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

134 CHAPTER 5
We see that even for 10 V the saturation velocity is not yet reached. The results we
presented (increase in demodulation contrast for increased PGL / PGR voltage) are
reasonable because an increase in electrical field still causes an increased drift
velocity and hence faster response of charge carriers to the electrical field, leading
to a better demodulation contrast at high modulation frequencies.
5.2.4 Influence of optical power and integration time @ 20MHz
We discussed in Section 3.2 that the charge transfer efficiency of CCDs strongly
depends on the number of charge carriers that are contained in the charge packet
to transport (c.f. Figure 3.20). Therefore we expect the optical power, i.e. how
many photoelectrons are generated (and hence have to be transported) per unit
time, to have a noticeable influence on the demodulation contrast. This is
investigated in the current section.
In order to measure the influence of the number of generated photoelectrons per
unit time on the contrast, two parameters are changed: (1) the optical power and (2)
the integration times. For a good reproducibility of the measurements we keep the
output power of the LED constant and use neutral density filters (ND) to attenuate
the power on the pixel. This has the advantage that the LED can be operated with
the same mean current (50 mA) for all measurements. (Because of a currentdependent
operating temperature, the LED changes its dynamic characteristics for
different control currents). For each integration time we chose a different spot size
(by adjusting the distance of the microscope’s objective to the sensor chip), the
exact parameters are summarized in MCD05 in the appendix.
We carry out measurements of the demodulation contrast for 4 different optical
power conditions (1) without ND filter, (2) with 27% ND filter (3) with 8% ND filter
and (4) with both intensity filters (2% attenuation). Since the total optical power and
the size of the light spot are measured, we can recalculate the optical power on the
light sensitive area of each pixel. The dark current is measured separately and
subtracted from the measured values before calculating the demodulation contrast.
These measurements have only been performed for the surface-channel realization
(SCCD); the results are shown in Figure 5.19 and Figure 5.20.