The intent of IMAX is to dramatically increase the resolution of the image by using much larger film stock at a resolution comparable to about 10000 x 7000 pixels (70 megapixels). To do this, 70 mm film stock is run "sideways" through the cameras. While traditional 70 mm film has an image area that is 48.5 mm wide and 22.1 mm tall (for Todd-AO), in IMAX the image is 69.6 mm wide and 48.5 mm tall. In order to expose at standard film speed of 24 frames per second, three times as much film needs to move through the camera each second.
Drawing the large-format film through the projector was a difficult technical problem to solve; conventional 70 mm systems were not steady enough for the 586x magnification. IMAX projection involved a number of innovations. William Shaw of IMAX adapted an Australian patent for film transport called the "rolling loop" by adding a compressed air "puffer" to accelerate the film, and put a cylindrical lens in the projector's "block" for the film to be vacuumed up against during projection (called the "field flattener" because it served to flatten the image field). Because the film actually touches the "field flattener" lens, the lens itself is twice the height of the film and is connected to a pneumatic piston so it can be moved up or down while the projector is running. This way, if a piece of dust comes off the film and sticks to the lens, the projectionist can switch to the clean side of the lens at the push of a button. The lens also has "wiper bars" made of a felt or brush-like material which can wipe the dust off the lens as it moves up or down to keep the show clean. IMAX projectors are pin stabilized, meaning 4 registration pins engage the sprockets at the corners of the projected frame to ensure perfect alignment. Shaw added cam-controlled arms to decelerate each frame to eliminate the microscopic shaking as the frame "settled" onto the registration pins. The projector's shutter is also open for around 20% longer than in conventional equipment and the light source is brighter. The largest 12-18 kW xenon arc lamps have hollow, water-cooled electrodes. An IMAX projector is therefore a substantial piece of equipment, weighing up to 1.8 tonnes and towering at over 70" [178 cm] tall and 75" [195 cm] long. The xenon lamps are made of a thin layer of quartz crystal, and contain xenon gas at a pressure of about 25 atmospheres; because of this, projectionists are required to wear protective body armor when changing or handling these lamps because the flying shards of crystal (should the lamp fall and crash) could be deadly when combined with the high pressure of the gas within.
IMAX uses a stronger "ESTAR" (Kodak's trade name for PET film) base. The reason is not for strength, but precision. Developing chemicals do not change the size or shape of Estar, and IMAX's pin registration (especially the cam mechanism) is intolerant of either sprocket-hole or film-thickness variations. The IMAX format is generically called "15/70" film, the name referring to the 15 sprockets per frame of 70 mm stock. The bulk of the film requires large platters rather than conventional film reels.
In order to use more of the image area, IMAX film does not include an embedded soundtrack. Instead the IMAX system specifies a separate six-channel 35 mm magnetic tape synchronized to the film. (This original system--35 mm mag tape locked to a projector--was commonly used to "dub" or insert studio sound into the mixed soundtrack of conventional films.) By the early '90s, a separate digital 6-track source was synchronized using a more precise pulse generator as a source for a conventional SMPTE timecode synchronization system. This development presaged conventional theatrical multichannel sound systems such as Dolby Digital and Digital Theater System. This digital source came in the form of a unit called a DDP (Digital Disc Playback) in which the soundtrack was recorded onto multiple CD-ROM discs which would play the sound which was recorded to the discs as a digital audio file. This DDP system has been replaced in almost all theaters with the newer DTAC (Digital Theater Audio Control) system which utilizes a computer running the IMAX's proprietary DTAC software. The software works in a similar style as the DDP except that instead of the audio file being based on discs, it is instead played directly off a hard drive in the form of a single uncompressed audio file containing the 6 channels which are distributed directly to the amplifiers rather than using a decoding method such as Dolby Digital. Many IMAX theaters place speakers directly behind the screen as well as distributing the speakers around the theater to create a three-dimensional effect.
IMAX theater construction also differs significantly from conventional theaters. The increased resolution allows the audience to be much closer to the screen; typically all rows are within one screen height. (Conventional theaters seating runs 8 to 12 screen heights) Also, the rows of seats are set at a steep angle (Up to 23 degrees in some domed theaters) so that the audience is facing the screen directly.