Machine vision using laser-based, 3D triangulation is a powerful way to acquire rapid, accurate measurements of product shape and dimensions, and it can
readily identify dimensionally nonconforming units on
high-speed production lines. As a result, this technique
is employed in industries as diverse as lumber mills and
But there are many different ways to implement a laser
line projection system. Each has its own characteristics,
advantages and disadvantages.
In triangulation (Figure 1), a laser line is projected
onto a part, and a camera views the line and analyzes its
shape dynamically to reconstruct the dimensions of the
object. There are four basic triangulation configurations
The most widely used configuration, called the standard geometry, positions the laser directly above and perpendicular to the object under test. The camera views the
line from an angle — usually about 45°.
The most important characteristic of this configuration
is that object height variations move the projected line
in the object’s Z axis only. This simplifies the calculations required to derive object shape, resulting in a system that is faster, more accurate and easier to calibrate
A disadvantage of this arrangement is that the camera
views the object from an angle, which increases the depth
of field needed to maintain focus as object height var-ies. Furthermore, magnification changes occur as object
height (and thus object distance from the lens) changes.
These disadvantages must be mitigated by calibration to
ensure system accuracy.
Standard geometry also can produce some occlusion.
In particular, whenever a camera views an object from
anything other than an angle perpendicular to the inspection surface, there may be some areas that cannot be seen.
This yields an inherent design trade-off: While measurement height resolution increases with camera angle (α) in
the standard geometry, so does occlusion.
The reverse geometry configuration switches the positions of laser source and camera. With a more oblique
angle of laser illumination, a given change in object
height produces a larger shift in laser line position. This
increases height resolution over the standard configuration. And, with the camera normal to the measurement
plane, there is no occlusion.
However, with reverse geometry, object height changes
(Z axis) move the projected line in both the Z and Y axes
of the object. This makes it more complex to interpret
results. Because of this, reverse geometry is generally
most useful with objects that have a relatively simple
The specular geometry configuration has both the laser
and camera at similar, non-normal angles to the object.
The camera therefore sees a larger line movement for
a given height change than either of the previous configurations. This yields greater height resolution, but it
becomes possible for the camera to see specular, or near
specular, reflections from the laser. These may produce
measurement errors if they cause saturation or blooming
in the detector. On the other hand, higher reflection levels
may increase signal levels from dark or highly textured
objects, making them easier to measure.
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Configuring a 3D triangulation vision system
BY DAN CALLEN
Figure 1. A projected laser line appears distorted when viewed from perspectives other than that of the projector.
This distortion is calibrated and used to derive the dimensions of the object under test.
resolution Useful with dark objects Offers highest measurement resolution
Requires large lens
depth of field
Specular reflections can
cause measurement errors Some occlusion
Use General purpose High-accuracy uses Dark-colored or highly textured objects Highly relective objects, (glass, metal, etc.)
A Comparison of 3D Triangulation Configurations