Using single lens projectors it is important to keep the projection perpendicular to the screen plane (i.e. the back of the projector being parallel to the screen), otherwise keystone distortion will occur (keystone = trapezoid shape). As rear projection usually employs wide angle lenses, the geometry of the screen/projector set-up is even more critical.
Using optical rear projection screens, the light dispersion depends on whether the projector is placed on or off-axis – particularly with regard to vertical viewing angles.
Applications requiring a specific vertical viewing angle (i.e. a specific angle of peak gain) can be achieved by shifting the projector location in relation to the screen. Shifting the projector upwards will angle the light dispersion downwards and vice-versa. When designing your installation, you can optimise projector position by drawing a line from the optimum viewing position, continuing through the centre of the screen. The projector should lie somewhere along this line. But remember you can only do this if the projector has vertical shift, so that the back of the projector is always parallel to the screen.