Selection Of Projection Screen Material For Different Projects
The selection of the projection screen material to be utilized during events are very important (2) as well as its increasing market size (1) and will continue to be used thanks to new developments (3) in spite of higher resolutions of LED walls (5), be it because the structural design (7) on-site does not allow the use of a heavy LED wall (6), government demands (4), legal regulations (8) or the requirements on the homogeneity of the display for the projection (10) (11), especially with viewers close to the picture (9), enforce this legally in some countries (DIN 19045).
Apart from this, a large image display can be implemented more cost-effectively as a projection. While the technical data of the projectors to be used in terms of resolution and brightness are often the focus of attention during planning, the properties of the screen surface itself and the projection screen material are often treated as secondary. However, both components lead to optimal image quality.
A suitable surface is required to project an image. Any wall that is as bright as possible could be used for this. However, if it is a question of a high-quality presentation, projection screens are more suitable. Only a smooth surface and a projection screen material matched to the respective purpose lead to optimal results.
When making the selection, we must therefore consider the projection screen material that will be utilized. For mobile use in the event area, screens stretched on frame structures are recommended, which are usually made of polyvinyl chloride (PVC). For larger areas, individual screens are connected to one another almost invisibly using high-frequency welding processes using special machines.
Luminance factor
The so-called luminance factor is an important quality criterion. This factor, also known as gain, describes the efficiency of a projection surface. In DIN 19045 the conditions are described how the light yield of a screen is determined in comparison to a defined reference surface.
This reference surface is a white chalk tile made of barium sulfate, a projection screen material for which the luminance factor of which is denoted by 1. All information on luminance factors from the individual manufacturers should always refer to this reference surface in accordance with DIN 19045.
The luminance is measured at different viewing angles. The luminance is usually highest when the projected light falls perpendicularly onto the screen and the viewer looks at the projection surface in the same axis. The projection axis and the optical axis are the same in this case, the viewing angle is 0 ° and the maximum luminance factor is measured here.
However, the further a viewer is outside the projection axis, the larger the viewing angle and the lower the luminance of the reflected light, as a rule.
A luminance diagram shows the luminance factor at the respective viewing angle. The flatter the curve, the more even the luminance distribution of the projection film. The steeper the curve is, the more the projection film tends towards hot spots and is unsuitable for certain purposes such as soft edge projections.
Another type of graphic representation of the luminance distribution depending on the viewing angle is a so-called luminance factor indicatrix. Professional providers of projection screen material surfaces provide this data so that interested parties can determine the optimal material for them. In most standard applications, the light incident on a screen should reflect widely and not have a pronounced preferred direction of reflection, so that viewers from the side can still see a good image.
However, there are also designs for special applications in which the luminance in the direction of the central axis is particularly high. So it is possible that there are luminance factors greater than 1. In these cases, a projection surface reflects or transmits more light than the white reference surface of DIN 19045.
However, the increase in the central axis is usually accompanied by a decrease in the side angles, so that such projection screen materials are particularly suitable when the viewer is preferably in the area of the central axis are located.
The immediate benefit of a projection screen with higher gain is obvious: In order to create an evenly bright image compared to a wall with a lower luminance factor, a lower projector output is sufficient – a factor to be taken into account in times of tight budgets.
Front and rear projection
Another possible distinction is the spatial relationship between projector, screen and viewer: Is it a front projection where the projector is on the same side of the screen as the viewer, or should it be a rear projection where the projector is from view behind the screen?
The German Institute for Standardization eV (DIN) has classified the various projection screens in DIN 19045-4 with the title “Projection of still and moving images” and divided them into different screen types with the designations D, R, B and S. With the various types of projection screens, the projection technology can be adapted to the spatial conditions, the room size and the seating arrangement of the viewer.
The screen types D and R cover the most common applications. D stands for diffuse and describes a projection screen that reflects the incident light in a wide-spreading manner and does not have a pronounced preferred direction of reflection.
Such films are also available in a light-tight design that do not allow incident light to pass through, so that this is not wasted through transmission, but is reflected as far as possible in the direction of the viewer. This is often achieved by joining different foils lying on top of one another. In addition, this also prevents disruptive light components from the rear.
Type R refers to a screen for rear projection, in which the light incident from behind is not reflected but is transmitted through (transmission). In the case of rear projection films in particular, a uniform, diffuse light distribution is necessary in order to avoid a so-called hotspot – differences in light and dark that the viewer can perceive.
The relevant providers offer films that can be used universally for both front and rear projection in order to offer the user the greatest possible variability. Professional manufacturers naturally provide two gain values for these products – for front and rear projection.
However, the same applies here – as is often the case with multi-functionality: those who can do everything can usually do nothing particularly well.
