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It is possible to differentiate between 1-
dimensional and polydimensional measurement
techniques. The former denotes a point measurement,
also called a whisk broom scanner (Green et al.,
1998), in which the full spectral data is obtained for
a single point, while the latter applies to line-scan
systems, area scanning and snapshot imaging. A
line-scan system, also known as a push broom
scanner (Weser et al., 2008), is able to acquire the
spectral data for one line instead of a single point.
For an image with a width of x pixels and a height of
y pixels, the push broom scanner obtains spectral data
(λ) for every pixel x in one line (y
i
) simultaneously.
To acquire the whole data cube, the push broom only
has to scan each of the y lines, making the method
faster than a whisk broom scanner which has to
scan x × y times in order to obtain the whole data
cube. However, the push broom scanner is not as
versatile as a whisk broom scanner, as the width of
the scanning unit is fixed and it is thus not possible
to freely choose the part of the sample for which to
acquire hyperspectral data. Area scanning, also called
spectral scanning (Dickinson et al., 2001), is another
technique which allows for surveying the whole
image monochromatically, meaning that a value for
every pixel of the image is obtained for one certain
wavelength λ. This can be achieved either by using
a broadband light source and an optical band-pass
filter, which rejects all but one wavelength and may
be tunable or has to be replaced in order to allow for
more wavelengths to be scanned, or alternatively by
using a light source with adaptable spectrum. With
this technique, it is easy to choose certain spectral
bands for the whole image. However, in order to
acquire data of several wavelengths, the station has
to be stationary, as otherwise spectral smearing can
occur, thus deteriorating correlations of an object at
multiple wavelengths. Finally, snapshot imaging, also
referred to as non-scanning (Volin et al., 2001), is a
technique which allows for obtaining the whole data
cube at once. This is naturally very time-efficient, but
computational effort and initial costs are high.
For the metrology platform presented in this pa-
per, a point scanner was chosen as it is the most ver-
satile of the described techniques and also the most af-
fordable. Due to the positioning robot, which allows
for nanometer-exact positioning, a high-resolution
spectral image can be obtained for heterogeneous
samples. Furthermore, the choice of the character-
istics of the spectrometers is flexible and almost any
wavelength range can be analyzed. The only draw-
back is the relatively slow capturing process com-
pared to the other recording approaches.
3 SYSTEM OVERVIEW
Atop of a vibration damped optical table, a three axis
Cartesian robot with nanometer-precision and a com-
bined measurement head for large area gray value
images as well as spectral measurements is placed.
A lighting system encompassing LEDs in different
colors and infrared along with an ultraviolet light
provides the means to acquire multispectral images,
while a broadband halogen light facilitates hyperspec-
tral image acquisition with the aid of spectrometers.
An overview of the system’s exterior, with the impor-
tant instruments and parts of CHRISS, as well as the
system’s main components of the interior, is presented
in Figure 1.
3.1 Enclosure
The core of the system is mounted atop the vibra-
tion damped optical table manufactured by the Opta
GmbH and is surrounded by a light-proof enclosure
construction with a canvas hull. Building vibration
is a problem when dealing with high-precision image
acquisition. In order to handle this problem, a table
which absorbs most of the vibration is used, thus en-
suring that the measurement of the sample is not cor-
rupted. A granite surface plate manufactured by the
company Johann Fischer Aschaffenburg is used as the
precise base for transacting measuring and inspection
tasks. It guarantees a flatness of the measuring sur-
face according to DIN 876 and is thus important for
nanometer-precise measurements.
Additionally, the influence of variable daylight
has to be minimized to ensure that measurements of
the same sample are always comparable. For that rea-
son the opaque enclosure is important, ensuring that
only the internal lighting system illuminates the sam-
ple. Since the user has to be able to access the interior
to, for example, replace samples, a roller blind has
been installed on the front of the canvas hull which
can be lowered to make the enclosure light-proof.
Due to the fact that the stages of the Cartesian
robot are only guaranteed to operate with nanometer-
precision at 20 degrees Celsius, the room is equipped
with an air conditioning unit. Since the lighting sys-
tem, especially the halogen light, causes the air within
the enclosure to heat up, a ventilation system inter-
changing the warm interior and the cool exterior air
is necessary to ensure the high-precision work of the
positioning robot. A thermometer system with three
measuring points provides information about the tem-
perature within the enclosure so that the user is able
to assess the precision of the positioning system.
PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology
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