What is “immersive”?

Originally published in the November 2008 issue.

By James Hyder, Editor/Publisher

As we recently learned, with its entry into digital projection, Imax Corporation now defines its theaters not as giant-screen, but as “the most immersive experience in the world.” (See LF Examiner, October 2008.)

So what does immersive really mean?

The word has been applied to everything from handheld video games to ultra-high-res digital presentations projected on huge hemispherical domes with multi-channel audio. Virtual reality systems with head-mounted displays are considered immersive, as are so-called “caves,” research systems that surround one or two users with 3D images projected on the faces of an 8–10-foot cube.

Most writers on the subject agree that the essence of immersiveness is giving viewers the impression that they are actually in the place depicted by the presentation. This illusion depends on successfully fooling several senses, principally sight and hearing, and eliminating or reducing any cues that would tend to break the illusion.

The basic principle in creating a visually immersive experience is to fill the audience’s field of view. As everyone knows, the closer something is to you, the larger it appears to be, that is, the more of your field of view it fills. A larger object farther away can appear to be the same size as a smaller one that is closer.

So with an image projected on a screen, a smaller screen can be made to seem bigger simply by getting closer to it. As long as the image quality is high enough that the viewer doesn’t begin to see film grain, digital pixels, or other distracting artifacts, the experience is nearly, if not precisely, the same as seeing a larger screen from farther away. As we will see, almost all efforts to create immersive motion picture experiences have involved increasing the amount of information presented, with larger film frames, higher frame rates, or both.

Early immersive cinema

In the cinema world, the first significant attempt to create an immersive environment was Cinerama, introduced in 1952. It used three strips of 6-perf, 35mm film, running at 26 frames per second, and projected onto a deeply curved screen. The special Cinerama camera had a field of view of 146°, nearly matching that of human vision, and an aspect ratio of 2.6:1. With three film frames, each 50% larger than a conventional 35mm frame, running about 8% faster than usual, Cinerama presented nearly five times as much information per second as the standard Academy-ratio 35mm movies of the day.

Subsequent formats such as Todd-AO and VistaVision offered aspect ratios approaching that of Cinerama on a single strip of 70mm or 35mm film, respectively. Both of these also had a frame that was significantly larger than 35mm, and ran faster than the standard 24 fps.

These, and the many other competing wide-screen formats of the 1950s and ’60s, were all intended to show feature films in existing movie theaters. They required the installation of wider and larger screens, but rarely entailed other changes to the basic geometry of the auditoriums, which, before the days of the multiplex, were usually quite deep relative to the height of the screen. Viewers who sat relatively close to the screen would have a very immersive experience, with the image filling a large part of their field of view. Those toward the back would naturally see a smaller, less involving image. In some Cinerama theaters, the ends of curved screen extended nearly to the first row of seats. People in those seats saw a picture that filled nearly 180° of their field of view.

Apart from films made for world’s fairs, theme parks, and other special venues, most of which were one-of-a-kind systems, the first widespread format to incorporate special theater geometry was the 15/70 system introduced in 1970 by Multiscreen Corporation, later known as Imax Corporation. Engineer and co-founder William Shaw described the essential elements of IMAX theater design in a 1983 article in the SMPTE Journal, co-authored with J. Creighton Douglas.

“The high-resolution picture is used in conjunction with a large screen and carefully organized audience seating to ensure that minimum and maximum viewing angles lie between 60° to 120° horizontally and 40° to 80° vertically for the farthest and nearest spectators respectively. The intent is to create an illusion of ‘being there,’ rather than present a ‘normal’ motion picture through a well-defined window. Most important, in our view, is to strive for audience viewing angles that approach those encountered in reality; that is, horizontal and vertical angles which extend well into the area of peripheral vision and which require eye/head movement to take in the entire picture. The feeling of a large window on reality is found to be enhanced if the screen appears to fill the entire front of the theater, wall-to-wall and floor-to-ceiling.” [Emphasis added.]

Later in the article the relationship between the seating area and screen size is quantified: “Because of the amount of information on the large format, we aim for a minimum eye-to-screen distance of 0.35 times…screen width and a maximum distance to the last row equal…to the screen width.”

Accompanying the article is a plan drawing of a theater in which distance from the screen to the last row of seats equals the width of the screen. The closest row is about 35% of screen width. These ratios, 0.35W and 1.0W (where W=the width of the screen), provide horizontal viewing angles of 110° and 53° respectively, close to Shaw and Douglas’ goals of 120° and 60°. To obtain an angle of at least 60° for all viewers, the last row must be no more than 0.86W from the screen.

