A new optical device has been developed in order to measure in-plane strain fields on front and lateral faces of rectangular cross-section specimens during tension tests. It is based on digital image correlation (DIC) technique, by using a mirror reflecting the lateral grainy pattern of the specimen. Local ''true'' stress and ''true'' volumetric strain have been calculated excluding classical incompressibility or isotropy assumption. The spatial resolution reached with this technique has allowed to identify high deformation gradients in transverse directions. Front and lateral in-plane measurement fields have been projected in respective perpendicular direction, allowing the analysis of the volumetric behaviour of the studied composites for any node of a three-dimensional mesh. Applied to impact-modified polypropylene composites, this 3D DIC technique has revealed the influence of talc content on volumetric dilatation gradients.