Laser triangulation

Laser triangulation, according to its low cost and simple implementation, is nowadays the most widespread technology on the market. Its applications spans from the biomedical area, for example for monitoring the cardiovascular system, to industrial applications, where needed non-contact measure and the other sensors, like accelerometers, does not work, for example in hot environments.

The main advantages of laser triangulation sensors are:
- non-contact and non-invasive measurements;
- good resolution and accuracy (with a trade-off on working distance);
- can measure on small targets since spot size of laser beam is very small.

The disadvantages of this technique are:
- the limited achievable range of working distance;
- it can be negatively affected by ambient light;
- it is difficult to measure on irregular and mirror-like surfaces.

From a basic point if view, a laser-triangulation system consists of a laser source with a lens that focalizes or collimates the laser beam on the target, and a CCD (or C-MOS) sensor or a PSD (Position Sensitive Detector), a second lens used to generate the laser spot image onto detector.

The idea behind this technique is very simple: the laser beam is aimed onto the target under test and the back-scattered light is focalised onto the PSD. The PSD  is an optical position sensor that can measure the position of a light spot in one or two dimensions. By changing the target position, the laser spot imaged on the PSD moves and the output signal from the sensor changes. Displacement or vibration measurements can be easily derived, by looking at the time-varying output signal from the sensor.