How do 3-D Glasses work?

How do 3-D Glasses work?

3D movies are really fun to watch as we get that amazing experience of something coming out of the screen! One kinda annoying thing about 3D movies – uncomfortable pair of glasses. If someone already has spectacles they have to wear 3D glasses above their spectacles which is even more uncomfortable – but worth it! So how do these glasses actually make things “look like” they are protruding out of the screen?

Lets go through few concepts first:

  • Let’s start by understanding what polarization of light is. The electromagnetic (EM) waves that compose electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields.
    Electromagnetic Waves.

    In short polarisation means the EM waves are oscillating in only one direction, typically we talk about only the electric field.So polarized light means the electric field oscillates along one axis called polarization axis. Any other light gets partially or completely blocked. Sunlight is a source of unpolarized  light which means its oscillating along different directions. When this unpolarized light is passed through a polarizer it allows only those waves which are oscillating along the polarisation axis.

Sunlight, bulbs are sources of unpolarized light.
  • Which brings us to the next question, what are polarizers? Polarizer is an optical filter that allows light waves of certain polarization to pass through it. The material used in these filters has a molecular structure that can oscillate in only one direction (i.e the polarization axis). So it allows only that light which oscillates along the axis.
  • Now these polarizers are called linear polarizers because the light can pass only through one axis, so if you tipped your head when you watch through a polarizer the light gets blocked which is not a good thing for us.
Polarized sunglasses polarize light linearly.
  • If light can be linearly polarized it can also be circularly polarized. If its possible for light to have linear momentum then its also possible for light to have angular momentum. In circularly polarized light the electric field goes around in circles.
Circularly Polarized light.

Which brings us to the next question, how the heck do you make electric field go in circles? We use something called as birefringent crystals, refringent means refraction and birefringent means the crystal refracts light in two different ways which creates two different images. This basically happens due the molecular structure of the crystal. Suppose we send linearly polarised light through the crystal, at the beginning the components of the wave are in phase with each other, but by the time they exit the crystal they are out of phase due to the crystal being birefringent. This makes the electric field go in circles with respect to the axis. How this exactly happens is kind of very complicated. Now if you make the material of just right thickness that it makes the light wave that comes out 1/4th of the wave out of phase then that piece of crystal is called quarter wave plate. Combining the components we find that its a circularly polarized wave. So circular polarizer is nothing but a combination of quarter wave plate and a linear polarizer.

Circularly polarized light enters quater wave plate and is linearly polarized and this light can be again circularly polarized using quarter wave plate.

We have to just make sure that the incoming light is at 45 degrees to the crystal.

Now finally lets get back to the main question. How do 3D glasses work? In reality we get the perception of depth due to the spacing between our eyes (which is about 2 inches). We actually look at things from two different perspectives and our brain combines these two images to give us the sense of depth (which mean 3D perception). Fun fact: In humans, each eye has a viewing angle (field of view) of about 150° but the binocular vision (i.e the image that can be seen by both eyes) is 114° which actually covers our nose. It means we see our nose continuously but our brain just chooses to ignore it for “convenience”. Back to 3D, so for the sense of depth we just need to look at images from two different perspectives and our brains will do the rest. This is where polarization comes in as we can project two different images (perspectives) of different polarization  on a 2D screen but by wearing glasses we can allow our eyes to see image of only one perspective and we get that feel of 3D image.

There are different methods of projection and types of 3D glasses. Here’s a list of some 3D glasses and their method of projection:

  • Anaglyph Glasses: These are the cheapest glasses that you can get. You might have seen these glasses as free gifts on some products. Anaglyph glasses have different colour filters for each eye (typically red and blue). The projected image has different colour elements and the filters allow both eyes to see different images. The quality of 3D is very poor as it is not able to resolve colours properly and its said to be very uncomfortable.
    Card-paper Anaglyph Glasses.

    These glasses are useful when there’s a large short term audience like in events or meetings.

  • LCD Active Shutter Glasses: LCD Active shutter means as the name suggests the glasses that we wear use LCDs (i.e Liquid Crystals) to make the glasses opaque or transparent at very high speeds like a camera shutter. It is commonly used in home theaters and 3D televisions. The television displays images with different perspectives at a frame rate of 120Hz which means it displays 120 images every second. The LCD active shutter is synchronized with the television which makes the LCDs in glasses become opaque at the same frame rate alternatively. So each eye gets effective frame rate of 60Hz (which is pretty good as we can see smooth video even at 27 frames per second).Due to the high frame rate, the brain however has the impression that it perceives both images at the same time and not in sequential order.
    LCD Active Shutter Glasses.

    Only disadvantage is that these glasses can be relatively expensive but nevertheless they provide very good quality 3D effect.

  • Glasses with Polarizing Filters: In these there are two types
    • Linearly Polarized Glasses: These glasses are used when two images are projected superimposed onto the same screen through orthogonal polarizing filters (Usually at 45 and 135 degrees). The viewer wearing linearly polarized glasses can see only one image in each eye (the one which has same polarizing angle). These glasses require the viewer to stay in the same orientation i.e if the viewer tips his head the image becomes darker as mentioned above in theory.
    • Circularly Polarized Glasses: These glasses are used when two images are projected superimposed onto the same screen through circular polarizing filters of opposite handedness.
      Right- Circular Polarized and Left Circular Polarized.
      Image credits: The science asylum.

      The viewer wears eyeglasses which contain a pair of Analyzing filters (circular polarizers mounted in reverse) of opposite handedness. Light that is left-circularly polarized is blocked by the right-handed analyzer, while right-circularly polarized light is extinguished by the left-handed analyzer.

      Illustration of Circularly polarized waves through glasses.
      Image credits: Science Asylum

      Thus the two different perspectives are projected on the same screen with different direction of circular polarization. Now in this case even if the viewer tilts his head he won’t lose the image and it becomes more comfortable. The quality and resolution of these glasses is excellent and these the typical glasses you use in movie theaters (and sometimes try to sneak out of theater with the glasses).

  • Virtual Reality Glasses: Everyone is familiar with the virtual reality tech that’s been blowing away people’s mind these days. These kinda give you the best 3D experience you can get. Both the eyes get different perspectives and when you tilt the head the images are displayed as if you are present in the scenario! These are used for realistic gaming, simulation, education and is kinda litt!
    Oculus VR.

    With virtual reality there’s also one other thing that is emerging – Augmented reality! This augments the reality in your virtual experience which means you can interact with real things and look at virtual animated things augmented into that vision! Microsoft Hololens is an example of Augmented reality and that tech is on whole another level! As technology gets better and quality of these virtual realities improve I don’t think we are far away from the point where we start doubting the reality! Kind of exciting and scary but who knows what new technology might appear and blow our minds!

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