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

DEMODULATION PIXELS IN CMOS/CCD 111
sampling point, so that 50% of the received modulated light is evaluated and only
50% is lost. This is an acceptable compromise when compared with the enormous
improvement in fill factor (20% rather than 5%). This integration time of half the
modulation period has no influence on the measured phase; it only attenuates the
measured amplitude to 64% of the real amplitude, which is a consistent and
predictable error (c.f. Section 2.2).
opaque layer
CCD gates
oxide
substrate
potential
intensity of incoming light
sampling interval
sampling interval
sampling interval
0V 10V 8V 0V 3.5V 0V 8V 10V
integration
direction
(a)
t
dump
direction
dump
diffusion
potential
i =
s(
t)
⋅
f(
t
+ τ)
f(
t
+ τ)
∫i = s(
t)
⋅ f(
t + τ)
= s(
t)
⊗ f(
t)
t=
τ
Q = ∫
Figure 5.7 One-tap lock-in pixel: illustration of (a) sampling and (b) correlation
process.
The more serious drawback is the fact that the sampling points have to be acquired
serially. This “serial tap-integration” reduces the application area of the pixel in 3Dmeasurement
to scenes with relatively slow movements. The integrated sampling
points Ai in Equation 2.17 contain both a fraction of integrated modulated light and
a fraction of background light. If the sampling points are all taken at the same time
(in parallel) they carry the same offset, even if the scene changes (moves) during
integration. With the subtraction of two sampling points the offset disappears. The
same is the case if the reflectance of an observed point changes during integration.
However, if we acquire the sampling points serially and the reflectance or
background intensity (offset) changes from one sampling point to the other, the
algorithm is no longer able to eliminate offset and reflectance. Therefore, the 1-tap
s(
t)
(b)
s(
t)