Why headset screens are a special case
In a virtual reality headset, the display sits very close to your eyes and is magnified by lenses. A television viewed from a sofa fills only part of your vision, and its pixels are too small to notice. A headset fills most of your view, and every pixel is enlarged. That makes headset screens harder to build than almost any other kind of display, and it explains why progress is steady but gradual.
This article focuses on four topics: sharpness, the screen-door effect, field of view and comfort. It describes ongoing directions, and where the future is concerned, it is open about what is uncertain. How the screens, lenses and tracking combine to create presence is covered in a separate article.
Resolution is spread across a wide view
Headset resolution is usually quoted per eye, and a high number can still look less sharp than a monitor with fewer pixels. The reason is that those pixels are stretched across a very wide field of view. What matters to the eye is closer to angular detail, meaning how many pixels cover each degree of your view, than the raw count. A monitor on a desk covers a small angle, so its pixels are packed densely into what you see. A headset spreads its pixels over a much larger angle.
This is the same idea as pixel density on other screens, applied to the angle instead of the inch. Text is where the limit shows most: small print that is easy to read on a laptop can look fuzzy in a headset unless it is made larger.
Rendering adds another cost. A headset needs a separate image for each eye, usually at a high refresh rate, so the computer or headset processor has to work hard. Some systems concentrate detail where you are looking and draw the periphery more coarsely, a technique that relies on eye tracking. Whether it becomes standard is an open question.
The screen-door effect
The screen-door effect is the faint grid pattern you can see when looking through a headset, as if through a mesh. It happens because there are small gaps between the lit parts of each pixel, and magnification makes the gaps visible. It is easiest to notice on bright, plain areas such as a sky.
Several approaches reduce it. Packing in more pixels makes each smaller and the grid finer. Designing the pixel layout to leave smaller gaps helps, as does using optical techniques that slightly blur the grid. The relationship between pixels and subpixels explains why layout matters as much as count. Reducing the effect is generally not free, and some solutions trade away brightness, sharpness or cost.
Panels and microdisplays
Headsets have used a few kinds of small screens, including liquid crystal and organic light-emitting panels. Smaller microdisplays are of particular interest because they can pack pixels very densely into a tiny area. Different technologies offer different strengths. Organic emitters tend to give deep blacks and fast response, while liquid crystal screens can be bright and cost-effective. Each also has weaknesses, such as the way some panels flicker or smear fast motion. Research on new approaches continues, but which will prevail is uncertain.
Fast response matters more in a headset than on a desk. Smearing when you turn your head can cause discomfort, so designers often show the picture only briefly in each frame, which reduces smear but can lower brightness.
Field of view
Field of view is how much of your surroundings the headset covers. Humans see a very wide span, and headsets generally cover less than that, so the image can feel a bit like looking through goggles. A wider view adds immersion but is hard to deliver. It requires larger lenses or screens, which can add weight and make the optics more difficult to keep sharp at the edges.
Designers have to balance a wide view against sharpness, size and weight. The lens types used can also affect how bulky the headset is. Some newer designs use thinner lens arrangements, but how well they will work at scale is not settled.
Comfort and eye strain
Comfort is the practical limit on long sessions. Several things contribute:
- Weight and balance on the head, which are affected by the size of the screens and lenses.
- Motion smoothness, since low or uneven frame rates are linked to discomfort for some users.
- Focus: in most headsets the image appears at a fixed distance, even when a virtual object looks near or far. The eyes’ focusing and aiming systems can disagree, which some people find tiring. Ways to address this are an active research area.
- Individual fit, including the distance between the lenses and the user’s eyes.
People differ widely in how they respond, and some are more prone to nausea or eye fatigue than others. Taking breaks and stopping when uncomfortable is sensible, and anyone with a medical concern should ask a qualified professional.
Related directions
Ideas from augmented reality, such as compact optics that guide light to the eye, overlap with headset design. Headsets that blend camera views of the real world with virtual objects also rely on the same displays. How these categories will converge is uncertain.
Reading claims carefully
Marketing for headsets often emphasizes a single number. It helps to ask what the number is measuring: pixels per eye, per degree, or total; the refresh rate at which the full resolution is available; and the field of view in which the image is actually sharp. Improvements in one area frequently cost something in another, and nothing here is a prediction of what future headsets will do.