A time-of-flight system using a laser projector is illustrated in Figure 7

The instrument has 3 principal functional components, a transmitter, a scanning mirror and a receiver. The basic idea is to measure the time difference between the reference signal and the transmitted/received signal; knowing the speed of light as it is possible to calculate the distance to the object. Measurement of the time difference is usually done either by comparing two very short pulsed bursts, or using a continuous amplitude or frequency modulated wave and measuring the phase difference or beat frequency. In figure 7, above, the laser beam is amplitude modulated and scanned over the scene by the mirror. When the beam strikes an object, a portion of the light is transmitted along a direction approximately coaxial to the optical axis of the receiver ( the exact amount depends on the reflectance characteristics of the object, as illustrated in figure 8, below), and captured by the photo-sensor, using an interference filter to reject ambient light. The amplitude of the received waveform is proportional to the intensity of the received light, but the phase difference between the received beam and the reference beam gives a direct measurement of the distance to the object point. The use of a phase-sensitive detector can give a more accurate indication of time-of-flight, and hence distance, than a pulsed system, in which a direct time measurement is made on an intermittent burst of laser energy. The accuracy of these systems is of the order of 1mm.

Figure 8:   Reflection of the laser signal at a surface

[ Three-Dimensional Imaging | Laser ranging using triangulation ]

Comments to: Sarah Price at ICBL.
(Last update: 4th July, 1996)