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

140 CHAPTER 5
Measured delay in ns
50
40
30
20
10
0
Measured delay Distance error
0 10 20 30 40 50
Delay in ns
Figure 5.23 Phase delay for 736 fW optical power on one pixel (time error:
260 ps rms � distance accuracy: 3.84 cm rms). [MCD07].
For this reason we use the setup described in Section 5.2.2 for the phase accuracy
measurements presented below. With an analog delay line (EG&G-ORTEC 425A)
we can delay the optical 20 MHz modulated signal in steps of nanoseconds. This
allows different delays of the same light power to be measured with the
demodulation pixels. As an example, one measurement is shown in Figure 5.23.
The time error is converted into an equivalent distance error in this figure
(50 ns=7.5 m, 500 ps=7.5 cm). For the optical power of 736 fW per pixel, we get an
accuracy of 260 picoseconds.
By attenuating the LED light (with ND-filters) the optical input power can be varied.
For higher optical power, the distance accuracy increases and for lower power it
decreases. This gives the possibility of measuring the range accuracy (or time
resolution) versus the optical power received in each pixel. The results of these
measurements are shown in Table 5.3 and in Figure 5.24. All measurements have
been performed (1) without averaging and (2) with an average over 16 runs.
50
30
10
-10
-30
-50
Accuracy in cm