How do 3D movies work?

Photo of Mars viewable with 3-D glasses

This photo from the surface of Mars was taken by the NASA's Phoenix Rover on Oct. 28, 2008. It can be viewed in three dimensions by wearing 3D viewing glasses with red and blue lenses. Photo in the public domain courtesy NASA/JPL-Caltech.

The key to being able to project an image in three dimensions is the human capability for binocular depth perception. What this means is that when we look at something with both of our eyes, the image that the left eye sees is slightly different than the image that the right eye sees. The closer the object is, the more different the two images will be; distant objects, such as a plane in the sky, will be seen with no perceptible differences between the eyes. In most people, the brain is quite capable of blending the images together while also sensing the differences between the two images to determine the object’s distance. We also use other cues such as shadows and size to determine distance, but this use of our two eyes coordinating is the one that 3D movies rely on. That explains why 3D movies won’t work for people who have vision in just one eye.

When a three-dimensional movie is filmed, then, it is necessary to produce two sets of images, one through a lens that records the images that the left eye will see, and one through a lens that the right eye will see.

An obvious problem to solve, then, is how to project a moving picture onto a screen so that the left eye sees what was recorded through the left lens and the right eye sees what was recorded through the right lens. There are at least three ways of doing this that have been in common use.

The oldest method is through the use of colored glasses. In this method, the left camera films everything through, for example, a red filter, while the right camera films through a blue filter. The movie is projected through similar filters. To watch the movie, the viewer dons colored glasses with a filter for each eye, red for the left eye and green for the right. Other color pairs may also be used. For obvious reasons, this method affects the colors in which objects are perceived and is a less than a satisfactory solution.

A more common method these days is through the use of polarizing filters. The concept of polarized light is a bit complex to explain in an article this length, but think of light waves as having directions, such as as vertical and horizontal. A polarizing filter might let up-and-down light waves through but block horizontal ones and vice versa. The concept in filming a 3D movie using the polarization method is much the same as with the color method. The left camera records the parts of an image that are polarized in one direction, and the right camera records the parts that are polarized at a 90-degree angle (a right angle) to that. The movie is displayed using polarized light, and the viewer watches through corresponding polarizing filters.

One advantage of the polarization method is that the viewing glasses are inexpensive. If you’ve ever watched a 3-D film wearing cheap-looking glasses with gray filters in a cardboard frame, this is the method that was used.

The newest method has been made possible by modern technology. As you may know, movies aren’t made up of moving images but a series of still images that come so close together that you can’t distinguish when one image begins and the next begins. In this third method, an image for the left eye is flashed on the screen for a split second, then an image for the right eye, then for the left, and so on — much faster than you can blink. The user wears special active filters — an infrared signal in the theater tells the electronic glasses to let light through the filter on the left side when the image for the left eye is displaying, then tells the left side to go dark for a split second while the light is let through the right side. All this happens incredibly fast, and the filters will actually look gray even through they’re switching between black and transparent.

When 3-D movies were shown in the 1950s, viewers often reported headaches and eyestrain because it was very difficult to precisely coordinate two film projectors. But with today’s digitization, many of the problem of the past have been overcome. You can expect to see an increasing number of 3-D films in theaters as the movie industry competes to provide an experience that most viewers can’t get at home.

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1 Comment

  • eiffel says:

    A great overview! Just one point on the polarizing system: the two cameras don’t need to record polarized light; they just need to record left-eye and right-eye views as with the red-green system.
    It’s only on playback that polarization is needed: a polarizing filter in front of each projector, and a matching polarizing filter in front of each eye.
    The advantage of the polarizing system over the red-green system is that it allows full-color images, whereas the red-green system is effectively projecting black-and-white images that have been tinted red or green.
    The advantage of the “electronic glasses” system is that only one projector is needed, therefore the system could even be adapted to television. The disadvantage is that the glasses are expensive compared to the polarizing glasses.

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