3d scanning accuracy

Even an expensive 3d scanner can provide different accuracy depending on a task it is used for, on action correction and on user’s qualification as well. May be due to this, some leading developers have removed the “accuracy” line from their technical descriptions at all and changed it with “noise”.
The following quality features of 3d scanners are used besides accuracy:

By accuracy one understands a possible measurement error, i.e., evaluating measured value deviation of an object surface 3-d coordinate from its true value.
Table data by their accuracy usually correspond to an ideal object and ideal scanning conditions. If white plasticine is considered as hard, then a buttered plasticine piece might be considered as ideal. Main factors that lead to accuracy loss are given in the table below:

An object is hard;An object is deformable;Fragments taken from different perspectives are skewed and match poorly An object is still;There is object/scanner motion during scanning;Any motion leads to result distortion (usually as small waves) An object is bright;An object is dark;A scanner works slower, the data noise increases. Matting becomes necessary sometimes. An object is matted;An object is non-glaring, semi-transparent or sparkling.;Without covering with a matting spray a result is poor or unsatisfactory An object is smooth;An object is with sharp ribs;The sharp ribs are always being smoothed a little. It is advised to set a scanner correctly against such ribs. An object is without holes, edges and grooves;An object is with holes, edges of sheet metal or with grooves (undercuts);An edge of a thin sheet on 3d data is always being torn without achieving the real object border. An object with good geometric features;An object with degeneracy along one or several directions, for instance, plane, sphere, rail.;Some fragments can’t be qualitatively matched based only on geometrical features. Accuracy becomes uncontrolled without using markers. An object doesn’t get into the scanning area;A big or lengthy object should be scanned by pieces;An additional error, which can grow, originates during matching parts. An object is being scanned perpendicularly to its surface;An object is being scanned at an angle. For instance, a bottom of the car standing on the ground;Decrease of detalization and accuracy. A scanner is heated up to its working temperature and calibrated.;A scanner is not heated up.;All measurements will contain small errors A calibration area is measured at actual temperature.;Scanning is done either in a cold/ hot room;Resulted scaling will slightly differ from the real object External illumination doesn’t obstruct scanning;Bright and directed illumination;Increased noise in data or impossibility to scan Cameras are spaced at correct distance;Cameras are shifted to scan hollows and undercuts;Increased noise in data

In order to achieve maximal accuracy, all components of a 3d scanner should be ideal. Of course, ideal components are impossible and those, which have better features, have high cost. That is why the understanding of tasks requiring different accuracies is very important practically.
The effect of thermal expansion limits the scanning accuracy. If special thermally stabilized rooms are not available, room temperature might alter within 10 C (for instance, from 18 to 28 C) or even more. Thus, length of a 1-m metal bar might alter by 0.1 mm. For aluminum alloys the alteration will make 0.2 mm and for some plastics it will make 1 mm or more. Therefore, in most cases there is no any practical value in accuracies better than 0.1-0.2 mm/ meter.
Our equipment has excellent accuracy for its price range. However, it is not designed to fulfill tasks of maximal accuracy requirements and neither has it corresponding certificates. Among such tasks are, for instance, control of fabricating turbine blades or artificial heart valves.
You may find our scanners’ data on the page “Products - > Features” of this site. The data are given for one scan of an ideal object taken under ideal conditions by a scanner adjusted for a corresponding area.

If fragments taken from different perspectives are compared, one can see than accuracy decreases due to matching errors. In the same time there are the following matching technologies:

Matching by geometric features of an object. This is the most accurate technology provided that there are enough geometric features on the object. Accuracy at such matching decreases a little, if the object doesn’t get in the scanning area completely. If geometric features are few, for instance, when scanning smooth fragments of a car, it is necessary to use matching by markers.
Matching by markers. Markers are being attached to an object before scanning (white round pieces on a black background). Their coordinates are being calculated by the scanner automatically and used for fragment matching. When scanning big and lengthy objects, errors of marker coordinate determination are being accumulated. Such objects should be scanned by means of photogrammetry. Markers should be stuck on correctly, i.e. chaotically and evenly over the whole surface as possible and not creating rows or lines.
Matching by markers through using a photogrammetric system. In this case matching is also done by markers, but markers’ coordinates are being identified by a photogrammetric system beforehand and with high accuracy. This allows to minimize accuracy loss, when matching big or lengthy objects.

In most artistic and design tasks there are no requirements to accuracy. Among the most important requirements there are: decreasing noise, increasing detalization, and complete scanning of the whole surface. Experts of a famous studio of industrial design pointed out to the fact that they very rarely need the scanning accuracy of 1 mm or better. Such accuracy is only needed, when it is necessary to match a processed part with already existing one, for instance, when scanning a car face bar.

When modeling objects based on scanned data, the following should be noted:
- dimensions of new objects are usually being rounded to integer values. For instance, a scanned plastic body dimensions are 100.18 x 220.03 x 194.87 mm. In this case a designer will most probably correct the dimensions in drawings as 100.00 x 220.00 x 195.00 mm.
- the CAD-object form built as per STL data from a scanner might be noticeably different from the real surface, as during its building with CAD-packages, we used math approximation, surface smothering and simplifying.
For the tasks which require accuracy, for instance, scanning car parts, it is suggested to use auxiliary means as photogrammetry or checking machines. It should be remembered that in spite of having good capabilities a scanner is not a measuring device and before production start-up at full scale, it is necessary to verify result correctness by either check measurements or making a prototype model.