1. Apparatus for computing a three dimensional model of an object, comprising: a portable, hand-held instrument comprising multiple directional energy sources arranged to directionally illuminate the object and an imaging sensing assembly having at least two spatially separated viewpoints obtained by respective cameras at known fixed positions relative to each other, the imaging sensing assembly arranged to record a series of images of the object at each viewpoint when the object is illuminated by each of the sources; at least one localization template having predetermined geometrical features, the localization template arranged in use to be visible to at least one of the viewpoints simultaneously with the object; and a processor arranged to analyze the images recorded at the viewpoints, so as to: determine a location and pose of each viewpoint relative to the template for each image in the series of images; generate photometric data for the object using the calculated location and pose of the viewpoints and the direction of illumination from each energy source; generate geometric data comprising a skeleton three dimensional model of the object by performing stereoscopic reconstruction using optical triangulation so as to provide the gross-scale shape of the object, wherein the optical triangulation is performed based on the known fixed positions of the cameras relative to each other; and combine the geometric data and photometric data by using the photometric data to provide high-frequency geometric detail onto the gross-scale shape so as to construct the three dimensional model.