<P> Planetarium domes range in size from 3 to 35 m in diameter, accommodating from 1 to 500 people . They can be permanent or portable, depending on the application . </P> <Ul> <Li> Portable inflatable domes can be inflated in minutes . Such domes are often used for touring planetaria visiting, for example, schools and community centres . </Li> <Li> Temporary structures using glass - reinforced plastic (GRP) segments bolted together and mounted on a frame are possible . As they may take some hours to construct, they are more suitable for applications such as exhibition stands, where a dome will stay up for a period of at least several days . </Li> <Li> Negative - pressure inflated domes are suitable in some semi-permanent situations . They use a fan to extract air from behind the dome surface, allowing atmospheric pressure to push it into the correct shape . </Li> <Li> Smaller permanent domes are frequently constructed from glass reinforced plastic . This is inexpensive but, as the projection surface reflects sound as well as light, the acoustics inside this type of dome can detract from its utility . Such a solid dome also presents issues connected with heating and ventilation in a large - audience planetarium, as air cannot pass through it . </Li> <Li> Older planetarium domes were built using traditional construction materials and surfaced with plaster . This method is relatively expensive and suffers the same acoustic and ventilation issues as GRP . </Li> <Li> Most modern domes are built from thin aluminium sections with ribs providing a supporting structure behind . The use of aluminium makes it easy to perforate the dome with thousands of tiny holes . This reduces the reflectivity of sound back to the audience (providing better acoustic characteristics), lets a sound system project through the dome from behind (offering sound that seems to come from appropriate directions related to a show), and allows air circulation through the projection surface for climate control . </Li> </Ul> <Li> Portable inflatable domes can be inflated in minutes . Such domes are often used for touring planetaria visiting, for example, schools and community centres . </Li> <Li> Temporary structures using glass - reinforced plastic (GRP) segments bolted together and mounted on a frame are possible . As they may take some hours to construct, they are more suitable for applications such as exhibition stands, where a dome will stay up for a period of at least several days . </Li>

In a projection when the source of light is at the centre of the globe then it is referred to as