Special applications
Types S and B are defined for special applications: With a type S (specular / reflective) screen, the light coming from the projector is reflected like a mirror. This mirror effect is achieved through a metal or silver layer on the screen surface. Such screens usually have an increased gain factor and a pronounced preferred direction of reflection in the direction of the reflected axis of incidence.
Screen walls with metallic pigments are required for 3D 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. The screen type B (beaded / beaded) works in a similar way to the type S described above, except that the light is reflected back “retroreflectively” in the direction of the incident light.
This is achieved through a crystal or pearl surface. In addition to a higher contrast, these screens also have a higher luminance factor, which is at the expense of the maximum viewing angle. Possible uses are the creation of the brightest possible image when using a not very powerful projector, Use in rooms that cannot be darkened, or the deliberate limitation of a lateral view. So-called bright room screens with a parabolic curved surface are offered for front projections with a gain of up to 26.
Material Colors
Normally, a projection screen material is offered in different colors – from white to almost black. Light-colored materials are particularly suitable for projections that are as loss-free as possible.
However, if the projection surface is to be integrated into a stage design, for example, and as inconspicuous as possible when not in use, darker designs are recommended, which usually also reflect less interfering light. The color of the material has an impact on the luminance factor and thus possibly on the necessary projector power.
When using dark foils, compared to bright surfaces, higher projector outputs will be necessary to achieve the same image brightness. But color rendering and contrast behavior of a screen can also be interesting aspects for a selection.
Color reproduction
The reflection or transmission of a canvas does not always have to be the same in terms of color reproduction across the color spectrum, but can differ in individual wavelengths of the light perceptible to the human eye – the range between around 380 to 780 nanometers.
If the requirements for exact color reproduction are particularly high, you should take a look at the color spectograms of the individual films provided by professional providers. Good color rendering can be recognized by the uniformity of the various wavelengths of the color range.
Contrast
There are also differences between the individual projection screen material specifications in terms of contrast: if the contrast is good, black parts of the image are perceived by the viewer as black and white parts of the image as white.
Naturally, a white image can be projected better on a white screen than on a dark one. Black image areas can, in turn, be created more optimally on a dark surface – you get a better black value.
A projector cannot project “black light”. Black only arises from the absence of light – including stray light – in these areas. A good contrast exists when the difference between the black and white parts of the image shown is as great as possible. In order to meet the various requirements of users, the manufacturers also offer products with different capabilities.
Fire Protection
According to DIN 19045-5, projection walls must be at least normally inflammable. The projection screen material should comply with all regulations. The classification of building materials is determined by American standards such as NFPA 70, or international standards, for example, according to DIN 4102 Part 1-4. Basically, this DIN regulates tests and requirements for the fire behavior of projection screen material and many other materials and components. The test procedure is described in detail in DIN 4102-1.
Many countries also have their own national test and classification standards for reaction to fire, for example NF P 92-503 applies in France, BS 5867 in England, IRAM 11910-1 in Argentina, RD 312/2005 in Spain, and NFPA 70 in the USA.
In most European countries, the European standard EN 13501-1 for the classification of building materials has meanwhile been established, whereby the national standards partly still apply in parallel to the European standards.
Professional providers provide information about an existing classification and its test basis for their products. When deciding on a projection screen material, you should therefore also consider the intended use in order to be able to meet the legal requirements.
Textile Projection Screen Material
Screens in standard formats such as 16: 9 or 4: 3 are only a part of the possible applications. In many cases, foils are custom-made for a specific application and installed. But not only the PVC canvas projection screen material described so far are suitable for projection: there is a wide range of materials with which impressive effects can be achieved. Semi-transparent, textile products have long been known in the theater sector, which enable projections depending on the lighting situation with a view of the space behind them.
This effect is also familiar to those who are not familiar with the industry: If you look at a house window with a curtain in daylight, you usually cannot look into the unlit room behind it. The same view in the dark outside and in an illuminated room reveals a view of the inside, depending on the density of the fabric. If you can influence the lighting situation in a room as in a theater or a hall, this results in a wide range of design options.
If you illuminate a format-filling gauze or tulle hanging in the front area of a stage at an angle from the front and ensure that the remaining stage space behind it remains dark, this front level becomes largely opaque and allows the audience, for example, to make hidden conversions or appearances.
If the space behind the fabric is illuminated and its direct lighting is removed, it becomes transparent depending on the fabric density. Another possibility is the combination of a semitransparent fabric, used as projection screen material, with a projection surface located at a distance behind it.
A front projection of an appropriately positioned projector is visible on both the front and the rear level. When projecting snowflakes, for example, on these two planes, three-dimensional impressions with a high effect of depth can be generated. Such a combination of semi-transparent fabric with a screen behind it is often used to keep unwanted interfering light – for example from stage lights – from the projection surface.
