Structured illumination

The projection of a light pattern on a surface can easily give information on its 3D dimensional features.

Structured light technique.
Structured light technique.

For example, if we observe a line projected from the vertical direction with a camera looking from a known angle, we can determine the height of the object where the line is projected. This concept can be extended using various different patterns, such as grids, crosses, dots etc.

Although both LED and laser sources are commonly used for pattern projection, the latter present several disadvantages (Fig. 36). The laser light profile of the line has a Gaussian shape, being higher at the center and decreasing at the edges of the stripe.

Additionally, projecting a laser onto a surface produces the so called “speckle effect”, i.e. an interference phenomenon that causes loss of edge sharpness of the laser line, due to the high coherent nature of the laser light.

With laser emitters the illumination decays both across the line cross section and along the line width. Additionally, lines from laser emitters show blurred edges and diffraction/speckle effects.

LASER vs LED in structured light illumination.
LASER vs LED in structured light illumination.

On the other hand, using LED light for structured illumination will eliminate these issues. Opto Engineering® LED pattern projectors feature thinner lines, sharper edges and more homogeneous illumination than lasers. Since light is produced by a finite-size source, it can be stopped by a physical pattern with the desired features, collected by a common lens and projected on the surface.

Light intensity is constant through the projected pattern with no visible speckle, since LED light is much less coherent than laser light. Additionally, white light can be easily produced and used in the projection process.

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