Stereo video, or stereoscopic video, is the practice of producing the illusion of a 3D image in moving form. There are a variety of methods used to achieve this effect, usually classified by whether the viewer needs to wear glasses, and, in turn, whether these glasses play an active role in the effect. The phrase stereo video could also refer to VHS equipment that records and plays two audio channels, rather than one.
The basic concept of stereo video is the same as 3D imaging. The viewer is shown a picture that combines two images, one viewed by each eye. These images show the same scene, but from slightly differing perspectives. The brain's attempts to reconcile this difference creates an illusion of depth. The concept of stereo video is to simply recreate this effect for every frame of the movie.
There are three main types of stereo video, the first two of which involve the viewer wearing glasses. The most well known in the 20th century was passive glasses, which mean the glasses are entirely static. One example of this is polarized glasses, which use one red lens and one green lens. The two images are shown on the screen simultaneously, with the lens for each eye filtering out the "wrong" image.
In the 21st century, active glasses have become more popular. One example is liquid crystal shutter glasses. These contain a liquid crystal layer in each lens that can quickly switch between being transparent and blocking light. The lenses switch back and forth in synchronization with the video, which displays images from alternating perspectives.
The final type of stereo video is autostereoscopy, or glasses-free 3D. Most variants involve a screen that has tiny ridges rather than being totally flat. This means that each eye can only see part of the screen, meaning different images can be shown simultaneously to the two eyes. The main drawback is that it has proven difficult to make this effect work well on large screens, or to cope with multiple viewers sitting in different locations.
In the 1980s and 1990s, the term stereo video was most closely associated with video cassette recorder, particularly the VHS format. In this context, stereo referred to audio rather than the video. During the heyday of VHS, stereo equipment became more popular than mono-only video recorders. In later years, movies were released in Dolby Pro Logic, which used the two audio channels to encode a multi-channel surround sound effect that could be played back on compatible sound systems.