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

DEMODULATION PIXELS IN CMOS/CCD 127
sinusoidal rather than square shaped input signal, the measured amplitude is
attenuated to 64%, independently of the actual demodulation performance of the
pixels. We do not make this correction, for two reasons: (1) We use exactly the
same LEDs (HPWT-DH00, 630 nm) modulated at 20 MHz for the range cameras
introduced in Chapter 6. The contrast values presented in Section 5.2.3 - 5.2.5
therefore represent the real measured contrast, including the modulation shape and
modulation depth of the optical input signal. And (2), as can be seen from
Figure 5.12, for 20 MHz modulation the optical signal is not exactly a sinusoidal
wave so that an amplitude correction by 56.25%, which corresponds to the pure
sinusoidal wave, would not be absolutely correct.
DC case: Stationary charge-separation/ shutter efficiency
Before presenting the first measured values, we would like to introduce a very
simple model, which gives a first feeling for the working principle of the pixel and for
numerical values we can expect for the demodulation contrast. Figure 5.14
illustrates why we do not get a 100% demodulation contrast even under ideal
stationary conditions.
potential
incident light
1/6 5/6
optical generation of
charge carriers
Figure 5.14 Simple model of shutter inefficiency (I).
(1) A fraction of charge generated under the left photogate will always travel to the
left and be integrated under the integration gate, independent of the
photogates’ bias conditions. This is because there is no potential barrier
between this photogate and the integration gate. The same is the case for the