Rear Projection


In a rear projection system the image is projected through a rear projection screen. The screen material should transmit the optical image with clarity and brightness and without visible hotspots (bright spot on axis of the projector beam). Also a higher intensity of the image source (projector brightness) than for front projection is needed as the light travels through the screen material.

Rear projection systems are the best choice in rooms with high ambient light because the degrading of the black level is much less than for a front projection system with direct illumination of the projection screen. The front projection screen is reflecting the ambient light in the same way as it is reflecting the light output of the projector. As soon as the ambient light reaches the brightness of the projected image of a front projection system (same amount of lux on the screen), the projected picture is nearly washed out. Ambient light on a rear projection screen is transmitted through the screen into the (matte black) rear projection room (position of the projector) and gets mostly absorbed. The projected image on the other way is transmitted through the rear projection screen in the opposite direction from the projector to the audience. The rear projection screen works as a two-way light transmitter. The typical viewing angle for rear projection systems is horizontal 90 degrees, vertical 60 degrees.

projected image with light diffusion

ambient light absorbed through the screen

Front Projection or Rear Projection, Contrast Ratios

A rear projection system achieves a much higher image contrast than a front projection system. Without any ambient light the front projection system generates always a better picture quality because the projected picture is reflected on one even surface, the screen.

front projection front projection and ambience light
rear projection rear projection and ambience light

In contrast the transmitted light beam in a rear projection system travels through the entire rear projection screen. The screen must be dense to avoid light hotspots and to spread the projected light to an useful viewing angle. This quiet difficult pathway through the screen material reduces the image quality to a certain degree. But by comparison with higher ambient light the advantage of the higher image contrast creates a generally much better picture quality than with the front projection system.

With a front projection system, a regular white screen (gain=1) and ambient light of 200 to 400 lux, normally only a contrast ratio between 2:1 to 10:1 can be achieved. For control room applications a contrast ratio of 50:1 is required. For a larger image size only a rear projection system can achieve this.

Contrast Ratings by INFOCOMM:
Below 5:1 unacceptable
5:1 - 10:1 bad
10:1 - 15:1 average quality
15:1 - 20:1 good
higher than 20:1 very good

Optical Screens or Diffusive Screens

There are two versions of rear projection screens: optical screens and diffusive screens. The optical screen can beam light with its optical structure (see below), the diffusive screen cannot bean light. Using just a diffusive screen a lower light output and visible hotspots in the axis between eye and projection lens must be taken into account.

Optical Screen / Fresnel Lens / Lenticular Lens

One or more optical lens systems are included, main part is a fresnel lens on the rear side of the screen (projector side). This lens will focus all light in the audience direction, creating a high projection gain. The typical gains of a rear projection systems are between 1.5 and 3.5. An optical screen is manufactured with one or more optical layers. The main part is always a fresnel lens on the rear side of the screen (projector side). The fresnel lens is focusing the light in the audience direction, creating a higher projection gain. Typical gain of a rear projection system is between 1.5 and 3.5.
The second part is the lenticular lens on the front side of the screen (audience side). This optical layer is used to disperse the light in a wider angle in the horizontal plane. The distance from one cut line of the fresnel lens or of the lenticular lens is called pitch. Front and rear side can have different pitches. The optical layers can also be combined in a different way for special applications like short throw projection.

spherical convex lensindividual parts (coaxial) moved down to create a fresnel lens

The distance between the circular rings (steps) is called the pitch of the fresnel lens. The finer the structure the smaller is the pitch and the flatter is the surface of the lens.

projection beam with a regular spherical convex lens

projection beam with a fresnel lens

Using a fresnel lens only the curve of the lens is important, not the body, if the curve parts are put on a plane flat surface, the lens characteristic is maintained. Because of their different optical properties these lenses are not suitable for optical applications like photo lenses or projector lenses but fit perfectly to create a light beaming surface on a rear projection screen. (another large application for fresnel lenses are lighting fixtures (fresnel spot lights, see lighting.

For instance a dnp rear projection screen has more that 10.000 circular cuts to create the fresnel lens surface. The center of the fresnel lens is normally located in the centre of the screen but can be also on the top or bottom edge of the screen. The fine structure of the fresnel lens is even adequate for 4K video projection.

The fresnel lens at the rear of the screen is combined with a diffusion surface (a lenticular lens) at the front side to create a wide light dispersion on the audience side.

lenticular lens (viewing from above)

smooth horizontal light dispersion on the front (audience) side

left: projector side, right: audience side

rear projection screen, single element design
with fresnel lens (left) and lenticular lens (right)
(viewing from above)

smooth horizontal light dispersion on the front (audience) side

left: projector side, right: audience side

Screen Focal Length

The most determining parameter of an optical rear projection screen is the Screen Focal Length. The screen focal length should match the projection distance for rear projection screens to ensure a right-angle light exit of the screen to the audience. If the projection distance is shorter than the screen focal length, the light exit is diffusing (light in the upper area of the screen goes up, light from the lower area of the screen goes down). This situation should be avoided in any case.
If the projection distance is slightly longer than the screen focal length the light output of the screen is focussing in one point. This can be helpful with large screens or with screens in long rooms.

The difference between screen focal length and projection distance should be not larger that -10% (projection distance too short) and +20% (projection distance too long).

correct projector position related to the screen focal length of the rear projection screen

too far projector position related to the screen focal length of the rear projection screen

too close projector position related to the screen focal length of the rear projection screen

Projection Angle

The projection angle should always be set in a way that the main light output is directed into the audience area. If the main light output is directed onto the ceiling of the room the brightness of the rear projection is unnecessary lowered.

horizontal projection, viewing from the sideprojection, angled up with vertical shift
projection, angled down, with vertical shift, better solution
same projection as on the left side, angled down, with mirror

slightly angled down with vertical shift, green arrow: main light outputsame projection, same light output, two-mirror-system

Rear Projection Mirrors

To use a mirror for a rear projection, it must be front-surfaced. Regular glass mirrors have their reflective surface at the back of the glass material. Using this kind of mirror will create double pictures, because the in-beam and the out-beam are not parallel. The light has to go through the glass twice and on the first surface a second reflection will occur.
Front-surface mirrors for projections can be glass and foil mirrors.
Glass mirrors have a reflectivity of about 90% to 95% and foil mirrors have a reflectivity of about 80% to 85%. The glass mirror is perfectly flat but heavy. It can even bend under its own weight. The foil mirror is much, much lighter, can be angled in any way but has not this perfect surface.

Screen Color

Coloring of the screen material creates a higher contrast but also lowers the brightness, reducing light transmission. Most rear projection screens are tinted with a medium grey color.

Rear Projection with Short Throw Projectors

For projectors with very shot focus of about 0.6 to 0.8, a double layer rear projection screen with inverted structures is the best choice. These screen compositions are more expensive but have also the advantage of a flat front surface.

sandwich structure with inverse fresnel lens and inverse lenticular lens for short throw projection

Suggested Projector Brightness for Rear Projection

For regular rear projection installations the following brightness of the projector is suggested in relation to the screen size (diagonal):