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

POWER BUDGET AND RESOLUTION LIMITS 89
to the surface normal. The reflection coefficient has values between zero and one
for Lambert reflectors (ρ=1 for a white sheet of paper). Retro-reflectors, for example
in traffic signs, and mirrors lead to a directed reflection, which depends on the
illumination angle with respect to the surface normal. In the reflection direction the
equivalent reflectivity can then be larger than 1 (retro-reflector: ρ=100-1000, mirror:
ρ=1000). Figure 4.3 shows the wavelength dependent reflection coefficients of
concrete, vegetation and blue cloth. Table 4.1 gives an overview over reflectivity
values of typical diffusely reflecting materials at a wavelength of 900 nm.
White paper up to 100% Carbonate sand (dry) 57%
Snow 94% Carbonate sand (wet) 41%
White masonry 85% Rough clean wood pallet 25%
Limestone, clay up to 75% Smooth concrete 24%
Newspaper 69% Dry asphalt with pebbles 17%
Deciduous trees typ. 60% Black rubber tire 2%
Coniferous trees typ. 30%
Table 4.1 Typical reflectivity values for various materials [RIG].
It is interesting to note that a given optical power density on the object always leads
to the same number of electrons in the pixel, independent of its distance. If the
same light power is used to illuminate an area of one square meter, one 5 m away
from the sensor and another 50 m away from the sensor, the number of electrons
generated in one pixel stays the same. This is because the angle of view for the
observation of a fixed area also decreases with the distance of the object. Hence,
the illumination beam has to have a smaller divergence to illuminate the object at a
greater distance with the same power density. Put another way, with increasing
distance of the target, assuming that the power density on the target remains the
same, only the number of pixels illuminated by the target changes, not, however,
the power in each of the illuminated pixels (c.f. Equation 4.3).