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

DEMODULATION PIXELS IN CMOS/CCD 137
5.2.5 Demodulation contrast versus frequency and
wavelength
The application areas of the TOF- range cameras are essentially restricted by two
important parameters: the required range resolution and the wavelength of the light
used. While almost all range measurement applications demand the best possible
range resolution, which requires a good demodulation bandwidth of the pixels,
some applications additionally require the use of near infrared light (invisible for the
human eye).
In this section we present the measurements of demodulation contrast for different
modulation frequencies and wavelengths of the light source. As light sources, we
use different kinds of LEDs. They all have a characteristic response time and
dynamic behavior. We operate them with the MICROCHOP module MV-LED1
[CS1], a fast LED driver module for the microbench. Figure 5.21 shows the
measured modulated output signal of all the LEDs that we use.
470 nm-LED signal @ 20 MHz
(20 ns/div)
630 nm-LED signal @ 20 MHz
(20 ns/div)
740 nm-LED signal @ 20 MHz
(20 ns/div)
850 nm-LED signal @ 200 kHz
(1 µs/div)
850 nm-LED signal @ 1 MHz
(0.5 µs/div)
Figure 5.21 Signal shape of the used LEDs for different modulation frequencies.
(Measured with FEMTO 500 MHz photodiode module [FEM]
PR-X-500M-SI-DC).
The 850 nm LED can obviously only be used up to 1 MHz, since its characteristic
20% / 80%-rise time of about 500 ns does not allow faster modulation. The output