This fabric is also used as a projection screen materialin laser shows, as they allow both a view of the laser beams and the display of graphic elements on the surface.
The pea and tapestry tulle fabrics are classics in the scenic area. The former has honeycomb cells with a hexagonal, almost circular hole shape and thus offers high transparency. Since it has less closed surface, the absorption capacity of incident projection light is lower.
The tapestry tulle weave leads to rectangular cells with a higher density and thus better visibility of a projected image with less transparency. In order to take account of the respective applications, these fabrics are available in white, black and gray and can even be colored in special colors on request. Since inherited tulle is not very dimensionally stable and tends to waist easily, this material is often stretched or guided to the side for use. Tapestry tulle is less sensitive here and can hang freely in the room.
Current production examples for textile projection screen material surfaces from the touring area are the last Adele tour, in which a 29-meter wide and 9.5 meter high goblet tulle was used, and the tour production by Xavier Naidoo, in which six roll-up screens with Trevira CS Voile arranged around a center floor were populated.
Visual Effects
Projection screen materials that can be assigned to the field of visual effects are somewhat more technologically complex. A self-adhesive projection film called “Inviscreen” transforms glass and window panes into projection surfaces.
Due to its crystal clear nature, the film cannot be seen during the day. At night or in dark rooms, you can project onto the highly transparent polyester film. Depending on the brightness behind the windows, objects – such as an illuminated high-rise skyline – are still visible and can thus become part of the image display.
Switchable systems can also be used for projections on glass surfaces, the surfaces of which can be switched transparent at the push of a button by means of applied voltage or opaque if there is an interruption. With these technologies, called LC film or LC glass, a liquid crystal film is either laminated between two panes of glass or applied on one side to an existing glass surface.
However, the maximum sizes for a 16: 9 format are limited to 3.4 x 2.2 meters. Due to their design, these systems are particularly suitable for rear projection. This LC technology is not to be confused with electrochromic glass, which is not a projection screen material, and is more commonly used in construction and whose optical change takes a certain amount of time.
Another projection screen material from the field of visual effects is the highly transparent, reflective special film “Pepper’s Ghost”. The desired animation is generated by a horizontal LED wall and mirrored on the film attached at a 45 ° angle. The audience only sees the illusion, the special effects film is invisible.
The special Holo-Gauze tulle is made of polyamide and a nine percent silver thread woven into it, making it ultra-transparent, highly reflective and polarization-retaining. This enables applications with hologram effects through to real, three-dimensional projections with polarization filters. The structure, however, is considerably simpler than that of the Peppers Ghost film, which has a comparable effect, since the Holo-Gauze is hung in front of the area to be played on like an inheritance.
The precise analysis of the planned application is decisive for the selection of the right material for a screen surface. In addition to properties such as luminance factor, viewing angle, color rendering and contrast behavior, other factors such as fire protection classification and processing options must be taken into account. The optimal projection screen material can certainly be found from the diverse product portfolio of professional providers.
Projection Screen Material Conclusions
Picture wall surfaces are often provided with black borders similar to a passe-partout in order to create a higher contrast impression. Apart from that, the black border simplifies the projector setup, as possible slight edge over-exposure disappears in it. Borderless image surfaces hanging in the room, on the other hand, give the impression of a flying image.
If loudspeakers are to be installed behind the screen, foils with perforations that allow sound to pass through can be used. A micro-perforation with 300,000 holes per square meter and a hole diameter of 0.5 mm leads to about 6% open area. From a certain distance, this is no longer perceptible, but in the worst case, it can lead to undesirable moiré effects in high-resolution images. From a certain height on a picture surface, this must be produced with fabric reinforcement, since a pure PVC film could no longer hold its total weight on the upper edge and would warp or even tear off.
Especially with mobile use, one should also pay attention to what should be done with the screen when the respective use is completed. Here manufacturers provide information on storage and cleaning types. When manufacturing a PVC film, plasticizers are also added to the plastic in order to make the material flexible. However, this effect diminishes with increasing age. Other enemies of the foils are, depending on the version, bent storage and processing at too low temperatures, which can lead to breakage.
There are also special projection wall paints to get an existing wall with better reflective properties and without picture format structures. The quality of the optical properties, however, is very dependent on the respective processing and can only be objectively checked by means of complex measuring processes after completion.
I am Bob. I work as an audio engineer and audio technician. I work in mastering and arranging bridges in existing songs and the arrangement and orchestration of the chorus. In Planet HiFi I test gear for a couple of days and write a review. I also write about AV topics, amplifiers, speakers, and headphones.
