Three-dimensional shape acquisition by fringe projection
Using at least three stripe patterns of sinusoidal shaped intensity distributions recorded in a fast sequence, the exact lateral stripe displacements ('phase values') are determined for all points on the surface. Intensity (grey) levels are produced digitally by high resolution micro mirror displays (DLP), a proven, robust and extremely durable technology by Texas Instruments.
This improves the triangulation and thereby the height resolution up to two orders of magnitude, at an extremely low temperature drift and with exceptional long-term stability. Typically, the height resolution with this method surpasses the lateral resolution of the measuring camera by a factor of at least ten. With an area averaging applied, this factor can be increased up to one hundred (right figure).
With fringe projection, the measurement area can be scaled over a wide range, from less than a millimetre up to more than one metre, whereby the resolution scales accordingly. The method suits small samples due to its high resolution and accuracy for precise detail measurements, as well as large area surveys due to its speed.
The volume of motor combustion chambers, for example, can be measured ten times more accurately than by classical volume gauging based on gases or liquids (following figure).
With very small measuring fields, the resolution is high enough to respond to the wavelength of light used. With monochrome, blue LED light sources, optics and cameras deliver optimal results. Moreover, the optical reflection characteristics can be improved for a wide variety of technical material surfaces.
The measurement directly delivers detailed height values over a large range (following figure).
he method allows for very fast measurements as well. Typical measuring times are ranging between several seconds and a fraction of a second. Specialised equipment, however, can deliver more than 30 frames per second. As the image is built from a number of fringe projections to gain the best accuracy the object needs to be stationary during the pattern sequence. The number of sequence projections also has an effect on accuracy so a trade off between accuracy and speed can be adjusted through camera configuration.
High speed, combined with the acquisition of large measuring volumes, also predestines fringe projection for a large number of industrial in-line control and inspection tasks, including form deviation, completeness, parts positioning or volume. To reduce the effects of light shadows and make the system more tolerant of light noise some fringe projection cameras include two sensors either side of the projector. Complete solutions using the technology can also be found in the vision systems section of this handbook.
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