When the diffraction pattern of the focused laser beam of the read-out system falls on the plane surface between the pits (or bumps), substantially all the light is reflected back to pass through the focusing objective. When it falls on a bump (or pit), phase differences are imparted to the light in the diffraction pattern, and some of the light returning from the disc is diffracted to pass outside the focusing objective; it is this reduction in the light returning through the objective that constitutes the read-out signal. The strength of this signal depends on the height profile of the bump or pit which (because of the photoresist process) will show more or less ‘rounding’ of the edges.