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

164 CHAPTER 6
Figure 6.10 Photograph of the 108 pixel TOF line camera. The modulated light
source consists of 64 LEDs (630nm, HPWT-DH00-00000), focused
by a cylindrical lens. Between both LED arms is the camera module
with the objective in the middle. On the top of the left photograph one
can see the sequencer board and the driver board plugged together.
Setup and performance of the modulated illumination unit
The measured beam profile is shown in Figure 6.11. Each LED has a light power of
approximately 8.5 mW at 630 nm (50° beam divergence). By focusing the LED light
with a cylindrical lens in the vertical direction the optical power density can be
locally increased by a factor of 7.5 in the (horizontal) observation plane, resulting in
a power density of 10 µW/cm 2 at 7.5 m distance. With the objective used (1:1.2,
4 mm) and a spectral filter with 50 % attenuation, this results in an optical power of
885 fW per pixel for a diffusely reflecting object of reflectance 0.7 (blue pair of
jeans) (c.f. Chapter 4). Such objects (reflectance of 0.7) at 7.5 m distance can be
measured with an accuracy of better than 5 cm for 50 ms integration time
(4⋅12.5 ms; 20 Hz frame rate). The accuracy increases for smaller distances or
longer integration times and decreases for less reflecting targets (c.f. Section 4.2).
This power budget is summarized in Table 6.1.