The vast majority of IMAX and other giant-screen theaters built between 1971 and 2004, when the MPX system was introduced, embody these principles. They are essentially square, with the back wall about as far from the screen as the screen is wide. In fact, the depth of many “classic” IMAX theaters is 0.9W or less, according to design consultant Mark Peterson of White Oak Associates, so as to attain that crucial 60% minimum angle.

IMAX in multiplexes

The first IMAX theaters in multiplexes followed these guidelines. They were purpose-built and used the same IMAX GT projectors installed in institutional and standalone theaters. Today some 37 multiplexes in 15 countries have full-blown GT projectors, and screens sizes ranging from 52×71 feet (16×22 meters) to 77×94.5 feet (23.5 to 29 meters). However, after the first rush of installations in the late 1990s and the subsequent implosion of the North American cinema industry in 2000, it became clear that exhibitors were unwilling to spend the $5 million or more such theaters cost. Imax responded with the SR system.

The SR projectors are smaller and intended for smaller theaters with smaller screens, although they maintain the standard 15/70 aspect ratio of 1.43:1. Virtually all SR theaters were also purpose-built, but the projection hardware and construction costs were substantially lower. Thirty-three are in operation in multiplexes today, with screen sizes from 46×57 feet (14×17.5 meters) to 52×73 feet (16×22 meters).

However, even this proved economically unfeasible for most theater chains, so Imax invented its third projector system, the MPX. Designed specifically to be retrofitted into existing multiplex auditoriums (hence the name), it fit into the footprint of conventional 35mm projectors, making it much simpler to install than the car-sized GT machines. The MPX was the first IMAX system to abandon the 1.43 aspect ratio in favor of the 1.85 ratio used by most Hollywood films. Traditional full-frame 15/70 movies shown on the MPX are cropped top and bottom by about 20%.

Although the hardware costs of the MPX were only slightly lower than those of a 2D SR system, the biggest savings came from not needing to construct and furnish a large new auditorium from scratch. The MPX was very successful for Imax: there are 60 systems in place today, with screens ranging from 24×47 feet (7×14 meters) to 46×74 feet (14×23 meters). The largest of these are purpose-built theaters, such as the three opened by Goodrich Quality Theaters, but the vast majority are retrofits into houses built for 35mm projection.

Rolled out in 2004, the MPX system served as a placeholder for the digital system that was finally announced in 2006, and many MPX contracts provide for an upgrade to the digital system

Imax had begun research into a digital projection system in the late 1990s and acquired one of the leading companies in the field, Digital Projection International, to develop a digital replacement for 15/70 film projection. But the technology of a decade ago proved unworkable, and in 2000, when Imax shares dropped 90% in three months because of the multiplex meltdown, R&D funds dried up. Imax sold off DPI and waited for technology to catch up.

Like the MPX, the Imax digital system that began rolling out this summer has a wide aspect ratio: 1.9, the ratio of the Texas Instruments DLP chip used in the Christie projectors that are at its heart. It too is intended to be retrofitted into existing 35mm houses with screens up to 70 feet (21 meters) wide and 37 feet (11 meters) tall.

The company has signed contracts for more than 200 of these systems, the vast majority of which are joint-venture deals with the exhibitor, under which the chain pays only about $100,000 for the conversion costs, and Imax bears the $500,000 cost of the projection and sound system. The parties proportionally split the income from the theater, where with its previous deals Imax collected a lease fee of only about 7% of the box office. These deals are attractive to the exhibitor because they reduce the up-front costs and the risk of poorly performing films, and put the burden of maintaining and upgrading the hardware on Imax. For taking that financial burden and risk, Imax receives larger continuing revenues than it had with its leased systems.

Converting a screen to IMAX digital

The process of converting a 35mm auditorium to MPX or IMAX digital projection usually entails removing the existing screen and the first few rows of seats, and installing a larger silver screen (for 3D) a few yards closer to the rear of the theater. The new screen is made to fill as much of the width and height of the room as possible, and typically has no masking on either dimension. A new Imax sound system, with new speakers and five-channel, uncompressed digital playback, is installed at the same time. The other fixtures of the theater, seats, seating deck, wall treatments, lighting, etc., are usually not changed, although some operators take advantage of the down time to upgrade the chairs.

