Left: convention used for the angles of incidence and diffraction. Right: pictorial description of the laminar and blazed gratings with relative nomenclature. d(x) is the period. α and β are the angles of incidence and diffraction, respectively, considered positive with respect to the normal. h is the groove depth for laminar gratings and W is the groove width. θ is the blaze angle and χ the apex angle. r is the source-to-grating distance and r′ is the grating-to-focus distance.