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

SUMMARY AND PERSPECTIVE 183
into the storage area. Additionally, the more sinusoidal rather than square shaped
optical input signal at this frequency leads to an additional amplitude decrease so
that the final operation limit in terms of modulation frequency is still not reached at
20 MHz. The reason why we did not further increase the modulation frequency was
the limitation of the available control and driving electronics and also limitations of
the light source we used. The measured time or phase accuracy strongly depends
on the power of the modulated light and the integration time chosen for the signal
acquisition. For an integration time of 12.5 ms and an optical power of 750 fW, the
measured performance displays an accuracy of better than 250 picoseconds and
1.8 degrees respectively, resulting in a distance resolution of better than 4 cm. This
is a surprising performance, because the above mentioned optical power implies
that one electron is generated, statistically speaking, only every 13 th modulation
period. This means that single electrons are moved, distributed and added in the
pixel with excellent reliability. The measured performance shows excellent fit to the
theoretical resolution predictions derived in Section 4.2. We are confident,
therefore, that we fully understand the relevant physical processes and their
influence on the distance resolution in any TOF ranging technique that relies on the
demodulation of modulated wave fields.
Based on the 1-tap demodulation pixel we have built two range cameras: one TOF
line camera (2D-range camera), working with the 108 pixel lock-in line sensor and
one 3D camera based on the 64 x 25 pixel lock-in imager. The measured range
accuracy is again in excellent agreement with our theoretical prediction. With the
3D camera we achieve a resolution of better than 10 cm for 100 ms integration time
(10 Hz) at 7.5 meters distance for non-cooperative targets of 70% reflectivity. The
range accuracy increases for shorter distances or longer integration times (more
optical energy). The range accuracy of the line camera is even better, since the
LED light can be focussed in one dimension, leading to an increase in optical power
density on the target. For this line sensor application, however, the illumination
setup and imaging optics can still be improved, since currently only about 5% of the
optically illuminated area is imaged on the pixels.
The drawback of serial sampling point acquisition of our current pixel realization,
that leads to measurement errors for quickly moving objects, can be overcome by a
new pixel structure described in Section 5.3. This solves the homogeneity problem
of accessing one light sensitive area within the pixel from four sides, as previously