Projection screen

A picture wall, screen or projection screen is a flat or curved surface collecting, for viewing the projected image. Auflichtbildwände have a diffusely scattering reflectance, projection screens, for rear projection need a diffusely scattering transmission.

Execution

The area can be for example a white cloth ( painted or sprayed canvas) or a wall in a room. A better image reproduction is achieved, however with special video walls. They are usually designed as a roller blind, fixed to a tripod or clamped in a frame. Previously, these tissues were actually made ​​of linen or cotton. Today coated fiberglass fabrics are used in order to avoid distortions and to make the cloths flame retardant.

Cinema curved surfaces are often used to provide a larger viewing angle of the viewers. Needed are curved screens in domes of planetariums and IMAX Dome theaters.

For the preparation of the speakers behind the screen there is Microperforation acoustically transparent screens.

For special applications concave mirror -like screens with metallic coatings ( aluminum tinsel in synthetic resin paint) are used, which have a much higher luminance factor for the central region.

Mobile outdoor screens are also used as inflatable version.

Transparent screens for rear projection are made of frosted or coated glass or plastic plates. For specific requirements, a Fresnel lens can be provided in addition to the defined lighting. Application in a compact projection units.

The fewer imperfections and the whiter the area, the more realistic the image is played back.

Screen properties

An incident screen is on the one hand have a high reflectivity, on the other hand, distributes the light evenly also on the side viewer places. It calls for a sufficient luminance and good luminance distribution. In addition, the reflections to avoid hotspots must be diffuse.

For transparent screens similar demands, high transparency, diffuse scattering at the sides and no hotspot apply.

Luminance factor, gain

One of the important properties is the reflectivity is described by the luminance factor. For comparison, barium sulfate, BaSO4, used as a white standard. The luminance factor is measured at almost the vertical projection in the center of the screen.

Luminance factor = luminance of a screen test / luminance of a white standard

The luminance factor, gain is on the screen for a normal Screen type D compared to white standard greater than 1, depending on the deviation from the normal screen, this value changes.

To achieve a better black in a bright room, you increasingly relies on screens with a gain factor of 0.6. These black screens use the same principle as CRT televisions or LCD displays. In order to achieve the necessary contrast to only use projectors with at least 3000 lumens.

Luminance factor indicatrix

The dependence of the luminance factor of the reflected angle is shown in the luminance factor indicatrix. Shows the distribution of the indicatrix of the light density of the various angles. A viewer from different positions a different impression of the uniformity of luminance.

The luminance factor indicatrix of a Screen type D showing a reduction in the central region of an increase of the reflected luminance and for the lateral region compared to the indicatrix of the ideal. Unlike the screens type S and B. The behavioral Screen type S, specular screen that reflects light from the projector more like a mirror. In the Screen type B, beaded screen, the light from the projector is reflected from the screen in the direction of the projector. The viewer in the center of the room provides an evenly illuminated image and the viewer on the page has a good impression of the uniform illumination of the image, albeit darker.

With a curved wall such luminance reduction is considerably improved at the sides. A curved projection screen gives the viewer side, a substantial improvement in the light conditions. The radius of curvature of the screen is equal to the distance of the projector to the screen. Concave mirror shaped executed rigid screens take advantage of a curvature for both horizontal as well as vertical alignment.

Optimization of the luminance should be for the vertical orientation. Depending on the placement location, the screen can be tilted so that the maximum of the luminance pointing towards the viewer and example, not illuminated the ceiling.

The half- angle is the angle at which the Leutdichte drops to 50% of the value relative to the screen normal. If on the side places a drop light is allowed a maximum of 50 %, the surface of the viewer is limited by this angle.

Types of projection screens

In DIN 19045-4 the " projection of still and motion picture" is defined and in Part 4, " Reflection and transmission properties of image walls; characteristic sizes, screen type, measurement, December 1998 ', the screens are divided. After the luminance factor of four main types are defined.

