The choice of front or rear projection depends on several factors
Front and rear screen surfaces
The material that the screen is made from is different for front and rear projection. A front projection screen is normally a matt white material and often they are designed with a backing that does not allow light to penetrate through it. This is useful if you have to place a screen in front of a window or other light source as it prevents the penetrating light from affecting the projected image. Rear projection screens are made from a semi-translucent fabric and they are normally grey in appearance. Our Stumpfl AV Vario screens and our Fastfold screens can normally be supplied with front or rear projection surfaces.
Do I need a special projector for rear projection?
No. All projectors have the ability to reverse/flip the image so that they can be used on a rear projection surface. However, if space is limited you may need a projector with a short throw lens.
How much space do I need for rear projection?
For rear projection you will need some unobstructed space behind the screen which sometimes means you aren’t using your available space as economically as you’d like. The amount of space required depends on the projector and lens that you are using. A short throw, rear projection lens will typically have a 0.8:1 projection ratio so you would need a distance at least roughly equal to the width of the screen you are using. If you are using a standard desktop type of projector they tend to have zoom lenses in the range of 1.5-2:1 so you would need much more space, typically one and a half to two times the width of the screen.
Will there be shadows?
One of the main advantages of rear projection is that you can stand in front of the screen without blocking the light from the projector and casting a shadow on the screen. This gives more options for stage and room layouts and provides a more professional presentation setup. With front projection you need to take care to setup the projector and screen where the light won’t be blocked by the presenter, members of the audience or other obstructions.
Brightness, screen gain and viewing angles.
Screen surfaces are passive devices, they do not produce light, they simply reflect or transmit the light from the projector to the viewer. Different surfaces behave in different ways and each type of surface is given a gain value and will usually have a chart that shows that gain at different viewing angles. The normal screen gain quoted for screens is measured at zero degrees (when viewed perpendicular to the centre of the screen). However, often the screen will be viewed from different angles so it is important to understand what effect that will have on the image.
Matt white front projection screens work by reflecting the light from the projector back to the viewer and will typically have a screen gain of a little more than 1 when measured perpendicular to the screen. As the viewer moves to the side and the viewing angle increases the amount of light being reflected back to the view will fall very slightly and the gain will normally drop just below one. For a matt white screen the difference is very small and these types of screen surfaces are generally very good for providing an even image at all viewing angles.
A rear projection screen works by transmitting from the projector, through the screen surface and directly to the viewer. Inevitably some light is reflected back to the projector so is effectively ‘lost’ and may appear dimmer than a front projection screen. However, when viewed straight on they will typically have a higher gain than front projection surfaces (our Stumpfl rear projection screens have a gain of around 1.2) but as the viewing angle increases the gain falls off quickly and when viewing at an angle of 30 degrees you would have a gain of around 0.65 so the image won’t look nearly as bright.
How does ambient light affect front and rear projection screens?
Ambient light hitting the screen surface will degrade the perceived brightness of both front and rear projection so where possible it is best to minimise it by positioning the screen away from windows and light sources or by dimming lighting in the room.
Rear projection screens tend to have a higher gain and the surface is designed to transmit light in the direction from the projector to the viewer so consequently, they will provide better colour and contrast in situations where the ambient light is higher. Minimising ambient light coming from behind the screen is essential to achieve a good image.