Nowadays, the use of optical 3D digitisation in metrology becomes more frequent and desired. Unfortunately, there are still no binding standards for determining measurement uncertainty of these systems and manufacturers of 3D scanners often use their own standards to define accuracy of their device. This paper introduces a methodology to assess the accuracy of digitisation using 3D optical scanners. The paper deals with practical implementation of an acceptance test for ATOS contact-less 3D scanners (from design and manufacturing of own test etalon, through the determination of its nominal dimensions, up to digitisation and evaluation) and publishes the results of several experiments demonstrating the impact of various factors on measurement accuracy.