Increasing the sensitivity of laser triangulation systems using structured optical surfaces

Laser triangulation is an optical method for measuring distances to an object. A laser beam is directed towards a measurement surface, and the diffusely reflected light is collected by an imaging system onto a detector. The absolute distance can be obtained by using known geometric relations of the system and the position of the laser spot on the detector. Therefore, a change in the measurement distance results in a corresponding movement of the imaged laser spot, defining the sensitivity of the system in pixels per millimeter. This value depends on the geometrical and optical design of the laser triangulation setup, especially the base distance between the laser and the imaging lens, as well as its focal length. As those parameters also influence the geometric dimensions and the possible measurement range of the device, the sensitivity cannot be increased arbitrarily. Thus, the sensitivity of a standard laser triangulation system is limited to a certain value. In this contribution, structured optical surfaces are applied onto the measurement surface to further increase the sensitivity. Through the spatial modulation of the imaged laser spot intensity distribution, the calculated laser spot displacement is larger than its actual geometrical displacement. This effect is examined through simulations with a bar structure, which leads to an improvement of the sensitivity by a factor of up to 5.7 at a distance of 1 m and a measurement range of 2 mm. Eventually, the concept is proven in measurements and feasible implementations of such a structure are considered.