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Monday, December 21, 2009

How 3-D Movie Projection Works

Wired Explains:

3-D Movie

Every few years you’ve probably watched a mainstream movie through a pair of glasses that make creatures, people and explosions pop out of the screen. And if you’ve bought into the massive hype, you were probably lining up this past weekend for James Cameron’s Avatar, which is screening in 3-D.

You might wonder, why can’t more movies be shown in 3-D? It would just take some post-production video rendering and a pair of stereoscopic glasses, right?

Actually, 3-D projection is a lot more complicated — and expensive — than one would think. In anticipation of Avatar, paid a visit to Dolby Laboratories in San Francisco to learn about the history of 3-D movie technology leading up to its current state.

Remember those junky glasses, with a blue lens for one eye and a red one for the other? They were tied to a 3-D-imaging method called anaglyph that dates back to the 1950s. With this system, the images on the screen were projected with two color layers superimposed onto one another. When you put on the glasses, each eye sees a separate visual, the red-tinted image through one eye and the blue-tinted one through the other. Your visual cortex combines the views to create the representation of 3-D objects.

Though it may have been impressive at the time, early anaglyph imaging suffered from many issues. The color separation on film was very limited, and thus it was difficult to perceive details in 3-D scenes. Another frequent problem was ghosting, which happened when the image that should be appearing in your left eye would creep over to the right.

And then there’s the screen. Theaters projecting 3-D movies with the anaglyph method had to install silver screens for an ideal viewing experience. That’s because the more reflective screen helped keep the two different light signals separated.

3-D movie technology has come a long way. Anaglyph imaging has improved: Glasses now are typically red and cyan, which, when combined, can make use of all three primary colors, resulting in more realistic color perception.

RealD cinema, currently the most widely used 3-D movie system in theaters, uses circular polarization — produced by a filter in front of the projector — to beam the film onto a silver screen. The filter converts linearly polarized light into circularly polarized light by slowing down one component of the electric field. When the vertical and horizontal parts of the picture are projected onto the silver screen, the filter slows down the vertical component. This effectively makes the light appear to rotate, and it allows you to more naturally move your head without losing perception of the 3-D image. Circular polarization also eliminates the need for two projectors shooting out images in separate colors. The silver screen, in this case, helps preserve the polarization of the image.


Dolby’s 3-D system, used for some Avatar screenings, is a little different. It makes use of an exclusive filtration wheel (above) installed inside the projector in front of a 6.5-kilowatt bulb. The wheel is divided into two parts, each one filtering the projector light into different wavelengths for red, green and blue. The wheel spins rapidly — about three times per frame — so it doesn’t produce a seizure-inducing effect. The glasses that you wear contain passive lenses that only allow light waves aligned in a certain direction to pass through, separating the red, green and blue wavelengths for each eye.

The advantages of Dolby’s 3-D system? There’s no need for a silver screen, thanks to the built-in color-separation wheel and the powerful bulb right next to it, ensuring a bright picture necessary for 3-D viewing. Also, a mechanism can be adjusted inside the projector to change the projection method from reflection to refraction — meaning theaters can switch between projecting regular movies and 3-D movies.

The cons? The glasses are pricey: $27 apiece, so they’re designed to be washed and reused (as opposed to recycled). (Although, this would be considered a pro for the environment.) Altogether, a Dolby 3-D projection system costs theaters about $26,500, not including the eyewear.

Updated 9 a.m. PDT with more details explaining circular polarization.

See Also:

Photos: Jon Snyder/, Brian X. Chen/


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