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Stereoscopic Vision

Humans, along with many animals, have the ability to see three-dimensional images through the use of stereoscopic (also referred to as stereographic) vision. Stereoscopic vision involves the use of both eyes. When you focus on an object each eye has a slightly different view of it. Your left eye tends to see a little more of the left side of the object, while your right eye sees a little more of the right side. Your brain automatically uses this information, plus the angle your eyes have to turn to focus on the object, to supply you with an estimate of the distance of the object.

Three-dimensional movies and pictures work by supplying each of your eyes with a different image. This way two "flat" images can look three-dimensional. At the unmuseum we use two different methods to show three-dimensional pictures. The first is red/blue glasses. With this method the images to be shown to the right and left eyes are either encoded in shades of blue or red, then combined. Since the colored lens of the glasses filter out any color but their own, the left eye, which has a red lens, only sees the red shades and the right eye, which has the blue lens, sees the blue shades. The brain combines them to give you the three dimensional images. The disadvantage with this approach is that only a monochrome images can be seen this way.

The second method is to get each of the eyes to focus on different images. With this the images are placed side by side. The viewer, by crossing his eyes, can interpose one on top of the other to generate the 3D picture. The advantage of this method is that the image has full color. The disadvantage is that it takes some practice to get the necessary coordination to do it unless you have a device called a stereoscope.

3D films often use a method similar to red/blue. Instead of separating the images by using color filters, the films depend on "polarization." Special filters are used to polarize the light differently from the left and right images. A polarization filter consists of a transparent sheet on which tiny lines have been etched. They run like blinds usually from the top to the bottom of the filter or left to right. Once light has been polarized it will pass through another filter with the same polarization orientation, but not one with an opposite orientation. The viewers are given glasses with one lens polarized up and down and the other one left to right so that each eye sees only the proper image. The advantage of this method is that it allows full color pictures, but requires both special glasses and projection equipment.

Some advanced computer systems use special glasses that "blink" to produce a 3D image on the monitor. As the viewer observes the screen each lens of the glasses he wears alternate as a shutter to close off light so the viewer can only see out one side at a time. This blinking action is coordinated with the computer which puts up the proper left or right image depending on the setting of the glasses. This way each of the viewers eyes only see the proper picture. The blinking action takes place so fast (usually at least 30 times a second) that the viewer doesn't notice that shutter action, but thinks he is seeing continuous images through both lenses.

In a "virtual reality" (VR) helmet each eye is given a small monitor of it's own to watch and the computer creates the proper left or right images for each. Since the computer can monitor the position of the helmet it can continuously change the images seen by the viewer to match his movements. This can give the viewer the sensation he is moving in a computer generated world, or reality, hence the name.

Not all animals use three-dimensional vision. For example, many fish find it more important to be able to look all around them for predators, than to accurately gage distance. For that reason their eyes' visual areas do not overlap, but point to the left or right. For animals like chimps, though, it is extremely important to be able to figure just how far you need to jump to catch the vine on the next tree. 3D vision is a must for all apes.

Stereoscopic vision isn't our only way to detect the distance of something. The brain can also use the "focal" distance of an object to estimate position. Like a camera, the eye must adjust itself to bring something into sharp detail (hold your finger in front of you about six inches and focus on it. Notice that objects farther out are blurry) The brain uses the amount the eye must change to estimate the distance of an object, too. The focal method isn't as accurate as stereoscopic vision, though.

Finally, the brain can estimate the size and position of objects by knowing what they are or how big they seem in relation to other objects of known size. This is what permits us to see a normal two-dimensional picture as a three-dimensional scene, instead of just a bunch of colors slashed up onto a flat surface. It also allows us to understand that the skyscraper in a picture is hundreds of feet high, even though the photograph itself is only inches high.

Although all three-dimensional pictures have sometimes been referred to as "holographic" only images encoded by a laser are truly holograms. The holographic process (which uses a laser) allows much more of an object to be seen than the above stereoscopic processes. Some holograms allow objects to be seen as three-dimensional even as the viewer shifts his position from left to right as well as up and down. With stereoscopic images the viewers' position is locked in.

Copyright Lee Krystek 1997. All Rights Reserved.