3D Technology in Cinema: IMAX 3D, RealD 3D, Dolby 3D and XpanD 3D

After we've made an overview of the 3D glasses kinds currently in use, let's take a look at the 3D technology systems used in cinema theaters. The most widespread technologies for 3D projection in movie theaters are IMAX 3D, RealD 3D, Dolby 3D and XpanD 3D.

IMAX 3D is the oldest of the 3D technologies covered here. Its most obvious characteristic is the big imposing curved screen. IMAX cinemas are typically analogue and project 70mm film to achieve the necessary resolution for such big screens. There is also a digital IMAX version which utilizes smaller screens. Due to the large and curved screen IMAX creates in-your-face 3D images with a lot of "pop". Some earlier IMAX cinema used active shutter glasses but current IMAX 3D uses linear polarization. Linearly polarized glasses require the viewer to keep their head level, as tilting causes the images of the left and right channels to spill to the opposite channel. This is the so-called "ghosting" effect which plagues to some degree pretty much any 3D technology displaying info for both eyes at the same time. The head movement restrictions have a couple of implications: first, the amount of seats in the cinema theater where the image quality is at its best is limited, and second, longer viewings may become uncomfortable. This is one reason why until recently IMAX 3D theaters mostly did show shorts.

RealD 3D upgrades the IMAX approach significantly. RealD 3D uses circularly polarized glasses which allow the audience to move their heads around without any loss of 3D perception. RealD 3D is a digital 3D technology which utilizes one of two possible projection systems. In one of these systems, each left-eye and right-eye frame of the typical 2 * 24 fps 3D movie (24 frames for the left eye and 24 frames for the right eye per second) is flashed alternatively three times for a total of six flashes per 3D frame and 2 * 72 = 144 flashes per second. For comparison, in regular 2D movie projection, frames are typically flashed twice for overall 48 flashes per second. The other system uses a single 4K projector but projects two oppositely circularly polarized 2K frames simultaneously through it. RealD 3D has another advantage over linearly polarized tech (IMAX 3D) and that's the reduced amount of ghosting. And as the screens are normal sized and not curved the action doesn't seem to pop out of the screen as much, which is less stressful on the brain although not so spectacular.

Polarization systems (IMAX 3D, RealD 3D) have an inherent problem: the loss of light. Light is initially lost during projection through the polarization filter at the projector lens and then lost once more when passing through the filters on the glasses. There is also some loss when light reflects from the screen (this is not 3D specific). IMAX battles this by using 2 projectors (one for each eye), which means double the light output. And RealD 3D combats it by using silver screens for better reflection. A cinema theater may also work to compensate that by using more powerful projection lamps. Still, there is an overall impression of a darker image.

Dolby 3D is another fully digital 3D technology. It is quite popular lately because the same projector can be used for both 3D and 2D movies and no special screens are needed. For 3D projection a special color wheel is attached to the projector to achieve wavelength multiplexing, thus splitting the wavelenghts used by the left and the right eyes. The glasses need highly accurate color filters to correctly filter color wavelenghts at the viewer's end. These filters are called dichroic (or interference) filters and are more expensive than the polarization filters. Unlike polarized glasses they are being reused. Cinemas still like this tech as they cut on the expenses by saving both on silver screens and additional 2D screens. Dolby 3D tends to preserve dark detail better than polarization systems. One disadvantage is that glasses are heavier and thicker than the light plastic polarized glasses.

XpanD 3D is also a digital 3D technology. Unlike all the others, it uses active shutter glasses synchronized with projection through infrared signals. As frames are flashed alternatively for the left and the right eye without any color or polarization filters, there is no light loss from filtration on projection. But the use of electronics in the glasses may lead to reliability problems and the batteries need to be replaced when they deplete. As there is no special projection or screens required, the XpanD 3D glasses can also be used with home cinema theater 3D systems.

So how do these 3D technologies compare after all?

Correctly configured latest generation XpanD 3D offers the best possible experience. A lot of reference theaters at the movie studios are equipped with this system. Public theaters may often suffer from technical problems (flickering has been reported) and poor maintenance though, which might make the next options preferable. Dolby 3D and RealD 3D should probably rank about equal and above the linear polarization of IMAX 3D. One consideration with 3D at cinemas is that it generally amplifies the lack of projection quality wherever this lack is apparent. Brighter lamps and dual projectors (where applicable) help alleviate brightness problems. But any kind of incorrect setup will usually appear more severely broken in 3D.

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