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

OPTICAL TOF RANGE MEASUREMENT 23
where an optical modulator works as an electrooptical shutter with varying
sensitivity over time. A similar realization, making use of the built-in electronic
shutter of a conventional CCD camera, rather than an additional optical modulator,
is described in [DAS]. This system works as follows: A light pulse of some tens of
nanoseconds is transmitted by a laser and it synchronously starts the integration of
a standard CCD camera. With the electrical shutter mechanism of the CCD
camera, only a very short period of time (also some tens of nanoseconds) is
integrated. Thus, depending on the distance of the targets, only a fraction of the
light pulse arrives before the integration stops. Performing two calibration
measurements, one without laser illumination and one with non-pulsed, continuous
laser illumination, enables distance calculation. However, compared to the total
power consumption of nearly 200 Watts, the distance resolution of ± 20 cm for non
cooperative targets in a 10 m range at 7 Hz frame rate is relatively poor. The
reason for this low performance is that no repetitive integration is possible; the CCD
has to be read out after every short time integration. This leads to a poor signal-tonoise
ratio of this realization that can only be compensated by very high optical
power of the laser. The power of laser sources, on the other hand, is limited by eye
safety regulations.
The restriction of conventional CCDs only allowing one short-time integration, as
described above, requires innovative, smarter 2D demodulation arrays realized as
customized PhotoASICs. The first such device, realized in CCD technology, was,
however, not intended to be used in an optical ranging system but for a 2Dpolarimeter
[POW]. For simplified 3D TOF measurement without mechanical
scanners, the lock-in CCD sensor was invented [SP1-SP4]. Based on these lock-in
structures, we present arrays of improved demodulation pixels in Chapter 5. In
these pixels, CCD gates are arranged such that light generated charge carriers
under the photo gate can be moved to different storage sites. This allows fast
sampling of incoming light, enabling the measurement of phase and amplitude of
the modulated light, as described later in Section 2.2. A related architecture, which
is operated with modified sinusoidal control signals, is the photonic mixer device
(PMD) [SC2, XU]. This device has a modified differential amplifier in every pixel,
which integrates the sum and the difference of two demodulation currents,
demodulated with 180° phase difference.