The lighting is supplied by an external source of light which is slanting (30°-60°) with respect to the surface of the object. The images become visible as the luminous rays, reflected on the surfaces with the same angle of incidence, assume a higher luminous intensity with respect to all rays reflected with different slants.
The shades created with this system are less stressed than those created with the grazing light system. The rays reflect even on deep parts of the surfaces, making this system suitable to visualize wider and deeper areas of the surface of an object. This type of lighting is used instead of the coaxial one because even the slanting surfaces which are lighted become partially visible.