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

OPTICAL TOF RANGE MEASUREMENT 13
profile can then be measured with one shot, and for 3D data the scan only needs to
be performed in one direction (light sheet triangulation). In industrial inspection
applications, such a 1D scan is often available free of cost, since the objects to
measure are moving anyway, for example on an assembly-line. [KRA] presents
such a scanning 3D camera working with active light sheet triangulation and
delivering 50 depth images per second in CCIR video format.
Further advanced techniques even use 2D structured light illumination and 2D
imaging to perform 3D triangulation measurements without scanning. The most
important members of this structured light group are phase shifting projected fringe,
Gray code approach [BRE], phase shifting moiré [DOR], coded binary patterns,
random texture [BES] or color-coded light. They typically use LCD (Liquid Crystal
Display) projectors for the projection of the structured patterns.
Triangulation systems are available for applications from mm-range (depth of focus)
to 100km range (photogrammetry). Their main drawback is that for a good
resolution they have to be large in size, since they need a large triangulation base.
On the other hand, the larger this triangulation base, the more they are restricted by
shadowing effects. Also, 3D triangulation systems are relatively expensive since
they require fast LCD projectors (only active systems) as well as large
computational power.
2.1.2 Interferometry
Interferometry is described by the superposition of two monochromatic waves of
frequency ν, amplitude U1 and U2 and phase ϕ1 and ϕ2, respectively, resulting in
another monochromatic wave of the same frequency ν, but with different phase and
different amplitude [SAL]. In the easiest interferometer setup, the Michelson
interferometer, illustrated in Figure 2.3, a laser beam (monochromatic and
coherent) is split into two rays by a beam splitter. One ray is projected to a mirror of
constant displacement x1 (reference path) whereas the other beam is targeted on
the object of variable distance x2 (measurement path). Both beams are reflected
back to the beam splitter, which projects them onto an integrating detector.