Author: Miss C Wise

How do 3D Displays work?

Hollywood is now producing as many blockbusters as possible in 3D; phones, televisions and games consoles are all trying to make displays and pictures that look more and more real as we tend towards the ultimate goal of virtual reality. But how do 3D displays work? In this article we’re going to look at the four main ways of seeing 3D images: using passive filters and polarisers, active shutters, head mounted displays and, the most impressive, autostereoscopic displays, ones which you don’t need special glasses to see.
How do we see in 3D?
But first, let’s have a look at how our eyes work and why we see in 3D at all. Look into the distance and close one eye. Now switch over to the other eye. You should see the image shift a little. We see slightly different pictures in each eye and our brain uses the differences between these two pictures to make up a 3D image. This also helps us with depth perception and working out how far away things are. This is called stereoscopic vision.
In order to create a 3D effect for the viewer of a film or a game, we need to send different pictures to each eye, offset by the correct amount. There are two main ways of doing this: filtering pictures so that each eye receives only one of two images or using a headset that provides a different image for each eye.
Anaglyph Systems: Red and Green 3D Glasses
The oldest form of 3D image is produced by colouring parts of the image green and the other red. It can also be produced using cyan and fuchsia or red and blue. These pictures are offset from each other. When viewed through glasses with a green filter on one eye and a red on the other, the sort given away with comic books or old horror films, each eye receives a different image and so we see a 3D effect.
Polarising 3D Glasses
Most cinemas now use polarised light to produce 3D films. You’ll find that your local cinema will only show 3D films in particular screens. This is because they need a special projector that produces light of the correct polarisations.
Box 1 explains how polarisation works. However, if you hold up two pairs of 3D glasses and rotate the lenses, like the picture shows with the polariser films, you’ll notice that the lenses don’t go completely black: they’re not blocking all the light. This is because, instead of using horizontally and vertically polarised light, the cinemas use circularly polarised light, see figure 8. One lens filters for right-handed circularly polarised light and the other for left-handed. This works better than horizontal and vertical as, because it doesn’t block all the light, the projector doesn’t need to be as bright. Why not try testing this next time you’re at the cinema?
Active 3D glasses
These are probably the chunkiest of all the 3D eyewear as these glasses need to contain a power source. The screen or display will alternate images for each eye and the glasses have a fast shutter set up to switch the eyes on and off alternately. If this is done fast enough, the viewer doesn’t notice the shutters and appears to be receiving different images for each eye at the same time.
Google Cardboard and the Oculous Rift
The Oculous Rift is a virtual reality headset that was launched on Kickstarter recently and Google Cardboard is its free equivalent, although has fewer gaming features. Both devices produce 3D landscapes by splitting the screen in two and using lenses to send different images to each eye.
Parallax and the Nintendo 3DS
Autostereoscopic displays are 3D displays which don’t require the user to wear glasses to filter out the images. These are probably the most technologically impressive displays.
While Sharp brought out the first 3D television a few years ago, one of the first mainstream devices to use 3D images was the Nintendo 3DS, a portable games console. It uses a liquid crystal layer to produce a parallax. You probably recognise this as the error in your experimental data caused by not reading an instrument at eye level. This means that light travelling through a liquid crystal has a very high angle dependence. You might remember that your parents’ old laptops used to have a very small viewing angle and the screen appeared black if you looked at them from the side. Normally, scientists work very hard to minimise this parallax when working with liquid crystal displays as a television that can only be seen from straight on isn’t very useful in a sitting room but Nintendo used it to its advantage.
Between the pixels, they placed a liquid crystal barrier. When switched on these crystals blocked the light travelling at an angle out of the pixel because of their parallax. This way they could send light from alternating pixels to each eye. This means that each eye can, again receive its own image and the 3D effect can be achieved. This is the principle by which most glasses-less 3D displays work. The clever thing is that it can be turned off for programs or games which don’t have a 3D component: the liquid crystal layer can be switched off and realigned so that there is no parallax and all the light reaches both eyes.
When the Nintendo DS first came out there were reports of users feeling a little sick as they looked at the displays but technology is steadily improving. The future of 3D displays certainly lies in this autostereoscopic technique as audiences really don’t want to have to don headsets and glasses. The difficultly now lies in finding a way for the screen to be viewed from many angles so that more than one person can use it at a time. I’m sure this exciting new technology will be available one day but for now it’s a challenge for you future optic- and electronic engineers.

Figure 1: Stereoscopic vision

Figure 2: A red-blue anaglyph image of the brain

Figure 3: 3D cinema glasses

Figure 4: Inside Google Cardbard

Figure 7: Rotating a polariser

Box 1: Polarisation
Light is an electromagnet wave whose electric (and magnetic) field is perpendicular to its direction of travel. However, there are many angles that the field would be at while still being perpendicular. Polarised light is light whose electric field is all in one direction: for example, horizontally polarised light has all of its oscillations horizontally and vertically polarised light has all of its oscillations vertically.
We can polarise light using polarising filters: they only allow one polarisation of light through the gaps. For example, if we send normal unpolarised light through a horizontal light polariser, we only get horizontally polarised light out. If we then send this light through a vertical polarisation filter, no light will come out. You can see this in action by rotating two polarisers.
Circularly polarised light, like that used for 3D cinemas is made using a special material called a birefringent material. This is something that has a different refractive index horizontally than vertically. This means that the horizontal and vertical polarisations of light travel through the materials at different speeds. If we can put them out of phase by π/4 or 90⁰, we get circularly polarised light.

Figure 6: horizontally and Vertically polarized liht

Figure 8 Circularly Polarised Light

Figure 5: the parallax barrier in a Nintendo DS

Box 2: Tester Question
Horizontal and vertical polarisers are made using grids of lines on a material, see figure 9. For example, scratching horizontal lines onto a piece of glass for sunglasses. Polarisers work best if the gaps between the lines are about the size of the wavelength of the wave travelling through. What part of the electromagnetic spectrum would be polarised by the glasses in figure 10?

Figure 9: Light travelling through a polariser

Figure 10: What kind of light will these funky glasses polarize?