Display glasses: what you actually get

We have worked with stereoscopic 3D for many years, and almost every day we receive the same question: Are these compact glasses with a huge virtual screen worth buying, and which model should you choose?

An ordinary person does not understand the underlying principles of optics and cannot properly evaluate the image quality of such devices. This is exploited by all kinds of scammers to sell low-quality products. We will explain how anyone can easily evaluate the image quality of these glasses. Rather than answer the same question over and over again, we decided to write this review.

Below, we will take a calm and objective look at the technology without hype or emotions. Only optics, physics, biology, and mathematics, based on what science actually knows. You can verify all of the conclusions yourself using a calculator. People who have already bought such devices say they feel deceived. Read about what you are actually getting before you buy, not afterward, when your money is already gone.

Manufacturers and marketing bloggers will tell you a very sweet honey storys. Their goal is to sell the device to you at the highest possible price, or to be paid for a review or a video. What happens after that is not their concern, it becomes your problem.

Formally, all the numbers in their advertising are true. But what they advertise are technical spec, not what the user will actually see in the real life. But what you really want is a high-quality image, not a bunch of numbers in an advertisement, right? The screen may appear huge. But 'large' and 'sharp' are two completely different things, and the second characteristic is conveniently left out of the advertising. Otherwise, they probably wouldn`t sell very well...

How it is built

Inside any such glasses there is a tiny micro-OLED panel, typically between 0.5 and 1.0 inch, and a lens that projects and magnifies a virtual image. Your eye does not see the panel itself, it sees a magnified optical projection of it.

There are two fundamentally different classes of glasses on the market, and they are constantly confused.

Birdbath, optical AR glasses.
The layout is a lens, a flat polarising beam splitter and a curved semi-transparent mirror. You see the real world through glass. This is how XREAL, VITURE, Rokid Max and their clones are built.

Pancake, essentially a VR headset in a spectacle frame.
The optical path is folded by polarisation between two films. Such a module cannot be transparent, because an opaque panel sits behind the optics. You see the real world through cameras, on a screen. This is how Meta Quest works in passthrough mode, and also the recent URXR One.

This distinction matters. If a product description uses the words "spatial", "AR" and "see-through", find out whether it means looking through glass or watching a camera feed. These are two completely different devices.

The key parameter they will not show you

What PPD is
Forget inches, forget panel resolution, forget the word "4K". Glasses have exactly one measure of sharpness, and it is called PPD, pixels per degree, the number of pixels that fall within one degree of your field of view.

Why this one? Because sharpness is determined neither by the pixel count alone nor by the image size alone, but by the ratio between them. The same 1920x1080 frame can be shown on a screen the size of your palm or on the wall of a house. The amount of detail does not change, only the size of each pixel in your field of view does.

The formula is simple:
PPD = horizontal pixels / horizontal field of view in degrees
A person with normal vision resolves detail of roughly one arc minute. That corresponds to about 60 PPD. Below this threshold you begin to see that the image is made of separate dots, of pixels.

Now let us calculate real devices.
That is the whole secret. Even the newest and most expensive display glasses deliver two to three times less PPD than an ordinary 150 dollar monitor. Every model of mini-display glasses has a PPD value at least two times below the threshold at which the human eye begins to perceive individual pixel structure. As a result, the image quality you see through the glasses can be several times worse than what you would get from a regular $50 smartphone.

Playing games with the numbers.
The manufacturer of the glasses in the last row of the table quotes about 36 PPD. That is not a lie, it is the vertical figure, 2064 divided by 56 degrees. But that is not what you need. You need something else entirely, the HORIZONTAL figure, the axis along which you read text or work with fine graphics. And there it comes out at about 31 PPD, a number they prefer not to print. Always check which axis a quoted figure refers to, and do the arithmetic yourself. Both numbers, resolution and field of view, must appear in the specification. If they are missing, someone is trying to mislead you.

Let's compare the image quality of the main models
The table is sorted by PPD, from best to worst. The main thing it shows: the wider the FOV, the worse the picture. XREAL One Pro and VITURE Beast are the most expensive models, advertised as the most "advanced" in their lineups. They have the widest field of view, and they are exactly the ones at the bottom of the quality ranking. All the expensive models are bad.

Same panel, wider angle, bigger pixels. Advertising sells a wide FOV as an advantage, but in terms of quality it is a straight downside! Almost all seven models use a panel with 1920 pixels horizontally. The very same one. The difference between a $400 and a $900 model is only the size of the field of view — that is, how far those same pixels are stretched. The XREAL 1S promises 500 inches and delivers 43 PPD, the XREAL One promises 70 inches and delivers the same 43. The number in the ad differs by a factor of seven, the quality is equally lousy.

