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

138 CHAPTER 5
signal of the 850 nm LED for 1 MHz is more saw-tooth shaped rather than a square
wave. This also lowers the demodulation contrast. The real demodulation
performance of the pixel for 850 nm light at frequencies higher than 200 kHz is
therefore better than we can show with this slow LED.
Contrast in %
Contrast in %
70
60
50
40
30
20
10
70
60
50
40
30
20
10
470nm
630nm
740nm
850nm
0
1000 10000 100000 1000000 10000000 100000000
0
1kHz
10kHz
100kHz
1MHz
10MHz
20MHz
Modulation frequency in Hz
400 500 600 700 800 900
Wavelength in nm
Contrast in %
Contrast in %
70
60
50
40
30
20
10
470nm
630nm
740nm
850nm
0
1000 10000 100000 1000000 10000000 100000000
70
60
50
40
30
20
10
0
10kHz
100kHz
1MHz
10MHz
20MHz
Modulation frequency in Hz
400 500 600 700 800 900
Wavelength in nm
SCCD BCCD
Figure 5.22 Demodulation contrast vs. modulation frequency and wavelength.
Note that the optical output signal for 850 nm and 1 MHz is saw tooth
shaped rather than a square wave. We obtain a better performance
for faster NIR light sources. [MCD06S; MCD06B].
The optical power for the 630 nm measurement at 20 MHz is 900 femtowatts per
pixel. The power is measured only for the red LED. All other LEDs are operated
with the same current and the optical power on each photogate is adjusted such
that the output amplitude of the sensor is similar for all measurements. (This can be
achieved by different sizes of the LED spot). This means that, independently from
the quantum efficiency at the specific wavelength, the power is chosen such that
nearly the same number of electrons is generated per unit time. The absolute