While the resolution affects the image quality in the x and y dimensions, the accuracy refers to the image quality in the z (or depth, or range) dimension. 3DD scanners calculate the range of each point on the target object. CCD noise, imperfect optics, and fundamental laws of physics all result in some error in the calculated range. It is not possible to design a scanner that does not have some error. The accuracy is a measure of this error. It is defined as the standard deviation of the difference between the measured range and the actual range to a target. The range error is approximately Gaussian in nature. This implies that if we scan a perfectly flat plane, at a fixed range of 245mm (10″), and if the accuracy of the scanner is 250microns (0.01″), and we build a histogram of the ranges of each of the points it would look like the one shown in the following figure. 65% of the range points will lie within +/- 1 standard deviation from the mean, i.e.: +/-250microns (+/- 0.01″). 95% of the range points will lie within +/- 2 standard deviations from the mean, i.e.: +/-250microns (+/- 0.02″).

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