so that the three separate images of the aperture can be stepped projected onto the three sub panel locations on the dmd panel the incidence angle of the primary illumination beam requires slight deviations in order to obtain optimum illumination efficiency in the lamp system the lamp optics applies a set of cylindrical lens so that the lamp beam can be formed to a shape that covers the aperture with most efficiency as shown in fig8 three projectors of the type of fig6 15 a 15 b and 15 c can be aligned with their projection beams 801 a 801 b and 801 c intersecting at a fixed distance and thereby superimposing the projected sub frames such a layout can be used in a volumetric 3d display based on moving screen projection by making the intersecting point on the moving screen 11 in the case of a rotating screen system for example fig1 of tsao et al u s pat no 5 754 147 the projection path length is always constant in the case of a reciprocating screen system for example fig4 b of tsao u s pat no 6 302 542 b1 the projection path length is kept constant by the reciprocating reflector 1300 as a result the projected sub frames from the three projectors will always superimpose on the moving screen one practical issue of using the discovery 1100 system is that data must be written into the dmd row by row so that sub panels have to be defined with their long side along the long side of the full.