Structured Illumination And Moiré Techniques
A further increase in image-capture speed can be achieved through the use of more sophisticated illumination. This illumination can take on many forms, but is typically an array of dots or a set of projected lines. An image of the structured illumination shown on an object can then be processed in much the same way as triangulation data, but a full frame at a time. It generally only takes a handful of these full-frame images to describe a surface three-dimensionally, making structured illumination techniques extremely fast.
One particularly interesting type of three-dimensional scanner that uses structured illumination is based on a phenomenon known as the moiré effect. The moiré effect is a fascinating visual display that often occurs when two periodic patterns are overlaid. It can easily be seen in everyday experiences such as overlapping window curtains or on television when a character wears a shirt with stripes that have nearly the same spacing as the TV lines. Moiré scanners typically operate by projecting a set of lines onto an object and then viewing that object through a transparency containing another set of lines. The resulting moiré pattern is an array of curves that trace out paths of equal object height, much like elevation lines on a topographical map. This image can then be used directly to check for surface features or combined with a few others and processed to give a true three-dimensional plot of the object.
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