• Type D: diffuse reflective cloth and wall places, with matt white paint, which are likely due to their wide dispersion for a wide viewing angle. In this case, wider viewing angle than 45 ° should be avoided for reasons of perspective distortion.

• Type B: retroreflective fabric types (B of beaded ). For glass beads are used, which preferably reflect the light toward the light source. Each glass sphere acts as a reflector, so preferably the scattered distribution of this screen acting in the direction of the light source. These screens have higher contrast in addition also a greater luminance factor, but this comes at the expense of maximizing the viewing angle. Small viewing distances require small, closely spaced beads, otherwise the structure of the screen is distracting. They are also known as beaded screen or crystal screen.

• Type S: Directional reflective fabric types (S from Specular ). They reflect the projected light like a mirror in a preferred direction in space. On the surface of the substrate, a thin layer of metal is attached ( aluminum tinsel in synthetic resin paint). In addition, more corrugations and grid structures can be attached, spread defines the scattering region for both horizontal as well as vertical direction. Screens with metallic pigments are required for 3- D projections beyond the polarization method. Only they are able to reflect the set of separating the left and right images of different polarization directions of light with unchanged polarization. You have by design a limited viewing angle. Under the heading of overhead screens versions with extreme gain settings (gain 18/ 23 Gain / Gain 26) have appeared in the market, with which it is possible to achieve a high-contrast image reproduction without darkening the room at normal room brightness.

• Type R: screens for rear projection ( back projection ) that do not reflect, but are permeable. They are made of glass (milk glass or opal glass ) or plastic. Depending on the mode of action of the rear screen, the RO and RS types can be distinguished.

• Type RO: The screen consists of an opal, with a single-sided or double-sided matte surface. Through various plastic blends can affect the scattering behavior. The washers are smooth and have no structure on the surface.

• RS: The light will guide this screen by a surface structure, eg a Fresnel lens affected. This gives the impression of a non-uniform light density distribution is improved when viewed from the side.

Note: The screen types P and M were changed to B and S, also type R was divided into RO and RS. (DIN 19045-4, December 1998)

Screen size, formats

The size of the screen depends on the room size, more precisely to the largest viewing distance. The viewer shall be able to recognize good from the greatest distance all the details. An observer on the front seats should contrast from a short distance, the image can still be recognized as a whole. Thus, the size of the wall from the largest viewing distance is set, the shortest viewing distance should be taken into account, however, by the arrangement of places viewers.

Largest viewing distance = screen width x 6

Minimum viewing distance = screen width x 1.5

The information on the minimum and maximum viewing distance refer to the " normal size " of about 2:3 or 3:4. For widescreen process, it is recommended by a normalized width of b = assumed 1.33 h. A secured recognition of the smallest elements is given when on the projection template, the conditions to "DIN 19045 projection of standing motion picture - Part 3: Minimum dimensions for the smallest elements, line widths, font and glyph size in original templates for the projection, Publication date: 1998 are -12 " respected.

If a screen for different projection methods to be used, with a square wall is appropriate. It All formats can be projected from the slide, portrait and landscape, overhead - formats, various film and TV formats on the wall. For the projection of rectangular formats, the screen can not be fully extended when rolling walls.

Projection

The luminance distribution for the viewer on the one hand determined by the screen, but on the other hand, it is also influenced by the projector. A projection with a longer focal length and a larger projection distance exceeds the vertical sides of the screen. This also makes the drop in luminance for the side viewer is less. The screen is illuminated by the light output of the projector. To achieve a good visibility of the image, has the illuminance of the projected image min. Be 5x higher than the illumination of the side light ( room light ) on the screen. The image projector ( cinema projector, slide projector, video projector) should be aligned with the center from the screen, otherwise the picture will be distorted. Some projectors have a possibility of correction for these distortions. ( Lens shift )

Note: The screen is only one component in the field of viewing and projection conditions. Based on the eye with its smallest resolution, continue on the design of the templates to be projected through the projector with a tuned to the room size luminous flux and the design of the observer space is only one coordinated system to an optimal success.