All VR headsets have the same broad function – they are supposed to beam sights and sounds at your sensory organs. While just about all of them use familiar headphone technology to let us hear what’s going on in the virtual world, their methods of beaming pictures to your eyeballs differ in some very important ways. All HMDs contain some sort of display technology, and which method of making pictures the manufacturer decides to go with has several implications for the final image. So every HMD maker has to think long and hard about the compromises they are willing to make when it comes to picture quality.
Obviously, as a consumer you should also be aware of the pros and cons a given display technology represents when you go to buy your HMD. We should judge such products as a whole, but the display technology implemented is a key consideration.
In this article I’m going to talk about the different display systems that you’re likely to see listed under the specs of modern HMDs. There are three main display technologies that on the market that matter: LCD, OLED, and retinal projection. Let’s look at each one now.
Liquid Crystal Displays – The Tried and Tested Option
LCDs have been around for a long time, but it hasn’t been that long since these flat display panels have become the norm. It was only in the early 2000s that big, heavy cathode ray tube TVs and monitors started giving way. The main reason for this is that LCD has been far inferior to other display technologies for most of its life. Laptops from the 90s were one of the first practical applications of consumer LCD screens for computers. Their light weight and low power requirements were worth the many visual tradeoffs. After all, a glass CRT is simply not practical for a portable computer, as early “luggable” PCs clearly demonstrated.
Having an LCD in the now-mainstream clamshell form factor meant that people could work anywhere. Unfortunately, they might also have needed glasses or migraine tablets. These monochrome LCDs were fuzzy, had extreme ghosting, and in general didn’t look all that great. Today things are very different. Modern HD and UHD LCD panels are thinner, lighter, and crisper than ever. The picture quality of even entry-level LCD panels is phenomenal, and even lower-end smartphones will have a 720p or 1080p screen built into them.
How LCD Technology Works
As the name implies, LCDs use a substance known as liquid crystals. Liquid crystals are not quite solid and not quite liquid – a discovery that challenges the idea that there can be only three states of matter: solid, liquid, and gas. Liquid crystals can take on a number of different “sub-states” between liquid and solid phases. These different subphases also happen to change the light that passes through the liquid crystals in predictable ways. This means you have the rudiments of a display method on your hands.
If you look closely at an LCD display you’ll notice that it’s actually made up of tiny individual dots known as “pixels”, or picture elements. On a CRT screen the pixels are virtual, since they are rendered by a scanning beam. On an LCD the display panel has a fixed number of pixels, which is known as its “native” resolution.
Thanks to the nature of liquid crystals, they change state when you apply electricity to them. Each pixel is made up of several substructures controlled by tiny electronic components. By cleverly manipulating these crystalline subpixels, the LCD panel can create images. Each individual pixel can take on one of millions of colors and different levels of brightness.
That’s a very simplified take on LCD technology, but it’s the basic principle of how it all works. That being said, different LCD technologies have developed over the years; each with their own approach to manipulating liquid crystals. All of them have aimed to improve the image quality in various ways. Some are better at some aspects of image quality than others, while perhaps trading off with image quality factors that are less important for specific use cases.
Of Back Lights and LED LCDs
One important thing your should know about LCD panels right from the start is that they can’t make their own light. The LCD pixels can only manipulate the light that passes through them, which means we have to provide that light.
From the early days of LCD technology until quite recently they’ve used cold-cathode fluorescent lights to provide the illumination. This worked OK, but caused uneven brightness across the surface of the LCD panel. Thanks to advancements in light emitting diode (LED) technology, we no longer use cold cathodes for light. Instead, most new LCD panels have incredibly pure and bright LEDs dotted around the edge of the screen. Modern LCD panels are therefore much more evenly lit, don’t have “yellowing” light with age, and just look a lot better!
There is no such as thing as an “LED screen”. Whenever you read that, it means it’s an LCD with an LED backlight. I mention this because we’re about to delve into the types of LCD panels and it’s important that you understand LED panels can be any type of LCD, it just refers to a specific type of backlight.
Incidentally, the fact that an LCD needs a backlight is one of the main reasons these screens can’t really display true black. Even if a given pixel is switched off completely, some light still makes it through. Leaving the black color closer to a dark gray.
LCD Panel Types
There are two main types of LCD panels that are really of interest these days. TN, or “twisted nematic”, panels are an affordable and popular choice for PC monitors. Then there are IPS or “in-plane switching” panels, which tend to be pricier and are favored by high-end users and creative professionals.
TN Panels for VR
TN panels are the most common type of LCD you’ll find in the wild. These LCD panels are known for their quick response times. In other words, the pixels in these panels can change state very quickly. High-speed gaming monitors that need to push high refresh rates tend to be TN panels. These panels are also the most power efficient, which makes them a good choice for mobile devices that have to run off a battery. Their overall brightness also tends to be superior to other LCD types.
On the downside, TN panels tend to have relatively washed-out colors, and the viewing angles are poor compared to other standards. “Viewing angle” refers to how far you can view a screen from its center before the picture gets all funky.
TN panels are popular for VR use because of their speed. A TN panel with a 1 millisecond response time, capable of showing at least 90 frames a second, is a great choice for VR. Viewing angles are a non-issue when it comes to an HMD, because the relative angle between your eye and the screen is fixed.
Color accuracy is a downer, but that only becomes an issue when VR moves into the realm of serious creative work that requires it. TN panels have also helped keep the cost of VR headsets down.
IPS Panels for VR
IPS panels are generally better than TN panels from a picture-quality perspective. The color reproduction is better, although with high-end TN panels this is no longer that apparent. Viewing angles are also much better. These days IPS panels are also better at showing black than they used to be.