Why "a 130, 200 or 300... more inch screen" is a deception

This is the most common manipulation in the industry, and it works because inches sound familiar while degrees do not.

A virtual screen has no size in inches.


It has only an angular size. The phrase "a 200 inch screen" is meaningless without the second half of the sentence, the viewing distance. 200 inches at 10 metres and 200 inches at 3 metres are completely different pictures in terms of quality. Manufacturers usually pick whichever distance makes the inch figure sound most impressive.

The governing rule: magnifying an image with a lens does not add a single bit of information. Optics can stretch a picture across any angle, but if the panel had 2000 pixels in a row, it still has 2000. Stretching the screen from "100 inches" to "200 inches" does not give you more detail or more sharpness. It gives you pixels twice as large and a PPD figure twice as bad.

An analogy anyone can follow: take a 2 megapixel photograph from an old phone and stretch it across the wall of a house. It will get bigger. It will never get sharper or better.

This is why the "enlarge the screen" mode in such glasses only degrades quality, never improves it. Anyone who has tried them knows that to read text comfortably the screen has to be made smaller. And then you squint at microscopic letters, which is exactly where your eyes give out fastest.

Where 85-90% of the light goes

No near-eye optical scheme delivers the full light output of the panel to your eye. The light passes through a polariser, which immediately costs 50% of the intensity, then reflects off a beam splitter, then off a semi-transparent mirror. Every stage costs optical power. Measurements by optics laboratories give the following figures:
This has practical consequences that never appear in the advertising.

Dark lenses
To make the image visible at all, birdbath glasses block roughly 70-75% of the light from the real world. That is the level of dark sunglasses. You are not augmenting reality, you are looking at the world through a filter. Outdoors in daylight the image becomes translucent, washed out and lifeless.

Black is not black
Micro-OLED by itself has excellent contrast, but in a birdbath layout the light of the room reaches your eye straight through the semi-transparent mirror. The black of a film turns grey. And once again you are being cleverly misled. The advertised 100000:1 contrast refers to the panel, not to what you will actually see.

Spatial glasses: you see the world through a camera

The new wave of pancake-optics devices deserves a separate discussion. They are sold with the words spatial computing, 6DoF and high-fidelity video see-through.

Technically this is honest: the 6DoF tracking really is there, and windows really can be pinned to a point in the room. But the phrase video see-through means literally this: an opaque panel sits in front of your eyes, and you see the world around you as a video feed from cameras on the housing. These are not AR glasses. This is a VR headset in a spectacle frame.

What that means in practice.

The world in low resolution
Your hands, your keyboard, the face of the person you are talking to, everything around you comes at the same poor PPD as the virtual screen. Few people enjoy that.

Camera dynamic range is far below that of the eye
A window in daylight turns into a white blob, and a dark corner of the room becomes a boiling mess of noise.

Latency
Manufacturers quote 10 ms or slightly less, which is a good figure, but it is nowhere near 1 ms. When you turn your head the world lags behind and swims. It will never line up perfectly.

Geometry
The cameras are not located where your pupils are. Because of this you will misjudge distances to objects. Pouring tea, picking up tools or walking down stairs while wearing such glasses is not advisable. You can injure yourself.

Sharp only in the centre

Perfect lenses do not exist. Good ones cost hundreds of dollars. A cheap lens weighing a few grams, built into a spectacle frame, is by definition a serious compromise. As a result you receive the full set of optical defects, none of which are ever listed in the specifications.

Chromatic aberration
Around lines and white letters near the edges you see multiple coloured false contours. Software compensation for this defect is partial and not very effective. The lens splits the light into its spectral components and you see the result. Think of a prism dividing light into a rainbow. The mechanism here is the same.

Distortion and pupil swim
Straight lines near the edges of the image bend, and the character of the bending changes as your eye moves. The brain perceives this as a slight swimming of the world.

A small eyebox
The sharp zone at the pupil is only a few millimetres across. If the glasses slip half a centimetre down your nose, the picture immediately blurs. This is why people constantly push such glasses back into place. You have to sit still like a statue, which is very uncomfortable.

You cannot read with your eyes, only with your head
On an ordinary monitor your gaze travels along the line of text. Here, as soon as the eye rotates, the pupil leaves the sharp zone, and you are forced to turn your head instead. An hour of reading like this tires your neck more than a session at the gym.

Interpupillary distance
People have different head sizes, and therefore different interpupillary distances (IPD). If your IPD does not match the optical centers of the lenses, your eyes are forced to converge unnaturally. Most inexpensive models of display glasses have no IPD adjustment at all.

