Structured illumination

The term structured light or structured illumination refers to the projection of light with a known shading pattern. The result is the projection of a known light pattern on the captured scene.

The main purpose of structured light projection is to detect and measure the deformation of the expected pattern on the scene. As an immediate consequence, structured light is used in 3D reconstruction of objects.

Structured light technique
Structured light technique.

Some examples of structured light projection are:

  • projection of a grid of lines on a surface in order to find the line crossing and reconstruct the surface shape
  • projection of a single line, called also sheet of light, and scan of the object with the purpose of the reconstruction of the surface of the object
  • projection of a pseudo-random cloud of dots for more accurate and fine 3D reconstruction
  • projection of sinusoidal patterns for advanced surface analysis

Although both LED and laser sources are commonly used for pattern projection, the latter present several disadvantages. 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 lighting 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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