A pulsed TOF distance measuring device consists of a laser
transmitter emitting pulses with a duration of 5 to 50 ns, a
receiver channel including ap-i-nor an avalanche photodiode, amplifiers, an automatic gain control~AGC! and timing discriminators. The emitted light pulse~start pulse!triggers the time interval measurement unit, and the reflected
light pulse~stop pulse!stops it. The distance to the target is
proportional to the time interval. A block diagram of a laser
range finder is shown in Fig. 5.
The selection of laser type depends on the intended measurement range and the required speed. For long distances
~up to several kilometers!, a Nd:YAG laser can be used,
giving peak powers extending to the megawatt level. Lowpriced pulsed laser diodes@single heterostructure ~SH! or
double heterostructure ~DH!-type#, capable of producing
peak powers of tens of watts, enable measuring distances
up to a few hundreds of meters—or even longer using coherent summing. The repetition frequency of YAG lasers is
low, whereas laser diodes can be used at rates of tens of
kilohertz, the DH-type may even reach the megahertz level.
In 3-D measurements, the laser range finder is equipped
with angle encoders to enable the definition of the coordinates of the measurement point. Scanning is mechanical
and is carried out manually or automatically. In some applications, manual scanning is adequate but for time-critical
purposes a servo system is required to increase the measurement rate. Basic techniques include scanning either the
measuring head or only the measuring beam by means of
galvanometer-driven mirrors.
Focal plane scanning can be used instead of a narrow
laser beam, which scans a surface mechanically, point by
point. This enables a range map to be obtained without
mechanical beam scanning. The result is highly improved
3-D mapping performance, particularly with respect to
measuring time, at considerably reduced mechanical complexity and thus reduced size and power requirements. The
principle of focal plane scanning is presented in Fig. 6.
The laser beam illuminates the total field of view on the
surface. A target’s surface is viewed using matrices of
separate detectors. Each detector covers its own fraction of
the field of view illuminated by the laser. Detector signals
are analyzed in the time domain, and distances to particular
points are calculated on the basis of time interval measurements. The system can simultaneously measure distances to
several directions without any moving parts.