You simply have to accept whatever spacing the manufacturer gives you. This can lead to loss of focus and significant image distortion. The optical centers of the lenses must be precisely aligned with your pupils.

Glasses over glasses
If you already wear prescription lenses, you will need custom optical inserts, which means extra money and weeks of waiting, or a built-in dioptre adjustment. Astigmatism, incidentally, cannot always be corrected with inserts.

Will You Really Be Able to Watch and Work While Traveling?

Don't worry - you won't... At least not comfortably. Text work is simply out of the question. There is another major problem that sellers rarely mention, and buyers usually discover it only after they are already on the road. And this problem destroys one of the main dreams people buy these glasses in the first place: watching movies on a train, bus, in a car, or on a plane.

The main problem is mechanical, not electronic
Remember the small eyebox we discussed above. The sharp-focus zone around your pupil is only a few millimeters wide. Yet the glasses rest on your nose and ears, soft tissue and weigh 100 grams, with a cable pulling them downward and sideways. Every bump, rail joint, or pothole makes the entire assembly move relative to your skull. Your pupil leaves the sharp-focus zone.

The image doesn't merely shake. It loses sharpness and brightness: it becomes blurry, darkens toward the edges, and develops colored fringes and geometric distortions. One second the glasses are back in position; two seconds later they've moved again. Your eyes are constantly trying to find a focus that simply isn't there.

This is fundamentally different from using a phone. When you hold a phone in your hand while moving, it shakes with you but remains in focus, and your eyes can easily follow it because the screen is relatively far away. With glasses, even a tiny movement of just a few millimeters can throw the display out of focus. These are completely different situations.

In the end, watching through such glasses while traveling can feel like watching a screen on a vibration table: even small external vibrations can cause ghosting, double images, and constant refocusing.

The only real way to solve this is to fix the device rigidly to your head essentially using a head strap instead of temple arms. But then they're no longer really glasses, they're a headset. And the whole point of having a lightweight frame is lost. There is no software fix for this problem. The optics have no way of knowing exactly where your pupil is and dynamically adjusting the image to match it. At this point, the only thing that would really help is a leather muzzle, like the one you put on a dog.

The second problem is virtual-screen drift
The gyroscope and accelerometer inside the glasses measure movement relative to the planet ground, not relative to your seat on an airplane. They simply cannot physically distinguish between you turning your head and the vehicle itself turning. Position-tracking cameras don't solve the problem either. They track the cabin around you, but the cabin itself is moving. The virtual screen, which is supposed to remain fixed in space, starts drifting sideways every time the vehicle turns, accelerates, or brakes. This will start driving you crazy within about 15 minutes.

The severity of the problem depends on the operating mode. If the image is rigidly attached to your head, there is no drift, but there is no virtual monitor either. The problem appears in Anchor, 3DoF, and 6DoF modes, precisely the modes for which people buy these devices in the first place.

Manufacturers themselves acknowledge the problem. Apple introduced a dedicated Travel Mode for Vision Pro. Without it, if tracking is lost, the display simply goes dark with a "Tracking Failed" warning.

Apple's official recommendations specifically cover airplanes and trains. Car travel is considered too unpredictable, while subways and commuter trains suffer from the same problem because of their frequent stops and starts.

VITURE advertises its car mode with the claim that the "screen stays stationary regardless of vehicle movement, preventing motion sickness." That's essentially a direct admission that without such a mode, the screen does not stay stationary.

You're going to throw up
Even if you manage to stabilize the image, there is still a conflict with your vestibular system. Your eyes see a stationary screen while your inner ear feels acceleration, braking, turns, and vibration. This is the classic mechanism behind motion sickness, and with glasses, it can be significantly worse than simply reading a book.

With a book, you can still see the real world around you. With glasses, you can't. The virtual world in front of your eyes may be moving in a completely different way from what your body is actually experiencing. After a while, you're going to feel sick.

Strain on the eyes: what is firmly established

What is proven and universally accepted

Vergence-accommodation conflict, VAC.
This is the central physiological problem of all such devices. In nature, when you look at a nearby object, your eyeballs turn inward, which is vergence, and at the same time the lens of the eye tightens to focus, which is accommodation. The two processes are firmly coupled from childhood. In display glasses the optics fix the focal distance at a single plane, usually 2 to 6 metres, while the convergence of your eyes depends on the apparent distance of the virtual object. The brain receives contradictory signals. This is a recognised cause of visual fatigue, headache and nausea during prolonged use.

Reduced blink rate and dry eyes.
When you stare intently at a bright source, your blink rate drops and the tear film is not renewed. A gritty sensation and redness are common. It is the same mechanism as working at a monitor, but more pronounced.

