Structured light scanning

Structured light scanning is a non-contact 3D scanning method in which a projector casts known light patterns, most often a series of stripes, onto an object while one or two cameras record how the pattern deforms over its shape. It is also known as fringe projection, or as white light and blue light scanning after the light source. The 3D coordinates of surface points are calculated by triangulation from the pattern's deformation and the known relative position of the projector and cameras.

In a single shot, the scanner projects a sequence of stripe patterns with different pitches and phase shifts, obtaining coordinates for practically every camera pixel. A shot takes a fraction of a second to a few seconds and captures the whole field of view at once, up to millions of points. Shots from several sides are merged using stuck-on reference points or the geometry itself. Modern scanners use white or blue light; narrow-band blue light is less sensitive to ambient light.

Stationary scanners stand on a tripod, often with a turntable, or on a robot in an automated inspection cell. Depending on the optics, the field of view ranges from a few centimeters to about one meter: the smaller the field, the finer the detail and the higher the accuracy, typically 0.02 to 0.05 mm and better still with the smallest fields. Handheld structured light scanners are more flexible but usually less accurate, around 0.05 to 0.1 mm.

Every point must be visible to both the projector and a camera, so deep narrow holes, grooves and internal cavities are missing from the data. Shiny, transparent and very dark surfaces reflect or absorb the light and are matted before scanning with a thin coat of powder or sublimating spray.

Structured light is the usual choice for small and medium-sized parts where detail and accuracy across the whole surface matter: plastic moldings, castings, forgings, turbine blades, molds, EDM electrodes, 3D printed parts and consumer products. It serves first article inspection, comparison with a CAD model and reverse engineering of freeform parts. Larger parts are handled with reference points or photogrammetry.

On small parts it is usually more accurate and detailed than a handheld laser scanner, while a laser is more practical on large objects, on the shop floor and on dark or shiny surfaces. Unlike CMM measurement, it captures the entire surface, but a coordinate measuring machine measures precise holes and fits more reliably.

A scanner's stated accuracy applies to ideal conditions and a specific field of view (manufacturers usually verify it to the VDI/VDE 2634 guideline). Final scan accuracy also depends on shot merging, vibration, temperature changes and whether the part moved during scanning. Ask about accuracy for your part size and the merging method.

The matting coat is on the order of micrometers thick, more if applied unevenly, and must be accounted for in precise measurements. Sharp edges and thin walls often come out rounded or incomplete, and small or deep holes may not be captured at all; for such features, agree on additional touch-probe measurement. In your order, state whether you want raw data, a cleaned watertight mesh or a comparison with the CAD model.