A screen or projection screen , originally and colloquially also a canvas , is a flat or curved collecting surface for viewing the projected image. Modern incident projection screens have a diffuse scattering reflection; Transparency screens for rear projection require a diffuse scattering transmission (→ ground glass ).
The surface can be, for example, a white cloth (painted or sprayed canvas) or a white wall in a room. However, a better image reproduction is achieved with special projection walls. They are usually designed as a roller blind, attached to a tripod or stretched in a frame. In the past, these fabrics were actually made of linen or cotton. Coated glass fiber fabrics are mostly used to avoid deformation and to make the cloths flame-retardant.
In cinemas, curved surfaces are often used to allow the audience to see a wider angle. Curved screens in the domes of planetariums and in IMAX dome cinemas are absolutely essential . For the preparation of the speakers behind the screen there are acoustically transparent screens through micro. For special applications, concave mirror-shaped projection screens with metallic coatings (aluminum tinsel in synthetic resin lacquer) are used, which have a significantly higher luminance factor for the middle area.
Transparent screens for rear projection are made from matted or coated glass or plastic sheets. For special requirements, a Fresnel lens can also be provided for defined light guidance. Use with compact projection units. The fewer bumps and the whiter the surface, the more realistic the image will be. For mobile screens in drive-ins and at outdoor events inflatable version (Airscreen) are used.
On the one hand, a reflected-light projection screen should have a high level of reflectivity, and on the other hand, it should distribute the light evenly to the observer's side. Sufficient luminance and good luminance distribution are required . In addition, the reflections must be diffuse to avoid a hotspot.
Similar requirements apply to transparent picture walls, high transparency, diffuse scattering to the sides and no hotspot.
Luminance factor, gain
One of the important properties is the reflectivity, which is described by the luminance factor. As a comparison, barium sulfate, BaSO 4 , is used as the white standard . The luminance factor is measured with an almost vertical projection in the middle of the 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 screen normal for a type D screen compared to the white standard. This value changes depending on the deviation from the screen normal.
In order to achieve better black in a bright room, screens with a gain factor of 0.6 are increasingly being used. These black screens use the same principle as tube televisions or LC displays. In order to achieve the necessary contrast, projectors with more than 3000 ANSI lumens should then be used.
Luminance factor indicatrix
The dependence of the luminance factor on the reflected angle is shown in the luminance factor indicatrix. The indicatrix shows the distribution of the luminance over the various angles. A viewer has a different impression of the uniformity of the luminance from different positions.
The luminance factor indicatrix of a type D screen shows an increase in the reflected luminance in the middle area and a reduction in the side area compared to the indicatrix of the ideal. Type S and B screen behave differently. Type S screen, specular screen, reflects the light from the projector more like a mirror. With the screen type B, beaded screen, the light from the projector is reflected back from the screen in the direction of the projector. The viewer in the middle of the room sees an evenly lit image and the viewer at the side has a good, even impression of the lighting of the image, albeit darker.
With a curved wall, this reduction in luminance is considerably improved on the sides. A curved screen results in a significant improvement in lighting conditions for the observer from the side. The radius of curvature of the screen is equal to the distance between the projector and the screen. Rigid projection screens designed in the shape of a concave mirror use the advantages of a curvature for both horizontal and vertical alignment.
The luminance should also be optimized for vertical alignment. Depending on where the projector is set up, the screen can be tilted so that the maximum luminance points in the direction of the viewer and does not, for example, illuminate the ceiling. The half-value angle indicates the angle at which the light density drops to 50% of the value in relation to the normal of the screen. If a light drop of a maximum of 50% is permitted at the side seats, the viewing area is limited by this angle.
Types of projection walls
In DIN 19045-4 the “projection of still and moving image” is defined and in part 4 “reflection and transmission properties of projection walls; Characteristic sizes, type of screen, measurement, December 1998 ”, the screens are divided. Four main types are defined according to the luminance factor. With DIN 19045-4 of December 1998, the screen types P and M were changed to B and S, and type R was divided into RO and RS.
- Type D: diffusely reflective types of cloth and wall, with a matt white coating, which are suitable for a large viewing angle due to their wide dispersion. Viewing angles greater than 45 ° should be avoided for reasons of perspective distortion.
- Type B: retroreflective types of cloth (B from beaded). For this, glass beads are used, which reflect the light preferentially in the direction of the light source. Each glass ball acts like a reflector, so the scattering distribution of this projection screen acts preferably in the direction of the light source. In addition to a higher contrast, these screens have a higher luminance factor, which is at the expense of the maximum viewing angle. Small viewing distances require small, closely spaced beads, otherwise the structure of the screen is perceived as disturbing. They are called pearl walls or crystal screens.
