Wednesday, November 30, 2011

Scanners and Shiny Stuff

Have you heard it? Have you felt it?
...the glaring silence when people talk about scanning a shiny object?
Sometimes the response is to jump quickly to questions of accuracy on parts that AREN'T shiny...
Sometimes the response is to jump quickly to a can of white powder spray...
Sometimes the response is to ignore the topic completely and hope it doesn't appear again...
None of the above responses are particularly flattering for the customer or the 3D metrology company. 
We've all seen the data: the glaringly high outliers, the strange reflections in the data, discontinuous data, the lack of data altogether, the high levels of noise, and the incomplete data coverage across the scanner's field of view.
But now it's time to write a rulebook on shiny objects. These are the "laws" that govern the 3D metrology of shiny objects.
Rule 1 - Scanning a shiny object will NEVER be as accurate as scanning a non-shiny one. As a rule of thumb, expect an order of magnitude up to several orders of magnitude of difference. This is because the "feature finding uncertainty" (whether a fringe, line, or pattern) shoots way up due to the specularity of the surface, and diffraction.
Rule 2 - A camera-projector setup that relies on cameras and projectors in fixed locations relative to a moving (non-fixed) part will suffer from severe systemic errors (several orders of magnitude of difference relative to a non-shiny part). Unfortunately, this setup applies to all "head" style scanners on the market -- which represents the vast majority of all 3D scanners. The pathological error in the case of fixed-camera-projector systems is known as multipath -- the phenomenon whereby the projected light reflects from one surface onto another, resulting in a "path" of light that takes "multiple" routes due to reflection.
Rule 3 - Scanning a shiny object MUST take place in a completely dark room/enclosure. The signal-to-noise requirements for shiny parts are much more demanding than non-shiny parts, and therefore complete blackness is necessary.
The 3 rules above necessarily indicate that when scanning shiny objects, a highly-conservative design in terms of accuracy must be employed, the light source must remain fixed with the part (as opposed to fixed with the cameras), and a dark environment must be built. Unfortunately, this is not the kind of 3D scanner that you will find at trade shows, and therefore most customers have never seen a working 3D scanner that can successfully scan shiny objects.
On a side note, some companies have advertised scanners that are "better at shiny objects," but this is generally a marketing angle rather than based on the laws of physics. The kinds of errors caused by shiny surfaces are not well-addressed by the claims of these companies, and by and large, the systems are not successful for high-accuracy work. For high-accuracy work, the 3 rules above must be observed. The reason these companies may advertise these claims is because, at a surface-finish level (i.e. surface porosity and microreflection level), some approaches are superior, however, they are useless for the large, dominant errors associated with shiny surfaces. In other words, solving a small problem does not solve the large one.

On a second side note, we have seen techniques to "mask out" portions of the projection to try to minimize multipath.  This is a valid technique, but it is not a catch-all.  It requires careful selection/deselection of very specific portions of the surface, and therefore requires very skilled operators.   
To measure shiny surfaces every time, no matter the surface, corners, or the shape, the three rules above must be upheld. 

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