When you put on a virtual reality (VR) headset, you are connecting virtually to another place whether that be real or imagined. Using AR you are able to overlay and view things on top of your real life. As these technologies become more and more engaging, people will start to connect VR and AR with not only gaming, but also other increasingly growing industries like healthcare, military, retail and the government which are finding new applications for the evolving technology already.
Today, the smartphone is a big deal when it comes to using a VR headset. But as this technology grows, there are still several more technical display requirements that must be met in order to fully achieve the affect and improve the user experience. It is important to understand how these work, and what Imaging Technologies already exist so that we can better understand how far we really can go with the technology.
Head-mounted devices for virtual and augmented reality come in different shapes and sizes from the minimal Google Glass to other fully immersive head mounted devices. If we really dig deep enough, we find that head-mounted displays consist of two primary structural elements: optics and image displays.
However, before going deeper into the fundamentals of Optics, we must properly understand how the human eye works, and the properties our eyes have.
Basics of the Human Eye
Field of View (FOV)
The Field of View is basically the total angular size of the image that would be visible to both the eyes. Binocular vision is a type of vision which basically explains how humans with two eyes are able to perceive three-dimensional images of its surroundings. Generally, the horizontal binocular FOV is 200 degrees in total and 120 degrees of that is a binocular overlap. The vertical FOV is around 130 degrees.
Inter-pupillary distance (IPD)
As the name suggests, it is the distance between the pupils of the eyes and is an extremely important consideration for binocular viewing systems. However, this can be a bit difficult to identify because this distance can vary depending on the gender and ethnicity of a person. Average IPD for adults is around 63 mm and the minimum IPD for children is around 40 mm.
It is very important to be accurate with the IPD or else there can be poor eye-lens alignment, image distortion, strain on the eyes or can cause a headache.
This is the distance from the cornea of the eye to the surface of the first optical element. It defines the distance at which the user can obtain full viewing angles. This is an important consideration especially for people who wear corrective lenses or glasses. Eye relief for spectacles is approximately 12 mm. Of course we want the customer to feel comfortable which is why enabling users to adjust the eye relief is extremely important for head mounted displays.
Augmented Reality (AR) displays
The idea of display for Augmented Reality (AR) is actually still fairly young and has a long way to go. Regardless, Augmented Reality displays can be broken down into two main categories: Optical see through and Video see through.
Optical see through
With Optical see through glasses, the user is able to view things directly through various optical elements that enable graphical overlay on the real world. Microsoft’s Hololens, Magic Leap One and the Google Glass are recent examples of optical see through smart glasses. Optical see through glasses can be further broken down into either Binocular or Monocular.
In a Bi-nocular view each eye gets a different view which creates a stereoscopic view. Binocular display types provide a lot of depth cues and a sense of immersion. On the other hand, they can be very heavy and also very complex.
Some examples of this kind of display includes: Microsoft’s Hololens, and Epson’s Moverio.
You probably guessed it, yes, this display provides a single channel for viewing. Generally, this channel is in front of one eye so that the user is able to view the real world completely through another eye. Monocular displays are often are good for information displays, mobile computing or video viewing with connectivity to mobile devices. However, these type of displays provide less stereo depth cues than binocular cues.
Google Glass and Vuzix Smart Glasses are both good examples of monocular display.
Video see through
Video see through basically means that the user views reality that is before captured by one or two cameras that are mounted on the display. These camera views are then combined with computer generated imagery for the user to see.
By now, most Virtual Reality headsets are fully immersive, which means that they are generating a three-dimensional image that appears to surround the user. These displays are usually combined with sensors which make the experience more fun. Many of the sensors on the headset are essentially tracking position and orientation.
Cave automatic virtual environment
A cave automatic virtual environment (CAVE) is an immersive virtual reality environment where projectors are directed to between three and six of the walls of a room-sized cube. These systems usually come in a variety of shapes and sizes from multi-walled displays to multi-projector hemispherical displays.
Hemispheres and Domes
Hemispheres and Domes are fairly common but mainly are popular defence and aviation. They can often provide considerable precision in graphics along with room for multiple users, and a huge advantage is that the user is able to freely move around anywhere without much restriction.
Optical Design System
There are two primary optical design systems that are widely used in AR and VR displays: pupil forming and non-pupil forming.
Non-pupil forming is made up of a single lens. This type of architecture uses a single magnifier to directly align the rays of light from the display panel.
The non-pupil using an effect known as pincushion distortion. In pupil forming architectures, another lens that produces a barrel distortion is used to nullify the effect. These are usually commonly seen in more non-immersive type displays.
Imaging and display technologies continue to and have already greatly improved in the past few decades. A few major imaging technologies include:
- Liquid Crystal Displays (LCD): A flat-panel display that uses the light-modulating properties of liquid crystals which do not emit light directly, instead using a back light or reflector to produce images in colour. This can be commonly seen in HD TV's.
- Organic Light Emitting Diode (OLED): A light-emitting diode (LED) which works without a back light because it emits visible light.
- Liquid Crystal on Silicon (LCoS) Microdisplay: This is a smaller reflective “micro display” which uses a liquid crystal layer on top of a silicon back plane. This is how LCD projectors use transmissible, allowing light to pass through the liquid crystal.
These displays and imaging technologies are growing rapidly and are making the VR/AR experience even more functional, realistic, and enjoyable. The possibilities are endless and who knows in a matter of years flat panel based HMDs might just become a thing of the past for many AR devices.
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