The first optical planetarium, built in 1923 by the Zeiss company in Jena, Germany, projected onto the white plaster lining of a 16-meter diameter hemispherical concrete dome. Solid domes such as this and the many fiberglass and glass reinforced plastic domes that followed, reflect sound as well as light, detracting from the viewing experience. Most large planetarium screens are now made of perforated aluminum panels fixed to a support in frame. The thousands of tiny perforations in the screen reduce acoustic reflection and actually allow speakers to be placed behind the screen so that sound can appear to come from the appropriate direction for the video. The perforations also allow for the flow of air through the screen for climate control, preventing the build-up of moisture on the surface, which was also a problem with solid screens. However, aluminum screens are expensive and although they can be built to very large diameters, require a complex supporting frame structure.
Inflatable domes, where the air pressure inside the dome is increased to hold the screen up offer portability but, when used outdoors are prone to movement in windy conditions.
Negative-pressure domes
The screen is made from lightweight blackout fabric (usually polyester) and is slightly smaller than the dome frame. There may be a black border area around perimeter of the screen where it attaches to the inside of the dome frame. Usually there are also straps connecting the screen to the frame at several points, so that it will not fall on the audience or the ground when the fan is off. Larger domes may use more than one fan.
The fan is connected to the outer cover of the dome either directly or via a duct. The type of fan used is designed to operate at high volume and relatively low pressure while remaining quiet. Attenuators can be added to the fan to make it even quieter and by using ducts the fan can be located away from the dome and the audience inside it.
Choosing the right screen type for your dome theater
From traditional planetariums to the latest high-tech rides, domes put the audience at the centre of the picture. But choosing the right type of screen depends on many factors.
Negative-pressure domes come in two main styles
- Enclosed domes can be used indoors or outdoors as the structure is free-standing and the cover goes all the way to ground level, forming walls. These domes are used for temporary events, expos, and festivals but can also be built as permanent, stand-alone, outdoor domes.
- Open domes are hemispherical without walls. They are for indoor use and can be supported on columns, wall brackets, or suspended inside a building. Open domes can replace, aging aluminum screens, be fitted inside an existing solid dome, or installed in new facilities.
Before Fulldome.pro revolutionized dome projection there wasn’t much call for high quality portable dome screens. The only portable dome projection was via low-resolution, single-projector systems in inflatable domes. When Fulldome.pro introduced high-resolution, multi-projector systems with fast and automatic calibration we realised we also needed to create portable dome screens that could make the most of this technology. Inflatable domes were not up to the task, solid domes are too heavy and laborious to be easily portable and perforated aluminum is expensive and only suited to fixed installations. So, we set about making something as portable as an inflatable dome, with the projection quality of an aluminum screen. Our purpose-built geodesic domes and fabric screens soon became the envy of the dome projection world. Our engineering and sewing teams have continued to innovate, making our domes even easier to assemble and selecting new fabrics for even better projection.
When designing a new dome, we carefully select the screen fabric and choose material with the right gain to match the projection system, resulting in a bright, high-contrast image. Our sewing team stitch panels together to form a perfect hemisphere with as few seams as possible. In fact, a fabric screen usually has fewer seams than a perforated aluminum screen of the same size.
When it comes to cleaning, a fabric screen can be removed and washed or replaced in a matter of hours, while an aluminum screen will require the dome to be closed for several days or even weeks.
Comparison of projection dome screen technologies
| Solid | Inflatable | Perforated Aluminium | Negative-pressure | |
|---|---|---|---|---|
| • Inexpensive for small domes | • Low cost • Very quick to install • Portable | • High quality • Good acoustics | • Inexpensive • Lightweight • Fast to install • Portable or permanent • Good acoustics • Easily replaced | |
| • Heavy • Poor acoustic properties • Prone to growing fungus and mould due to moisture | • Unsuitable for use outdoors in windy conditions • Will collapse on audience if fan fails | • Expensive • Difficult to install • Difficult to clean | • Screen needs to be looked after in extreme weather to prevent moisture build up • Fan consumes a small amount of electricty |
Frameless Negative-pressure Screens
The latest innovation from Fulldome.pro is a frameless negative-pressure screen. Instead of building a frame to support the cover and create a negative-pressure envelope for the screen the cover is suspended inside the room or building and stretched outwards towards the walls and ceiling. The screen is attached around the perimeter of the cover and has some support straps connecting it to the cover. A fan is used to create negative-pressure and the screen is pushed upwards and outwards towards the cover. In some circumstances, where the room or building is well sealed it may even be possible to eliminate the outer cover and just use the building itself to create the negative-pressure envelope.
Frameless negative-pressure screens can be made in different shapes such as an elongated ‘pill’ shape, or an open-ended dome or a tunnel.
What is Negative-pressure?
When we talk about air pressure, we usually mean positive pressure. Inflating a car tire, pumping up a basketball, and blowing up a balloon are all examples of positive air pressure – that is the air pressure inside the vessel is higher than the surrounding atmosphere.
Negative-pressure is where the air pressure in a space is lower than the surrounding atmosphere – a vacuum (or partial vacuum). One place we hear about negative-pressure is hospitals, where negative-pressure rooms are used to prevent the spread of disease. An infected person is placed in isolation in a room and the HVAC system is set up so that slightly more air is sucked out of the room than is blown in. When a door is opened for a doctor to enter, air rushes into the room to equalize the pressure, preventing possibly infected air from leaving the room. The HVAC system filters, cleans, and disinfects the air that is extracted from the room.
For a negative-pressure dome, a fan runs continuously to evacuate air from behind the screen, creating an area of low pressure. The higher air pressure inside the dome pushes the screen up towards the dome frame. The pressure required to hold a negative-pressure screen up is very low. At just 200 pascals it is less than 1 percent of atmospheric pressure, one thirtieth of the air pressure inside a party balloon, hundreds of times less than in a basketball, and thousands of ties less than the pressure of a car tire.
Frameless Negative-pressure Screens
| Pascals | Pounds per square inch | Atmospheres | |
|---|---|---|---|
| Atmospheric pressure1 | 101,325 Pa | 14.7 psi | 1 atm |
| Car tire | 227,500 - 262,000 Pa | 32 – 38 psi | 2.2 – 2.6 atm |
| Basketball | 51,700 – 58,600 Pa | 7.5 – 8.5 psi | 0.5 - 0.6 atm |
| Balloon | 6900 Pa | 1 psi | 0.07 atm |
| Dome | - 200 Pa | - 0.03 psi | - 0.002 atm |
¹ Atmospheric pressure is measured in absolute terms, whereas the other pressures are relative to atmospheric pressure. Therefore the absolute pressure in a car tire is around 350,000 Pa/50 psi/3.5 atm (relative pressure of car tire, plus absolute atmospheric pressure)