3D glasses add a new dimension to the movie-watching experience (literally!) (Source: slashfilm.com)

How Do 3D Glasses Work?

Learn how a simple pair of glasses manipulate our brain to see images in 3D

3D glasses have been around for a long time now. Watching a movie in 3D in a cinema hall is truly a much better experience than watching it in 2D. The simplicity of wearing a pair of glasses that allows us to see a movie projected on a flat surface, in 3D is amazing. The science behind these glasses is simple, yet ingenious. To understand how 3D glasses work, we need to first understand how our eyes see the world in 3D. But technically, they don’t!

Both our eyes see the same but slightly displaced images that our brain uses to interpret a 3D image (Source: Quora)

Both of our eyes see the world as a 2D image. The images formed by both the eyes are slightly off. You can observe this by lacing your thumb in front of your eyes and looking at it with only one eye. As you switch the eyes, you can see the position of your thumb shifting. These two images are interpreted by the brain, and it senses the depth of objects. So, basically, our brain takes two 2D images from our eyes and interprets them as 3D.

Red and blue lens 3D glasses (Photo by Luke Jones on Unsplash)

3D glasses try to replicate this. In movies, when the light is projected on a flat screen, both our eyes get the same image of the movie. The earliest 3D glasses used the colours red and blue to give different images to our eyes. The moviemakers would specifically create two different images in red and blue.

A 3D image meant to be seen by 3D glasses (Source: Locafox.de)

Image as seen by the different eyes (Source: Locafox.de)

Each of them meant for one particular eye so that the brain could interpret them as 3D. The glasses would let one colour pass through one eye, and the other through the other eye. This made sure that both our eyes saw the two different images that our brain could interpret as 3D.

Two images of the horse are overlapped in blue and red t give a 3D effect with glasses (Source: Pinterest)

But this had a major drawback. The colours of the movie were no longer relevant. Since only blue and red would pass through the filters, the movie could not be seen with the original colours. To counter this drawback, 3D glasses started using polarization.

What is the polarization of light?

Light is an electromagnetic wave (Source: BestAnimations.com)

Light is a wave. This wave has a particular direction of oscillation. Horizontal, vertical, or at any angle. This direction is called the direction of polarization. Normal light has light waves in all directions of polarization. But when this light is passed through a linear polarizer, light with only one direction of polarization passes through, all others get blocked.

A linear polarizer allowing only the vertically polarized light through (Source: Physics Stack Exchange)

Using the concept of linear polarization, movie makers made the same two images for 3D glasses, but this time instead of using colours, the made one with light that has vertical polarization and one with light that has horizontal polarization. Similarly, the 3D glasses had one lens that would have a vertical linear polarizer and the other would have a horizontal linear polarizer. This ensured that both the eyes had a different image for the brain and that the original colour of the image was maintained.

The working of two polarizers in the 3D glasses (Source: World of Science)

But this concept also had a major drawback. If the viewer would tilt his/her head the direction of polarization of the glasses would also tilt. The angle would no longer match with the angle of the polarization of the light. This would result in very dim or no vision for the viewer since the polarizer would not allow the light to pass.

In the first case, light passes through. In the second case, the axis of the polarizer is tilted, hence no light passes through (Source: Edmund Optics)

To counter this problem, the glasses needed to have circular polarizers instead of linear.

Different types of polarization of light (Souce: Gyfcat)

A light wave can have only two types of circular polarization — clockwise and counterclockwise. Just like linear polarizers, circular polarizers allow only one of these to pass.

Working of circular polarizer 3D glasses (Source: ThorLabs, Inc)

So, the movie makers make the same two images for the 3D glasses, but this time using light with clockwise and counterclockwise polarizations. The 3D glasses have one clockwise polarizer and one counterclockwise polariser, ensuring two different images for each eye. This would allow for images with proper original colour and also made sure that the viewer could view the same image even with a tilted head, as a clockwise polarized wave is still clockwise even if the viewer tilts his/her head.

This technology is currently used in 3D glasses and though they seem like normal glasses, they are much more than that.

On a side note, the concept of interpreting two 2D images for depth sensing is used in portrait modes of cameras in mobile phones. They use two different cameras and then use calculate the depth of each point in the image and give it an appropriate lens blur to make the image look more real.