused light
allows this larger tonal range needed for merging orthos. Our
goal is to offer both options in the DSW700.
Illumination systems have in the past been high cost items in
terms of both standard maintenance (for example, bulb
replacements) and failures that require trained support
engineers from. the vendor to resolve. They have been
expensive, short-lived and require constant recalibration.
Solid state light sources, such as LEDs, offer improvements
in all these categories and have been steadily entering
lighting design. LEDs have matured to offer sufficient
colours, intensities and longevities for practical use in high-
end scanners. After 130 years, the incandescent can be put to
rest while, after 65 years, the fluorescent lamp will also soon
be eclipsed, both technologies still common in scanners
today. The LED, demonstrated first almost 100 years ago, is
not new but its commercialisation is only 40 years old. It has
become attractive for scanners since it offers the potential as
a zero-maintenance, lifetime source. Expected half-life
approaches 10 years of constant 24 hours per day, 7 days per
week operation. Modern devices offer huge improvements
over their predecessors, which often failed prematurely,
through more sophisticated current controller technology as
well as package design to reduce heat and maximize
efficacy. Today’s device works by causing current to pass
through a semi-conductor junction or diode. This results in
light emitting from the junction in the spectrum wavelengths
consistent with the semi-conductor material used. Currently
AlInGaP is used for red, InGaN for green and blue, and GaN
for white. The final spectral output is often strongly
monochromatic and much narrower than the halogen/xeon
lamp/filter combinations which follow the more broadband
"blackbody" laws or the multiple-peaked and broadband
fluorescents. The narrower bandwidth has advantages in
measuring the precise amounts of photographic dyes in films,
and can even be tuned to match the representative Status A
or Status M film density sampling curves. Colour software
processing can be used to convert these primary
measurements to a standard colour space, such as SRGB now
commonly used in electronic displays, preserving neutrality
better than with broadband filter functions. .
Figure 2. Ring light LED light source
Internatio
EE
The use «
more effi
than has |
all emplo;
optical fil
stood stat
process \
required ¢
switching
constants
room for
brought
millisecoi
where tht
fast as bl:
the othe
susceptib;
figure 2.
3.3 T
The third
is the sei
lower in
improven
influence
current d.
film form
megapixe
high spee
improved
computer
look forw
Redlake
DSW700
Fig
This spe
readout [
several sl
Also the
preservin
of the ar
speeds ol
still con
comprom
time and
As a resu
declined
