Acquisition of 3D topographic images of tool marks using non-linear photometric stereo method

摘要

There is described a method and 3D image acquisition system for addressing the specular nature of metallic surfaces in general, and ballistic pieces of evidence in particular, using photometric stereo by identifying and solving a plurality of sets of non-linear equations comprising a diffusive term and a specular term to determine a surface normal vector field N(x,y), and using N(x,y) to determine a 3D topography Z(x,y).

主权项

1. A method for determining a 3D topography Z(x,y) of a surface of an object, the method comprising:
successively illuminating the surface of the object from a plurality of localized axis positions, the object having a specular reflection dominant over a diffuse reflection; acquiring at least one image of the surface of the object at each successive illumination of the surface of the object using a sensor having a sensing axis substantially perpendicular to a general plane of the surface of the object, thereby resulting in a set of radiance images having a same field of view (FOV); for each radiance image of the set, defining a measured radiance at position (x, y) on an image i as a sum of a diffuse reflection term and a specular reflection term dependent on a surface normal vector field N(x, y), a light source direction Li and a camera direction C, to obtain a set of non-linear equations; solving the set of nonlinear equations for at least the specular reflection term to find the surface normal vector field N(x,y); and integrating the surface normal vector field N(x, y) over the FOV to determine the 3D topography Z(x,y).