The Microscribe is a 3D digitizing precision mechanical arm in which high performance sensors embedded in its articulated joints track the position and orientation of the stylus tip as it is moved in space by the operator. By pressing a button or a foot pedal (depending on the accessory in use), it is possible to record the relative spatial position of the tip in any given moment and, through a serial connection, send such data to a computer. There are many CAD and/or 3D oriented softwares capable of making use of such input just as if it were coming from a mouse or digitising tablet (with the fundamental difference of the 3D nature of such data). Moreover, the system is highly portable: whether it is set on a table simply sitting on its base or on a tripod it can be placed just about anywhere provided there is a power source and a connection to a computer (a notebook for example). Not only it is possible to record single points but also streams of points by keeping the input button or pedal continuously pressed while the stylus tip is passed over the object to be digitised; this method is particularly indicated in association with softwares that make use of "point clouds". Since the first three points recorded during a digitising session are used to establish the reference plane to which all successive points are referenced, it is also possible to fix, in a different area, another set of three points during the same session to be used as reference points for a subsequent session; this enables to cover much wider areas, far beyond the actual reach of the digitizer. As a last consideration we must not underestimate the intuitive and easy operativity of the device, in which the operator is always in total control of all activity. 

The 3D digitizer was used to retrace the outer contour of the only remaining portion of the mosaic placed on the vault, point by point. A series of parallel lines running over the vault’s surfaces were also digitised to be used as a basis for their reconstruction. This procedure was chosen given the insufficient clearance between vault and total station for the laser rangefinder to work properly. All the frescoed designs and the traces of figures, left on the lost portions of the mosaics, were digitised with the Microscribe Arm and, as a further test, a detail of the face of the Christ was digitised tessera by tessera before the restoration operations. 

The software used in connection with the digitizing arm (Rhinoceros 3D) not only gave an interface between instrument and user, but it also permitted us to digitally recreate, shapes, lines, curves, surfaces in their exact spatial position and, most of all, in real time. This data, through further processing, was instrumental in the construction of our 3D model, to be used either for the extraction of two dimensional representations by projecting it onto whichever plane we desired to observe it on or, by retaining its 3D characteristics, use it in one of the many applications that made use of spatially defined entities 

• To complete the surveying of the wall and vault surfaces where the use of the total station proved unviable as for example on the top of the vault due to the lack of clearance for the laser EDM to be used successfully

• To "retrace" all the hard-to-see details of the mosaic sinopite otherwise not distinguishable in photographs

• To position and document all conservation relevant features such as voids, cracks, painting detachments etc.

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