3D theaters have recently proliferated in large quantities. Not far behind in their prevalence and 3d-TVs lately. However, what exactly is behind the marketing “3D” in each case is not always clear and obvious.

It is worth noting that it would be more correct to call it “stereo” cinema, but the term “stereo” has long and firmly (just apparently by right of primacy) entrenched for sound (in this regard, for example, is illustrated by the title of the magazine “Stereo and Video”). Therefore, marketers had to use the term “3D”, which is associated with a three-dimensional image in one sense or another. In this case, it refers to the brain’s perception of volume by giving each of the eyes an image slightly different from the image for the other eye, similar to the way the images received by the eyes in life are different.

So, in order to create a sense of volume, it is necessary to give each of the eyes a different picture.

This can be done in the following ways:

1. Shutter technology.
   Each eye has its own frame, and the frames are interspersed with each other. In order to separate the frames one from another, you need glasses which would skip one frame and show the other, synchronously with the display of these frames. Such glasses always contain some kind of electronic stuffing, require batteries (which means regular replacement) and, most annoyingly, flicker. This technology is pretty old, back in the days of CRT NVidia produced graphics cards that doubled the frame rate and had special glasses attached to the video card, which closed (with LCS – Liquid Crystal Shutter) one of the eyes synchronously with the image. When they replaced the LCD it was not realistic, because the refresh rate of the first LCD was way below the necessary 120 Hz.
2. The second way is to superimpose the image for both eyes simultaneously on one screen and divide it with filters in glasses. In this case the filters on the glasses are passive, do not contain electronics, but divide the luminous flux based on some physical properties of that flux. Dividing can be done in different ways:

(a) by color:
are the long-known blue-red (or some other color with a non-overlapping spectrum) glasses. The easiest and most accessible way. The disadvantages of this method is that colors are lost, besides, after a long time of sitting in such multicolored glasses for some time after taking them off, the eyes see different colors, because they had time to adapt and adjust the “white balance” as they could.
b) on the spectrum:
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This is a somewhat complicated first method: each eye is given all three colors, but in slightly different non-overlapping frequency ranges corresponding to each of the primary colors.
c) by polarization [5]. In this case two sub-variants can be considered:

Linear polarization:
Linearly polarized light is an electromagnetic wave in which the oscillations of the field vector lie in the same plane. In this case, each eyeglass lens is a linear polarizing filter that allows light with polarization in one plane and blocks light with polarization in the plane perpendicular to the first. In the intermediate planes some part of the light passes through, depending on which of the main planes the polarization is closer to.

Accordingly it is possible to picture a picture where for the left eye will be for example vertical polarization and for the right eye horizontal polarization (or vice versa). Then glasses with appropriate polarizations instead of glasses will filter the image for one eye from the image for the other.

There is a nuance here: if the glasses are rotated 90 degrees, the images passed through will swap places. And at 45 degrees there will be no separation at all: both equally darkened images (with double “three-dimensional” objects) will pass through the glasses. Thus glasses with linear polarization are very sensitive to head tilt.

Circle polarization:
Light with this polarization has a vector of field strength running in a circle. The fact that there are only two eyes is very convenient here, as well as the directions in which this vector can run (clockwise and counterclockwise). The filters in the corresponding glasses are circular polarics. No matter how you rotate them, they will filter the light the same way. Of course you will not be able to see 3D while lying down, but you can tilt your head to 30 degrees.