U.S. patent #7,106,411, “Conversion of a cinema theater to a super cinema theater,” issued to Imax Corporation on Sept. 12, 2006, claims (without providing any absolute or relative dimensions for the theater or screen) that for a viewer in the front row (position Z on the drawings) this process increases the apparent size of the screen by 115%, and that for a viewer at roughly two-thirds of the way to the back wall (position X) the apparent size is increased by 100%. The patent states that the change increases the minimum horizontal field of view (from the last row) from 45° to 55°.

The illustrations in the patent document show a theater that starts as 1.3W (1.3 times as deep as the width of the screen), and ends up as 1.0W after the conversion. This would appear to achieve the standard for immersiveness that Shaw established in the 1983 article. However, according to Clyde McKinney of McKinney Technical Services, most multiplex theaters these days start out with a depth of 1.5W or more. A proportional change to a 1.5W theater would yield one that is a little over 1.2W deep.

LF Examiner has visited the first seven IMAX digital theaters to open (conveniently, they were the ones closest to our offices in Columbia , MD ), and used a laser rangefinder to take accurate measurements of their screens and other dimensions. (Naturally, we did so only between shows, so as not to distract other customers.) In the new IMAX configuration, the depths range from 1.24W to 1.48W, providing minimum horizontal viewing angles between 37° and 45°. None that we have visited, including the AMC Empire 25 in New York City, the site of the first open industry demonstration in September (see LFX, October 2008), approaches the 1.0W ratio shown in the patent drawing, much less the 0.9W ideal of Shaw’s classic IMAX theaters.

The maximum horizontal viewing angle from the front row of these theaters ranges from 103° to 122°, but most are under 110°. So the seats in the front half of these houses fall within the range of horizontal angles Shaw and Douglas specified. But in theaters of conventional design, movie goers rarely choose seats that far forward, most preferring to sit in the back half. The design of a classic 0.9W IMAX theater enforced a closer seating position relative to screen size, and thus guaranteed a more immersive experience for all viewers.

So far we have been dealing only with the horizontal field of view. With the change to Hollywood ’s wide aspect ratios, IMAX digital and MPX theaters have dramatically reduced the viewer’s vertical field of view. For any given width, a screen with a 1.9 ratio is 25% smaller than a 1.43 screen.

In a classic IMAX theater with a 1.43 aspect ratio screen, the vertical field of view from a last-row seat at 0.9W is 42.5°. (Shaw’s goal was between 40° and 80°.) In a digital theater with a screen aspect ratio of 1.9, and its last row at 1.2W, the minimum vertical angle is 25°. And this is the best case: as we see from the examples above, most digital theaters seem to have shorter screens and deeper houses, yielding minimum vertical angles as small as 20°.

Even if a digital theater were given a depth of 1.0W, which would call for sacrificing at least 100 seats, it would still have a minimum vertical viewing angle of only 30%, because of the screen’s shorter aspect ratio.

In short, the IMAX digital theater design increases the horizontal field of view of the average seating position slightly when compared to the previous configuration, but leaves many seats — including those where most people choose to sit — well outside Shaw’s ideal parameters for horizontal field of view. And the wide aspect ratio of the system (and Hollywood films) reduces the total proportion of viewer’s field of view taken up by the image by 25%, compared to a 1.43-ratio 15/70 image.

A key element of Imax’s theater conversion patent is that, in addition to the changes in screen position and size, the projection system should be replaced with one that has greater “image fidelity.” This prevents viewers, who are now closer to the screen, from seeing projection artifacts, such as “lack of image resolution,…film grain…, or in the case of digital projectors, image pixels.”

According to Imax’s Brian Bonnick, the resolution of the IMAX digital projector is “somewhere above 2K, somewhere less than double that of 2K.” Doubling 2K yields approximately 2.9K, because resolution increases as the square of the horizontal dimension: 4K has four times as much information as 2K. It is widely acknowledged that a 35mm film frame has at least as much information as a 4K digital image. Thus, an IMAX digital conversion would appear to reduce, not increase, the amount of on-screen information.

As we said in our report on the IMAX digital system last month, with two projectors, the IMAX digital image is brighter and has better contrast than conventional digital systems and average 35mm projection in a multiplex (although it is not significantly brighter than the best 35mm projection). But the IMAX system also suffers from a “screen-door effect,” a noticeable dark grid separating the pixels, that is more obvious the closer one is to the screen. Thus it might be argued that IMAX digital conversions don’t meet the criteria of the Imax patent.