3D Time-of-flight distance measurement with custom - Universität ...

OPTICAL TOF RANGE MEASUREMENT 21
proportional to the time of flight. The difficulty of this detection mechanism is the
definition of a trigger level for the detector, because the amplitude of the received
light strongly depends on the distance, background and the surface to measure.
For 1D-TOF ranging systems, high dynamic, high sensitivity PIN photo diodes or
APDs (avalanche photo diodes) are used. PIN photo diodes have a very fast
response. Typical cut-off frequencies are 10 GHz and beyond. After the fast
detection, the modulated light, now converted to an electrical signal, is electrically
demodulated (using sophisticated special purpose ICs) leading to the desired
phase difference between transmitter and receiver. This electrical demodulation
often suffers from temperature drifts of the electric components involved. Therefore,
regular reference measurements and calibration are necessary to ensure
reproducible measurements. APDs and photomultiplier tubes [PAR] can be
modulated in their sensitivity, enabling a direct demodulation or mixing of the
incoming light. Today, TOF rangefinders and TOF laser scanners are available with
mm accuracy for cooperative and several cm resolution for non-cooperative targets
over a distance of some 10 up to 100 meters and more [LEI, KAI].
All these components only allow a 1D measurement, i.e. the distance measurement
of one point in the 3D scene. The operation of many such receivers in parallel
appears to be impractical due to large size and enormous demand on additional
electronics. Therefore, 2D depth profiles or 3D depth images can only be obtained
from such 0D detectors by scanning the light beam over the observed surface.
This, however, requires time, because every point has to be measured serially. It
also requires mechanical scanners of very high precision. Those scanners are
bulky, expensive, and sensitive to vibrations.
III. 3D TOF RANGING
Instead of scanning a laser beam and serially acquiring the range data point-wise,
we can illuminate the entire scene with a modulated light surface in order to
perform a 3D measurement, as illustrated in Figure 2.5. This, however,
necessitates the use of a 2D-electrooptical demodulator and detector to measure
the distances of some hundreds or thousands of points of the observed scene in
parallel.
The 2D detection itself can be performed with CCDs or (active) photodiode arrays,
so called active pixel sensors (APS). However, in contrast to discrete photodiodes,