1. Automatic process for inspecting and sorting non-metallic objects that belong to at least two different categories and pass in an essentially single-layer stream on a conveying plane of a conveyor belt, whereby said process consists essentially in temporarily subjecting a surface or outside layer ( 4 ) of said objects ( 1 ) to the caloric radiation of at least one remote heating means ( 5 ), so as to deliver to each of these passing objects ( 1 ) a non-altering heat pulse that is identical for all of the objects in terms of heat energy applied per unit of surface area in the conveying plane ( 2 ), then to acquire at least one thermal image of each of said objects by means of at least one linear or matrix thermal sensor ( 6 ), for example a thermal camera, this after a determined length of time has elapsed following the application of the heat pulse, then to classify or to categorize each passing object ( 1 ) based on data contained in its thermal image or images and to deliver a control or actuation signal for each object and, finally, to separate the passing objects ( 1 ) based on their class or category and/or the corresponding control or actuation signal that is delivered, process characterized: in that the data of the image or the thermal image(s) from each passing object ( 1 ) are processed to perform discrimination or characterization of the objects in terms of the thickness of the surface layer ( 4 ) affected by the radiation, whereby the constituent material of said surface layer ( 4 ) at least of the passing objects ( 1 ) is identical for all of the objects, in that for a given object ( 1 ), the length of time that elapses between the application of the heating radiation and thermal imaging is, on the one hand, sufficient for ending in an essentially homogeneous distribution of the caloric energy that is absorbed in said surface layer ( 4 ) such that the temperature difference after heating said objects on the surface is essentially inversely proportional to the thickness of this surface layer ( 4 ), while being, in contrast, short enough so that the phenomena of lateral thermal diffusion, cooling by radiation and convection are negligible.