A Beginner’s Guide to … 3D entertainment
Don't know your passive display from your active glasses, then read on for everything you need to know about 3D.
The downside is that the picture is in monochrome, and not even a simple greyscale. The fact that each eye sees different colours is distracting, and some people find it fatiguing or headache inducing. Full-colour anaglyph techniques have been developed, but in truth they’re only partly full-colour and partly 3D.
Another technique that has been tried in cinemas is to use battery-powered glasses that use LCD shutters to block the light to each eye in turn. The timing of the alternating lenses was synchronised with frames on the screen, so when the projector flashed up a frame for the left eye followed by a frame for the right eye, only the correct eye saw each image.
This gave a full-colour 3D image and a convincing 3D effect, but the alternating shutters could be distracting. They weren’t comfortable to wear, either, resembling a futuristic welder’s mask. Because the technology was built into the glasses, it was expensive to equip each person in the theatre, so it’s not surprising that this approach never caught on in cinemas.
POLAR EXTREMES
By far the most enduring technique in cinemas is polarisation. This works on the principle that light is a wave, oscillating at right angles to its direction of travel. A polarising filter can absorb light rays that oscillate up and down, while letting through rays that oscillate left to right. Rotate the filter by 90 degrees and it has the opposite effect.
Polarised glasses, the most common way to see 3D at the cinema today
By using two carefully aligned projectors and fitting a vertically polarised filter to one and a horizontally polarised filter to the other – and equipping the audience with corresponding polarised glasses – a full-colour 3D effect is created. Despite the iconic status of those red and green anaglyph glasses, it was actually polarisation that was most widespread in the 3D cinema booms of the 1950s and 1980s.
The main projector at BFI Imax, Waterloo, uses this long-established technology. It’s essentially two projectors in one, with separate reels of film for each eye. They pass through two projectors, each one having been fitted with a polarising filter that corresponds to the filters in the glasses worn by the audience. Not all 3D films are distributed in Imax 3D but the biggest blockbusters generally are, as well as films made exclusively for Imax.