Measurable short-term effects.
Studies on children and adolescents record fatigue after VR sessions, increased esophoria, reduced near visual acuity and a temporary myopic shift. These effects pass shortly after the device is removed.

The bottom line on health
Such glasses will almost certainly not ruin your eyesight quickly, but they are very likely to cause fatigue, soreness and headache after 30 to 60 minutes of use. That is precisely why most buyers use them a handful of times and then leave them on a shelf for good.

A direct comparison with an ordinary monitor

The only things a monitor cannot do: fit in your pocket, work on an aeroplane, and keep your screen private. Those are the honest advantages of display glasses. Everything else in their advertising is about small size, not about quality.

The real price: what the price tag leaves out

The light weight of such glasses is achieved not by engineering brilliance but by a simple trick: the processor and the battery have been removed from the housing. A weight of 90 to 100 grams means a frame with no signal processing of any kind. The device still needs a powerful processor to drive the video, and you will still have to carry it on you.

DisplayPort Alt Mode
The connection runs over USB-C in this specific mode. Support for it is far from universal among phones and laptops. Check your particular device by model number, not by the presence of a USB-C connector. There is no Wi-Fi here: the glasses contain neither a processor nor a battery, and wireless transmission would require both, as well as adding compression and latency.

External battery
Phones cannot supply the power that bright displays and this kind of load demand. Manufacturers sell the battery pack as a separate paid accessory, typically around $70, and that means another 300 grams in your pocket and a second device that needs charging.

Cables
Nothing works without cables, which should be obvious to anyone. You will constantly snag the cable on things around you, and the glasses will be pulled off your head and damaged. You will not be able to watch lying down either, because the connector presses against the pillow and pushes the glasses off your nose. There is only one option left, to sit still like a statue.

The vendor application
The full feature set often works only through it, and only on some platforms. iOS and Android versions are usually promised for later. Once the money has been collected, however, those promises are frequently forgotten.

The small items
Prescription inserts, a headstrap, a case, adapters, all of that adds to the bill. Count the final total, not the number on the banner.

Questions to ask before buying

Put these questions to the seller. If there is no sensible answer, that is an answer too.

What is the resolution and the horizontal field of view?
Divide one by the other and you have the PPD. Below 45 and text will be hard to read. Do not accept the word "4K", ask for the physical pixel count per eye.

Is the see-through optical or through cameras?
A single question that separates AR glasses from a VR headset in a frame.

What is the brightness at the eye, not the brightness of the panel?
Indoors you need around 300 nits, outdoors around 1200.

Is there IPD adjustment?
And what range of interpupillary distance is supported. You need the optical centers of the lenses to align precisely with the distance between your pupils.

What about prescription lenses?
Are inserts included, sold separately, or not available at all.

Is my particular laptop or phone supported?
And does it require the additional battery pack.

Are there real photographs taken through the lens?
Not renders, not a screenshot of what you are supposed to see, but an actual photograph taken by a camera placed where the eye would be. If neither the manufacturer nor the reviewers have such images, that is a clear red flag. Do not buy.

What are the return terms and the return period?
Comfort in these devices is highly individual, and there are already plenty of complaints. The right to return the product after an hour of use is worth more than any discount.

A separate word on crowdfunding
Kickstarter is not a shop. Nobody guarantees you a result. In many cases you receive a low-quality, half-finished device with no way to get your money back.

Conclusion

Display glasses are engineering full of compromises, and they are sold with the drawbacks left unmentioned, without any guarantees. Largely relying on the technical illiteracy of buyers. They can see that the image quality is poor, but they are unable to explain what exactly is wrong with it.

The law of optics from which everything else follows reads as follows: magnifying an image with a lens does not create information that is not in the panel. A huge virtual screen is not a large high-quality screen like a cinema, and not even the equal of an ordinary display.

Do not buy them if you want to:
Get TV-level image quality - you won't get it
Replace your monitor for working with text and code - it won't work
Use the glasses comfortably for several hours at a time - you won't
Watch media while traveling or in a moving vehicle - you won't be able to
Get a sharp image across the entire frame - you won't get that either

You may buy them if you need to
Watch movies while sitting in a chair, have a private screen in a shared space, or experiment with this technology while understanding its limitations. They may also make sense if you have the option to return the product quickly.

If your goal is high-quality image, including 3D
An ordinary display beats any display glasses on every measurable parameter,except portability. This is not the author's personal opinion. It is dictated by the laws of optics and the mathematics of PPD. Want to argue with the math? Go ahead and try...

Sources
KGOnTech, birdbath optics teardown and light-loss measurements
Information Display SID, optical efficiency of AR and MR architectures
Systematic review, virtual reality and myopia
Road to VR, specifications of current pancake-optics glasses