The downside of IPS is that it has significantly longer response times and lower refresh rates in general. While it’s not uncommon to get TN screens that can run at 90Hz, 120Hz, and even 144hz, you’ll be hard-pressed to find IPS screens over 75Hz that won’t ruin your wallet. As you can imagine, that doesn’t make IPS panels very popular for VR HMDs. That doesn’t mean they are never used. For example, the VR Union Claire uses IPS panels, since the goal of that HMD was for high-end cinematic VR experiences. Predictably, however, it’s an expensive piece of kit, and for HMDs aimed at gaming or general VR, IPS isn’t yet worth it in terms of price.
Organic LEDs – A Colorful Alternative
Organic LED panels are fairly new compared to LCD technology; on the surface they can look very similar to each other. Both technologies are digital flat panels that have a fixed native resolution. Both come in panel sizes that work for smartphones, tablets, and even televisions.
That similarity really is only skin-deep, since OLEDs use a completely different principle of operation. OLED technology uses an organic material that emits light when it gets an electrical current passed through it. There we already have a major departure from LCD technology. Remember that LCDs don’t generate their own light; they need a backlight for that. OLEDs don’t have that problem since they make their own light. It also means these screens can be incredibly thin, since they don’t need that additional layer of electronics dedicated to lighting.
On top of this, OLED panels can display a true black, because individual pixels can be switched off. Since they are their own source of light, that means it’s dark for real.
Just as with LCD technology, there are subtypes of OLED. There are two main types of OLED you need to know about – PMOLED and AMOLED.
PMOLED – Passive OLED
PMOLEDs have a simplified display control design. The rows of the display are controlled in sequence. Basically, this means these OLEDs are the cheapest to make and can be found in many small devices such as watches or other products that have three-inch or smaller screens. The downside is that the lifespan of these screens isn’t that great and they aren’t very power efficient. Not a problem on a tiny smartwatch, but phones, tablets, and TVs are out of the question. As far as I know, there are no VR headsets that use PMOLED. But if you ever see one, avoid it.
AMOLED – Active Matrix OLED
AMOLED technology is what you’ll find in things like phones and (incredibly expensive) TVs. The physical structure of an AMOLED places a matrix of OLEDs onto a TFT or “thin-film transistor” layer. Usually, each OLED has two transistors that stop and start a storage capacitor.
AMOLED displays have amazing black levels and very fast response times, and are generally perfect for VR. That is why headlining HMDs like the Vive and Oculus use this technology. Colors tend to be oversaturated and AMOLEDs cost a little more, but as a total package they are perfect for consumer VR.
What if we didn’t need to stick a screen in front of our eyes to see an image? That’s the idea behind retinal projection technology. The photons are beamed onto your retina, which acts as the projection screen.
There’s already one head-mounted display on the market that makes use of retinal projection. It’s called the Avegant Glyph, but it’s not a VR headset. Yes, it has head tracking, but the Glyph is not meant to fool your senses into thinking you’re somewhere else. Instead, you see a big monitor floating some feet ahead of you. Retinal projection is, however, a technology with a lot of potential for VR. It can ultimately do away with many of the issues that plague current HMD display technology.
Let Me Bounce Something off You
One of the main draws of retinal projection is that it provides your eyes with imagery in a much more natural way. In nature, photons reach our eyes after bouncing off something else. That is, in fact, how we are able to see. Everything is a projection of reflected light. When we look at a screen however, we are staring directly at a bright light source. The claim is that this increases eye strain and is generally not good for the old peepers.
In the case of the Avegant Glyph video headset, LEDs project into millions of micromirrors, which then render the image directly on your retina. There is no screen to speak of. According to the company there are no “pixels” to see and the image looks like an object in nature. After the low-power LED projects onto the micromirror array, the photons are focused through a set of adjustable optics. This also means that you don’t have to wear glasses, as each eye’s image can be adjusted to compensate for abnormalities in each the eye of the user.
Retinal projection holds a lot of promise and is superior to other display methods when it comes color, latency, and image detail. However, as a consumer technology it is still pretty expensive, and it still needs various improvements for true mainstream viability. Products like the Glyph are very much first-generation consumer hardware, but the proof of concept is undeniable.
Light Fields – The Future of Displays
When you take a photo with a camera, the image sensor captures the position of the photons as they strike it after being focused through the lens. That’s why we get a single, 2D image. One angle, one picture, that’s it. Light is, however, not two-dimensional. In a given volume of space where photons are present, they are all over the place – bouncing off stuff, bouncing off other stuff, and generally getting into everything from every angle.
A light field is a description of the light flowing in every direction through every coordinate in that volume of space. It’s a way of looking at light in the same way that we look at magnetic fields – as a holistic end product with complex properties.
Light fields are very important to the future of VR. There are already cameras that capture light fields, which means true 3D recordings can be produced for viewing on a VR system. Perhaps one day we’ll have something beyond retinal projection that recreates the light field so that our eyes see the imagery as indistinguishable from real life. Light fields are already getting the mixed-reality people excited, since it would allow for truly seamless blending of the real and virtual, but it also means a revolution in completely virtual image generation.
Getting the Picture
The evolution of display technologies is a fascinating roller coaster. Just 100 years ago we figured out how to painstakingly create analog film and project it. Now it seems like every day there’s a new way to generate the imagery we love so much as a visual species.
VR is carving out its own legacy in the field of display technology and (for now) is about as intimate as the relationship between eye and screen can get. What will tomorrow bring? No one can say for sure, but it will probably be bright, crisp, and very realistic.