Photogrammetry applied to small and micro scaled objects: A review

Digital close range photogrammetry as 3D optical measurement system has achieving a great success due to the several advantages which meet the ever more complex market requests for free-form components. The capability of the sensor to be moved automatically around the scene, varying the distance from the object and the magnification ratio in a single scanning session allows to reconstruct the 3D model of the sample even in presence of undercuts or complex geometries. The latter are typical of most industrial sectors, e.g. automotive and aerospace, biomedical sector, nature science, e.g. entomology, as well as cultural heritage and archeologic field. Photogrammetry based scanners can be designed in a simple, robust and very cheap way, obtaining results similar to more complex and expensive scanners based on other technologies. In the last years, several steps have been carried out to consolidate and metrologically characterize this methodology with the final aim of making it comparable with other ones already accepted. Although, there are some relevant questions which still represent criticalities for this technique. This paper would provide a review of the main improvements obtained in testing photogrammetry applied to very small object with sub-millimetric features. Firstly, the improvements of using completely automated and controlled scanners were analysed in terms of time saving and precision of the instrument, then, the photogrammetric process has been decomposed in its main phases to allow the emergence of the respective critical issues, and, finally, all the methodological approaches for performance verification have been reported using the existing standards.