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

166 CHAPTER 6
Boundary conditions Calculation
Optical power at 7.5m distance 10 µW/cm 2 Projected pixel size on the target 30 mm x
(The area, projected to one pixel, is only
22.5 mm high, therefore the optical power
density is 10 µW/cm 2 )
25 mm
Power on projected pixel area 73.1 µW
Target Reflected power of projected pixel
area
51.2 µW
Distance 7.5 m Effective diameter of objective 3.3 mm
Reflectivity of target
0.7 Power on pixel in fW 885
Objective (CS-mount) Energy in pixel 11.1E-15 J
Focal length 4 mm Energy of 1 photon 3.16E-19 J
F/# 1.2 Number of photons per pixel 35,000
Transmission of lens: 0.7 Number of electrons per pixel 22,800
Transmission of filter:
0.5 Sensitivity of output stage 3.6 µV/electr.
Sensor Output voltage 82 mV
Pixel size
16 µm x #modulation cycles for generation 11
13 µm of 1 electron
Operating conditions Resulting distance accuracy < 5 cm
Integration time 12.5 ms
Wavelength 630 nm
Quantum efficiency 0.65
Conversion capacitance 40 fF
Amplification of source follower 0.9
Modulation frequency 20 MHz
Table 6.1 Optical power budget for the 64-LED modulated light source.
Phase homogeneity of the LEDs
The optical and electrical performance of the LEDs we use for the modulated light
source has already been discussed above and in Chapter 5 (c.f. Figure 5.11). In
addition, for the distance measurement, it is important that there be no phase
difference in the optical output of the LEDs. Such a phase difference could either be
caused by electrical delays on the PCB that holds the LEDs or technological
variations between the single LEDs. The next graph shows the measured phase of
the single LEDs. In Figure 6.12 we can see that the measured phase homogeneity
of the LEDs is better than ± 0.5 ns. This is sufficient for a homogenous phase of the
LED illumination in the far field.