Although polarization is used for many of the latest movies, it’s quite an old technology. As far back as 1894 a John Anderton of England was using polarizing filters over the lenses on a pair of projectors, a silver screen, and polarized glasses worn by the viewers to show stereo images. The process had been discussed earlier, by Charles Wheatstone and others, but Anderton was reportedly the first to make the process practical enough to be used. Decades later Edwin H.Land, the founder of Polaroid, invented inexpensive polarizing filters and gave the first public demonstration of their use in projecting 3D stills and movies in 1936.
Types of Polarization
There are two kinds of polarization used in stereo photography—linear
. The advantage of circular polarization is that you can tilt your head and change the viewing angle when watching a movie or slide show. If the images are linearly polarized, as they once were, tilting your head causes ghosting, lowered contrast and brightness, and even loss of the 3D effect. This happens because when you tilt your head the orientation of the glasses no longer matches the orientation of the light reflected from the screen.
With linear polarization light oriented in the same direction as the filter (top) passes through. Light oriented at an angle is increasing reduced as the angle increases until at 90 it’s blocked entirely. The problem with linear polarization is you can’t tilt your head or you’ll loose the 3D effect.
Polarization does not reduce the 3D resolution of a projector by half as some systems do. However, projectors using this technology require brighter lamps to offset the light lost as it passes through the two sets of polarizing filters—one set for the projector and one for the viewer.
StereoJet imaging makes it possible to have high quality 3D polarized prints hanging on your walls or backlit in displays. A stereo pair is printed back-to-back using a standard inkjet printer with left-and right-eye images polarized at 90° to one another. The two images are superimposed and laminated together and a retarder layer is added to convert the polarization from linear to circular, affording a wide viewing angle with no crosstalk or distortion and providing freedom of head position and comfort in viewing. Both prints and transparencies are viewed using the same kind of polarized glasses used in cinemas.
Circular polarization is hard to illustrate but this is one way to think of it. Waves in space rotating clockwise (left-handed) or counter clockwise (right-handed). Courtesy of Dave3457 on Wikipedia.
IMax started by using 70mm film with three times the size of regular 70mm and ten times the size of conventional 35mm which you would see in a normal movie theatre. For 3D they use a digital format with a polarized projector and glasses.
Polarization technology, widely used in cinemas to display movies such as "Avatar," is supplied by companies such as RealD and MasterImage 3D. Their systems have three basic components—a projector that can project polarized images, a screen that preserves that polarization and image brightness, and polarized glasses that let each eye see a different image.
In the 1940s Polaroid developed a 3D process called vectographs that is now being developed further under the name StereoJet. Each StereoJet print or transparency
has the two images from a stereo pair superimposed with opposite polarizations. The two images are printed on opposite surfaces of a single transparent support sheet. A viewer wearing polarizing 3-D glasses views only the assigned image with each eye. Just as in natural binocular stereo vision, the brain processes the information so that the observer perceives the pair as an image in depth. Lamination to a reflective aluminum-coated paper backing converts the image to a reflection print.
There are two approaches to projecting polarized images and movies:
- Using two projectors the images in a stereo pair are projected on the screen superimposed. Each of the projector lenses has a filter over it that circularly polarizes its projected image—clockwise for the right image and counter-clockwise for the left. When not wearing polarizing glasses, the images on the screen look like out of register images, one slightly offset from the other. One advantage of a two-projector setup is its increased brightness. When used to project movies on film this setup was called a dual strip because the left and right image film was fed through two linked and synchronized projectors. This setup is now used mainly to project 3D images in non-theatrical settings.
- Using a projector with a single lens the two images in a stereo pair are displayed alternately. A Z-screen modulator in front of the lens is synchronized with the projector so its filters alternately change the circular polarization of each image as it’s projected—clockwise for the right eye and counterclockwise for the left eye. (Some projectors don’t require external filters because they are internally polarized.) By just removing the polarizing filter from in front of the lens the projector can be used for 2D movies. When used to project movies this setup is called a single strip system because it only requires a
The source video is usually produced at the standard 24 frames per second. However, if the movie is projected at that rate each of the viewer’s eyes sees only 12 frames per second and will experience flicker, ghosting and stuttering when the camera pans horizontally. To reduce or eliminate these problems, the source is divided into 24 frame per second left and right eye image streams and each frame is alternately projected three times, a process called triple flash
. The result is
72 frames per second for each eye or a total of 144 frames per second.
The screen must preserve the polarization as it reflects the image to the viewer. It must also retain image brightness so images aren’t too dark when seen through the glasses. The type of screen normally used is referred to as a silver screen
but is actually a fabric screen with a thin aluminum coating. Screens made from other materials, such as beaded screens, destroy the polarization and hence the 3D effect.
The viewer’s glasses have alternate polarizations so each eye sees only one image in each stereo pair (polarized sunglasses won’t work). Since these glasses are not electronic they are called passive glasses
. Their lack of electronics makes them less expensive and since they don’t use batteries they can’t die at inappropriate times. In theaters, RealD glasses are for onetime use. This saves the theater money but creates an environmental problem unless the glasses are recycled.
Polarization with TVs and Monitors
Although rare until recently, there are computer monitors and TV sets that use polarization for 3D effects. One advantage of such displays is that the viewing glasses are not only inexpensive, but are also interchangeable between displays from different manufacturers. One approach used in these displays is what’s called a patterned retarder
made from glass or increasingly from film. It is bonded to an LCD display and circularly polarizes the light passing through it. The left and right images in each stereo pair are interlaced horizontally before being displayed. Right-eye images are presented on the odd lines and polarized clockwise. Left-eye images are on even lines and polarized counter clockwise. Viewers’ right and left eyes therefore see different images through their polarized glasses, making them 3D. Because two interlaced images are shown on the display in the height normally used for one image, vertical resolution is reduced by 50%, but horizontal resolution is unaffected.
A patterned retarder displays a stereo pair from an interleaved signal with odd rows on the display used for right images and even rows used for left images, each with opposite circular polarization. Viewers wear passive polarized glasses (the small rectangles) to separate the images. The advantages of this approach are the lower cost of the passive glasses, higher brightness and wider color gamut. The disadvantages are higher cost and lower resolution per eye since only half the rows are used for each image and there may be some distortion when viewing 2D content.
The Main Drawback
The main drawback of polarization is the way it cuts the light, giving you a darker image compared to shutter glasses. For example, the RealD projector divides the light between the two images—reducing it by 50%. The glasses reduce it another 50% so you are getting only 25% of the original projector light. The brighter silver screen improves things a bit but still only 35% or so of the light you would see in a 2D movie with a beaded screen reaches each eye.
A vintage movie poster for "Bwana Devil" illustrates the polarizing technology used to show it. This 1952 film was the first in a wave of 3D movies, almost all of which used linear polarization. Some of these movies were later converted to the anaglyph format so they could be broadcast in 3D for TV.