A Beginner’s Guide to … 3D entertainment

Inevitably there are downsides to the polarisation technique. One is that a silver rather than white screen is required to maintain the polarisation as light bounces off the screen. That also means a brighter picture for those sitting in the best seats, but a considerably darker one for those sitting at the sides. If you’re watching a 3D film in the cinema, or even watching a 2D film on a screen that’s designed for 3D, be sure to get a good seat.

Another problem is that the two projectors must be aligned perfectly and in sync to avoid the 3D effect going disastrously wrong. Even with careful alignment film can jitter, whereby the picture moves up and down very slightly on the screen. These problems aren’t easy to overcome, and were particularly tricky to handle in the 1950s and 1980s, and they’re further reasons why 3D cinema is associated with headaches. It explains why Imax cinemas, with their highly specialised equipment and staff, were the only ones to keep 3D cinema alive throughout the 1990s and 2000s.

Imax 3D uses over 10 miles of film for each eye for a 150-minute film

The third boom in 3D cinema that’s happening now is largely down to the convenience of digital projection. Rather than grappling with twin projectors, cinemas that use the RealD system – which is most 3D screens in the UK – receive films on a 500GB hard disk that plugs into to a single DLP projector. An active polarising filter in front of the projector’s lens uses liquid crystals to change polarity 144 times per second – that’s six times the speed of 24fps film. Each frame is shown six times, alternating between the left-eye and right-eye images to avoid perceptible flicker.

Another difference between Imax and RealD 3D systems is that RealD uses circularly polarised filters. Rather than filtering the light on vertical and horizontal plains, it’s filtered in clockwise and anti-clockwise twisting motion, like a corkscrew. The advantage of this technique is that it still works when viewers tilt their heads. In an Imax 3D film, it’s important to keep your head straight or else the polarising glasses become ineffective. Neither system is perfect, though, and there will always be some ghosting in polarised systems where a little of the left-eye image reaches the right eye, and vice versa. It’s most noticeable in high-contrast lines such as title sequences and end credits, but pale faces against a dark background can also suffer this problem, and it can be quite distracting.

Digital cinema projectors are certainly more convenient than dual film projectors, but they’re not as bright or as detailed. The Odeon chain installed NEC NC1600C digital projectors in its cinemas back in 2009, but its 17,000-lumen brightness and 2,048×1,080 resolution means it’s only suitable for small and medium-sized screens, especially because polarisation halves the amount of light projected. NEC’s latest NC2500S projector is rated at 26,000 lumens, and Sony’s SRX-T110 has a 4,096×2,160 resolution, but compare these to the 60,000-lumen brightness of dual Imax projectors and the theoretical resolution of 12,000×8,700 of its enormous film frames, and it’s easy to see why Imax still claims to offer the best 3D cinema experience available.