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

100 CHAPTER 5
be read out at a frame rate of some tens of MHz. As the system uses active
illumination and the information of interest is carried by the modulated light, the fill
factor has to be as high as possible for good sensitivity (c.f. Chapter 3). Also the
dynamic range of the pixels is of high importance. The intensity of back-scattered
light decreases with the square of the target’s distance to the range camera (c.f.
Chapter 4). Another boundary condition for the pixel realization is that, due to a
limited research budget for this project, we only have access to semiconductor
technologies offered as MPWs (multi project wafers) that are more or less standard
processes. We can summarize the physical demands on our detector into four
principal tasks:
1. Energy conversion.
� Conversion of photons into electron-hole pairs.
2. Fast separation of optically generated charge carriers (photoelectrons).
� Avoidance of temporal blurring of the received time critical information.
3. Repeated, noise-free addition of separated photoelectrons.
� Improvement of SNR and insensitivity to spurious frequencies.
4. In-pixel storage of the acquired sampling points.
+
∑
A0
= a0,
i
PIXEL
Energy conversion Fast Separation Repeated addition In-pixel storage
Figure 5.1 Physical demands on a demodulation pixel.
All these demands, illustrated in Figure 5.1, can be realized excellently with the
CCD principle. Better than any other electronic principle used in integrated circuits,
the CCD principle allows nearly loss-free addition and directed transport (spatial
distribution / shutter mechanism) of electrical signals in the charge domain.