To perform a deeper comparison analysis of the features present in the samples that we have fabricated from original bullets a technique providing also information about the real topography is required.
The best balance between resolution and versatility is provided by the white light optical profilometry (WLOP).
This technique, based on the interferometry of the light reflected by the sample surface, can achieve in the vertical scale (topography) a resolution of few nanometers, while on the X–Y plane the resolution is the same of optical microscopy.
Recently this kind of technology has been also widely applied to forensic ballistic measurements (see the latest models of the Integrated Ballistics Identification System, IBIS, for example).
Compared to a bare optical image, an optical topography prevent all the artifacts due to the illumination issues [10] and allows a further level of comparison of the identified features [24].
As an example in Fig. 5, two profilometric images of different bullets are reported, the top views are combined with their 3D rendering for appreciating their three dimensional nature.
We have here chosen to superimpose a section analysis of the two images along the direction perpendicular to the striae to prove that the lateral colocalization of the features is flanked by the association of their vertical sizes.
The analysis herein reported show the accuracy of the fabrication and comparison of planar replicas from curved ballistic samples.
Nowadays several microscopy techniques allow going to resolutions far better than those reported above.
Scanning electron microscopy micrographs have been made on two replicas of bullets shot by the same weapons, where one can see at low magnification the sequence of micro-striae and at high magnification their local morphology [25].
These data show that our method is compatible also with the searching and comparison of very small features on the ballistic samples [26].