only the first order diffraction 102 but also the second order diffraction 103 occurs in the first order diffraction the red light exits the light guide plate 11 at an angle of 30 u00b0 the green light exits the light guide plate 11 at an angle of 45 u00b0 and the blue light exits the light guide plate 11 at an angle of 50 u00b0 in the second order diffraction the red light exits the light guide plate 11 at an angle of u2212 23 u00b0 the green light exits the light guide plate 11 at an angle of u2212 8 u00b0 and the blue light exits the light guide plate 11 at an angle of 0 5 u00b0 fig1 shows the angular distribution of the first and second order diffraction light in fig1 it can be seen from fig1 that the second order diffraction is to be suppressed because it lies in the clear viewing cone and the second diffraction light will disturb the reader as glare light when he reads the pages under the light guide plate 11 the second order diffraction can be suppressed by a proper design of the grating shape a sinusoidal grating performs less well than a square shaped one this is illustrated in fig1 a and b note that ambient light that passes along the surface normal will be weakly diffracted it should also be noted that the shape of the gratings only determines the diffraction efficiency and does not.