IMAX
On travels to and from Central Mumbai, a sure attention-grabber is the huge green and white striped dome. Called the IMAX dome, this huge
theatre has lead to a whole new movie-going experience with a huge (21.5 X 15.6 meters) screen made of perforated stretched
vinyl materials in Mumbai.
The screens huge size can be likened to a 7-8
storied building which is broader than it is tall. There also exists a hemispherical dome which is approximately 30 meters
in diameter!
No matter whether you are in a theatre or a dome,
the effect is amazing, and it fills your field of vision. It also increases your perception
of motion as nothing else can distract from you viewing the screen.
IMAX Dome produces a picture extending 180 degrees
horizontally, about 100 degrees above the horizon and 22 degrees below. The dome surface area is 1000 square meters; 800 square
meters are used for IMAX Dome projection. The screen is made of many small plates of aluminum with 20% of the area perforated
with holes.
IMAX premiered at the Fuji Pavilion at EXPO 1970
in Osaka, Japan. The first permanent IMAX
projection system was installed in Toronto in 1971. Dome screen Omnimax, its' sister system,
debuted at the Ruben H. Fleet Space Theatre in San Diego on March 10, 1973. A few years ago the trade name Omnimax was changed to IMAX Dome.
IMAX and IMAX Dome Projectors:
IMAX and IMAX Dome projectors are the most advanced,
highest precision and most powerful projectors ever built. The IMAX projector uses what is called a "rolling loop" mechanism.
The following make up the Rolling Loop film transportation system:
àRotor: A 37.5 inch diameter drum containing eight
windows or gaps, each of which forms a loop or wave in the film as it passes the input sprocket, and then advances the film
by carrying the loops past the aperture.
àInput Sprocket: A sprocket is driven in synchronism with the rotor to
feed one frame of film (15 perforations) per rotor gap. It positions the film so that the cam pins enter the intended perforation
at a precisely determined point.
àCam Unit: A mechanism which oscillates
the film-engaging pins or claws (two pins at each edge of the film) to catch and slow down the film, fed by the input sprocket,
and to preposition the perforations which are engaged by the registration pins at the aperture.
àRegistration Pins. Four fixed pins (one at each edge of the aperture) which engage the film perforations
to precisely position the film during projection.
àOutput Sprocket:
Like input sprocket, it rotates in sync with the rotor.
àRotary Air Valve:
This valve, which has one outlet for each rotor gap, pulses air to a row of jets at the rotor gap as it passes the input sprocket
in order to help shape and accelerate the loop of film which forms in the gap at that point.
The advantages of this technique are:
àGentle film
handling - you don't have to accelerate or decelerate a foot or two of film by pulling on the sprocket holes, it's just pushed
along. The film lasts many times longer than those in 35mm projectors.
àThe
film gate has 4 steel registration pins to hold each frame in place during projection (side-to-side and up-and-down directions).
Although IMAX images are about 3 times larger than 35mm in linear measure, the projector is about 10 times steadier, which
is why IMAX films are steady and since 35mm projectors just hold the film in place with friction. The film is actually held
against the rear element of the projection lens (which is cylindrical) by a vacuum during projection. So focus
is stable.
The negative side of this is that the mechanism
is large, noisy, heavy (almost 2 tons), and expensive to build.
The 68 per cent shutter (using LCD shutter technology)
transmits one-third more light than does the 50 per cent shutter in conventional projectors.
The IMAX projector is equipped with a 15.000
watt water cooled Xenon lamp. In smaller theatres a 4.000 watt lamp is enough.
IMAX Film:
Film, in normal theatres comes in the 35mm format
(and almost square, with the width of the screen being more than the height, the projector adjusts the width to fill the screen).
In 70mm (70mm wide frame) there is no compensation
in the width resulting in better resolution.
In IMAX films however the frame is 70X15 ie:
70 mm high and 15 perforations wide (almost 10 times bigger than the standard 35mm films!).
The frame rate is also 48 frames-per-second almost
double that of the standard (24.5 fps).
IMAX
theaters use a 6-channel sound system, and in some theaters you also wear a headset that provides two extra sound channels
for each viewer.
Some IMAX movies are filmed and shown in 3D (stereoscopic
perspective) using one of these projection techniques:
3D IMAX type 1: You wear Polarized glasses (except
one lens is rotated 90 degrees
so that one eye sees vertically polarized light,
the other sees horizontally polarized light).
3D IMAX type 2: You wear battery powered headset
visor with 24 hz LCD shutter in front of each eye. Infrared detectors pick up
signals in the theater to alternately darken either eye
as the appropriate frame is on the screen. This still uses polarizing technology in the LCD itself, but could be considered active
rather than passive.
The powered shutter headsets are known as E3D
(Electronic Liquid-Crystal shutter 3D). A version with built in headphones is
known as the PSE (Personal Sound Environment. The PSE does have the advantage in that it has small speakers that can be adjusted
to give individuals private alternate language audio tracks during the movie. The built in speakers can also be used to enhance
the surround sound audio experience. The 3D LCD shutter system uses a single 48fps projector with alternate left and right
eye images fed through the projector(effectively giving 24fps 3D).
IMAX theatres have very steep seating enable
everyone to watch the screen unencumbered. This allows for multi-directional viewing as in daily life.
References:
http://www.howstuffworks.com
http://www.geocities.com/TEG2/IMAX/
http://www.1570films.com
http://www.imax.com
http://www.in70mm.com/newsletter/1997/48/tbp_projection/projector.htm