- Type S: Directionally reflective types of cloth (S from Specular). They reflect the projected light in a preferred spatial direction, similar to a mirror. A thin layer of metal particles (aluminum tinsel in synthetic resin varnish) is attached to the surface of the carrier of these so-called silver screens. In addition, corrugations and grid structures can be attached, which distribute the scattering area for the horizontal and vertical direction in a defined manner. Screen walls with metallic pigments are required for 3-D projections using the polarization method. Only these are able to reflect the different polarization directions of the light used to separate the left and right image with unchanged polarization . Due to their construction, they have a restricted viewing angle. Under the heading of bright room screens, versions with high gain values (Gain 18 / Gain 23 / Gain 26) have appeared on the market, with which it is possible to achieve high-contrast image reproduction with normal room brightness without darkening the room.
- Type R: screens for back projection, which do not reflect but are permeable. They are made of glass (frosted glass or opal glass) or plastic. Depending on how the rear projection screen works, a distinction is made between types RO and RS.
- Type RO: The screen consists of an opal pane with a surface matted on one or both sides. The spreading behavior can be influenced by different plastic mixtures. The discs are smooth and have no structure on the surface.
- Type RS: The light guidance of this screen is determined by a surface structure, e.g. B. influenced a Fresnel lens. This improves the impression of an uneven light density distribution when viewed from the side.
Screen size, formats
The size of the screen depends on the greatest distance from the observer 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 still be able to see the picture as a whole from a short distance. In this way, the size of the wall is determined by the greatest viewer distance; the shortest viewing distance, on the other hand, should be taken into account by the arrangement of the viewer positions. The conditions in the open-air cinema pose a challenge to the screen .
A rule of thumb:
- Largest viewer distance = screen width x 6
- Smallest viewing distance = screen width x 1.5
The information on the smallest and largest viewing distance refer to the "normal format" of approx. 2: 3 or 3: 4. For wide-screen processes, it is advisable to assume a standardized width of b = 1.33 h. The smallest elements are guaranteed to be recognizable if the conditions according to "DIN 19045 Projection of still and moving images - Part 3: Minimum dimensions for the smallest image elements, line widths, font and image character sizes in original templates for the projection, issue date: 1998- 12 “are complied with.
If a screen is to be used for different types of projection, a square wall is appropriate. All formats, from slides, portrait and landscape, overhead formats, various film and TV formats can be projected onto the wall. For the projection of rectangular formats, the screen cannot be fully extended with roll-up walls.
The luminance distribution for the viewer is determined on the one hand by the screen, on the other hand it is also influenced by the projector. A projection with a longer focal length and a larger projection distance hits the sides of the screen more perpendicularly, so the decrease in luminance for the side viewer is less. The screen is illuminated by the luminous flux of the projector. In order to achieve a good readability of the image, the illuminance of the projected image must be min. 5 times higher than the illuminance of the secondary light (room light) on the screen. The image projector (cinema projector, slide projector, video projector) should be centered on the projection surface, otherwise image distortions will occur. Some projectors have a way of correcting this distortion. ( Lens adjustment )
Note: The screen is only one component in the area of viewing and projection conditions. Starting from the eye with its smallest resolution, then via the design of the templates to be projected, via the projector with a luminous flux matched to the size of the room and the design of the viewing room, only a coordinated system leads to optimal success.
- DIN 19045-4: 1998, projection of still and moving images; Part 4: Reflection and transmission properties of projection screens, characteristic variables, projection screen type, measurement
- H. Naumann, Schröder: Components of optics . Carl Hanser Verlag, Munich 1992, page 295, ISBN 3-446-17036-7 .
- Gottfried Schröder, Hanskarl Driver: Technical Optics , Vogel Buchverlag 2002, page 106, ISBN 3-8023-1923-0 .
- Wolfgang Grau: Technique of optical projection , Beuth Verlag GmbH Berlin, 1994, page 215, ISBN 3-410-13194-9 .
- Wolfgang Grau: Technique of optical projection , Beuth Verlag GmbH Berlin, 1994, page 187, ISBN 3-410-13194-9 .
- H. Naumann, Schröder: Components of Optics . Carl Hanser Verlag, Munich 1992, page 296, ISBN 3-446-17036-7 .
- Wolfgang Grau: Terms of photographic recording and playback technology including video and LCD projection . Beuth Verlag GmbH, Berlin 1994, page 51, ISBN 3-410-13099-3 .
- H. Naumann, Schröder: components of optics , Carl Hanser Verlag, Munich 1992, page 297, ISBN 3-446-17036-7 .
- Wolfgang Grau: Technique of optical projection , Beuth Verlag GmbH Berlin, 1994, page 50, ISBN 3-410-13194-9 .
- Wolfgang Grau, Hugo Heine: Technique of projection , Beuth Verlag GmbH Berlin, 1980, pages 41-45, ISBN 3-410-11227-8 .