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

172 CHAPTER 6
In order to adapt the camera to different application needs, we have produced the
LED module as a stacked 2 PCB- module, one carrying the driving electronics and
the other one carrying the LEDs. So it is possible to easily change the LED type
and thus the wavelength of the light source. Figure 6.18 shows the 630 nm and an
820 nm LED module, each containing 160 LEDs.
Figure 6.18 LED modules for the 3D camera (160 LEDs). Left: 630 nm module
(visible), right: 820 nm module (NIR, invisible). The 630 nm module
offers a total optical power of 900 mW.
The optical power of the HPWT-DH00 LEDs used for the 160-LED 630 nm module
is, unfortunately, only about 5.5 mW (@ 50mA), rather than the 8.5 mW (@ 50mA)
for the line sensor module (the distributor could only deliver LEDs of a lower power
category). The total optical power of the 630 nm module is therefore about
900 mW.
Optical power budget
Since, in contrast to the line camera, we cannot increase the light intensity on the
object by additional optics, in general we receive less light in each pixel than with
the line camera. Therefore, we increase the integration time to 100 ms (4⋅25 ms,
10 Hz frame rate). With the 2.6 mm 1:1.0 CS-mount lens used, we can calculate
the number of electrons received per pixel as a function of the target reflectivity and
distance. This optical power budget is given in Table 6.2. The range resolution for a
target at 7.5 m distance with a reflectivity of 0.7 is better than 9 cm.