A projector screen is a flat or curved projection surface prepared for projection purposes for viewing the projected image or video from projectors. Modern reflective projection screens have a diffuse reflection property; transmitted light screens for rear projection require diffuse scattering transmission. The projector-screen assembly is one of the so-called objective optical instruments. The screens can be scrolling, fixed, mobile, and of different sizes depending on the capacities of the projector lamp.
Projector Screen Properties
The projection surface can be a white cloth or a white wall in a room. However, better image reproduction is achieved with special projection screens. They are usually designed as roller blinds, attached to a tripod or stretched into a frame. In the past, these fabrics were actually made of linen or cotton. Mostly coated fiberglass fabrics are used to prevent deformation and to make the cloths flame retardant.
In cinema, curved surfaces are often used to allow a wider viewing angle for viewers. Curved screens in domes of planetariums and IMAX dome cinemas are absolutely necessary. For the installation of loudspeakers behind the projection screen, there are acoustically permeable video walls through micro-perforation. For special applications, concave-mirror-shaped projection walls with metallic coatings (aluminum flakes in synthetic resin lacquer) are used, which have a significantly higher luminance factor for the middle range.
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The screen “diffuses” the luminous flux (towards a set of spectators) more than it “reflects” it (only one direction). The projector-screen distance and the power of the lens will define the “magnification” between the dimensions of the object and those of the image projected on the screen. The expression that characterizes it is called illumination of the screen; it is the ratio between the incident flow and the illuminated surface area, of unit “flux per unit of area”. The illuminance of its surface is inversely proportional to the square of the distance from the source (a double distance will decrease by 4 the amount of light received at one of its points).
The illuminance is maximum in normal incidence (axis of the projector perpendicular to the screen). A distinction can be made between point source and extended source for the calculation of the flux emitted and received by the screen (Lambert’s formula).
Traditional PVC screens are made of PVC materials (mostly made in Asia), matte white, and without gain. The canvas receives the light level and then returns the same amount of brightness. The majority of these traditional screens are divided into 2 categories: fixed screens for dedicated rooms, and screens rolled up on the ceiling (tend to curl around the edges or making a kind of V facing down the screen, which completely deteriorates the projected image). Gain screens are used to increase the image quality emitted by the projector (or video projector in the case of home theater). New home cinema projectors require new screens, because video projectors, once limited to company meetings and then to the home reconstruction of a private cinema room, are now in high definition, or even in 3D. Full HD video projectors now make it possible not only to project films, but also high-definition video games, HD television via boxes (sports broadcasts on very large screens, nature and wildlife reports) as well as all high-definition digital devices (camcorder, tablets, computer, etc.). It is then advisable to have screens that will not curl (problem of traditional electric screens rolled up on the ceiling) and that integrate into the interior decoration of each (living room), thanks to “tensioned” screens and designs integrated into living room tables.
Transparent Screens for rear projection are made of frosted or coated glass or plastic plates. For special requirements, a Fresnel lens can also be provided for defined light guidance. Application with compact projection units. The fewer unevenness and the whiter the surface, the more realistically the image is reproduced. On the one hand, a reflective projection screen should have a high reflectivity and, on the other hand, distribute the light evenly to the lateral viewing positions. Sufficient luminance and good luminance distribution are required. In addition, the reflections must be diffuse to avoid a hotspot. Similar requirements apply to transparent screens, high transparency, diffuse dispersion to the sides and no hotspot.
Luminance Factor and Gain
One of the important properties is the reflectivity, which is described by the luminance factor. For comparison, barium sulfate, BaSO4, is used as the white standard. The luminance factor is measured with almost vertical projection in the middle of the projection screen. The luminance factor is the luminance of a screen sample divided by the luminance of a white standard. The luminance factor gain is greater than 1 on the projection screen standard for a type D screen compared to the white standard. Depending on the deviation from the screen normal, this value changes.
In order to achieve a better black in a bright room, screens with a gain factor of 0.6 are increasingly used. These black screens use the same principle as CRT TVs or LC displays. In order to achieve the necessary contrast, projectors with more than 3000 ANSI lumens must be used.
Size and Format
The size of the projection screen depends on the largest viewing distance and ultimately on the size of the room. The viewer from the greatest distance should still be able to see all the details. A viewer in the front seats, on the other hand, should be able to grasp the picture as a whole from a short distance. This determines the size of the wall from the largest viewing distance, while the shortest viewing distance should be taken into account by the arrangement of the viewer seats.
If a projection screen is to be used for different types of projection, a square wall is appropriate. All formats, from slide, portrait and landscape, overhead formats, various film and TV formats can be projected onto the wall. For the projection of rectangular formats, the projection screen cannot be completely extended on roller walls.
