With dormer windows of any inclination and orientation, it should be possible to adjust the sunshade effect throughout the year and to have at the same time a good transparence to daylight. To this effect, there is used a prismatic plate, flat at the outside, and which is provided inside with parallel prismatic bars (P) having a surface (P1) free of reflecting layer on the right angle side. According to the invention, the other surface on the side (P2) is provided with a reflecting layer and forms with the non reflecting surface a right angle. Furthermore, the prismatic bars are arranged perpendicularly to the intersection line (L) which is defined by the intersection plane (M) with which the solar beams, during the longest day, form the maximum incidence angle, psi max with respect to the plane of the window (F). For the angle alpha of the base of the non reflecting surface (P1), a value may be selected with which there is obtained a sunshade effect and at the same time optimum conditions of transparency to daylight.