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A sensor which has these characteristics could be used
as indicated in Figure 2. The basic idea is to have a small
sensor which can easily be moved step by step around
(or along) an object, while a computer processes the
information from the sensor to generate a dense point
cloud of coordinates representing the surface of the
object.
3-D sensor
Figure 2 : Basic model for a "free-flying" 3-D sensor.
At Daimler-Benz, such a sensor is under development.
This sensor was designed using a combination of sensor
principles, namely photogrammetry and active optical
techniques. This sensor does not require expensive,
high-precision positioning systems, and it is expected
that the cost of this sensor will be considerably less than
CMMs. Hopefully, it will also lead to greater use of, and
new applications for, digitization of physical objects.
The rest of this paper is divided into two parts. In the
first part, the focus is mainly on providing a brief descrip-
tion of sensor principles and techniques relevant to the
digitization of physical objects. In the second part, the
particular combination of principles used in the Daimler
Benz sensor is described. The second part also contains
some measurements made using the sensor to illustrate
its use.
2. SENSOR PRINCIPLES FOR
OPTICAL 3-D SURFACE MEASUREMENT
There are many 3-D measurement principles used in
different scales (see Figure 3). This chapter compares
several sensor principles and discusses their fundamen-
tal constraints and practical assumptions.
There are some principles, which need no mechanical
positioning (left side in Figure 3). For example, acquisi-
tion of dense and accurate 3-D data from multiple views
with reconstructed camera positions has been realized in
many passive remote sensing applications using natural
textures and cooperative features [Kon96].
In close-range applications, digital photogrammetry can
measure some (10' v 10°) retroreflecting target points
with high precision (e.g. 1 : 10°). However, it is not possi-
ble to obtain the dense point clouds required for realistic
Surface descriptions and accurate CAD model recon-
Struction.
337
E
$ "free flying" sensors mechanical controlled
= À with non-mechanical sensor positioning
a | navigation with high accuracy
6 +
10
i 5 (not used
0 i in this range)
10
3
Field of Reverse
-2 3-D microscopes of
10 | different principles
(stripe projection,
coherence radar )
with mechanical
positioning and
scanning tables
-3
10 4 1 (not used n
10 In this range)
Figure 3: Applications for 3-D digitizing.
On the other hand, there are many active optical sensor
methods used in medium and small scale applications
that use well defined artificial illumination in combination
with high accuracy mechanical positioning systems. For
example, 3-D sensors consisting of a matrix camera and
a coded stripe projector can measure a large number of
coordinate points (10°. 10”) from unprepared surfaces
with moderate resolution (e.g. 1 : 10°).
However, to fully digitize 3-D objects using such active
sensors, it is necessary to move the sensor or the sur-
face while maintaining relative orientation information. In
addition, the object area, which is measurable from a
single view can not, in practice, be larger than approxi-
mately 1 m’, because of limited projector light energy.
It should be mentioned here, that sensing principles,
which are not based on matrix cameras, will not be dis-
cussed in this paper for several reasons. Those princi-
ples, for example laser spot triangulation with linear
photo diode arrays or posicons, time-of-flight sensors or
laser heterodyne interferometers with single photo diode
detectors always need a scanning device of high accu-
racy. It is envisaged that devices with acceptable lateral
accuracy and long-term stability will be too expensive
compared to digital matrix cameras in the next few years.
2.1 Passive Sensing Principles and Their Limitations
2.2.1 Natural Features and Natural Illumination: Be-
cause active lighting is not applicable from satellites or
airplanes, passive stereo photogrammetry based on natu-
ral textures and features is used in remote sensing appli-
cation at optical wavelengths. To measure corresponding
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
