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

DEMODULATION PIXELS IN CMOS/CCD 139
optical power, used for the other wavelengths (470 nm, 740 nm, and 850 nm), can
be recalculated from the ratio of the measured mean values of the sampling and
the quantum efficiency of the specific wavelength, which is known. The
measurement results are summarized in Figure 5.22. With another infrared LED,
emitting at 810 nm (manufacturer: EPIGAP), we obtain a demodulation contrast of
better than 25% at 20 MHz.
The measurements show the expected contrast decrease for high frequencies and
longer wavelengths (c.f. Section 5.2.2). Especially for high frequencies, the BCCD
performance is superior to that of the SCCD. For 20 MHz modulation and 630 nm
wavelength we still achieve a modulation contrast of better than 40% with the
BCCD version, which is still quite good compared to the DC performance of about
60%. Furthermore, we can state that the simple model described in Figure 5.14
and Figure 5.15 makes an astonishingly good performance prediction for the DC
case.
5.2.6 Phase accuracy measurements
In the previous sections, we presented measurements of the 1-tap pixel’s
demodulation contrast versus several parameters for the BCCD and the SCCD
realization of the 1-tap pixel. In the following we will only consider the BCCD
version. In fact, however, we are interested in the time or phase resolution offered
by the demodulation pixels. For a real distance measurement, the received optical
power varies with the distance of the target. It is therefore not practical to
characterize the time resolution with a distance measurement setup, because at the
same time the time delay (distance) changes; also the received power would
change. With such a setup one cannot measure the time resolution isolated from
the received power.