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

DEMODULATION PIXELS IN CMOS/CCD 99
5. Demodulation pixels in CMOS/CCD
In Section 2.1.3 and 2.2 we have explained that we can measure distances by
measuring the phase of an RF-modulated optical radiation field. We have
suggested measuring these phases in an array of so-called demodulation pixels,
which are additionally capable of measuring the modulation signal’s amplitude as
well as the background brightness. Such an array of demodulation pixels would
make possible the realization of a 3D range camera without moving parts. This is
because, rather than scanning the laser beam over a scene, the scene can be
illuminated simultaneously with a light cone and each pixel in the imager array
individually measures the distance of the corresponding point in the scene. We also
suggested realizing the demodulation task by temporal sampling of the modulated
optical wave.
To realize such a fast sampling demodulation pixel, an efficient electrooptical
shutter mechanism is needed. A quick estimate gives an idea of the required speed
performance: Using a sinusoidal wave as modulation function for the TOFapplication
restricts the non-ambiguous measurement range to half a modulation
period π, not 2π because the modulated wave has to travel the distance from the
3D camera to the target twice: forth and back. If we postulate a distance resolution
of 7.5 cm and expect λ/200 phase resolution of the system, a reasonable value
compared to existing systems, we need a 15 m rf-wave. With f=c/λ we obtain the
modulation frequency of 20 MHz. For these values we have to take the single
sampling points Ai at a repetition rate of 20 MHz, corresponding to an effective
sampling rate of 80 MHz for 4 sampling points.
For a better SNR, the pixels should offer the possibility of successively adding
short-time integrated sampling points without additional noise. This technique also
makes the system insensitive to frequencies other than the modulation frequency,
and that’s why we also call our pixels lock-in pixels (c.f. Section 2.2 and
Figure 2.10). All sampling points (four, to keep the previous definitions) have to be
stored within the pixel because the complete high-resolution detector array cannot