“It can’t be comfortable or healthy to stare at a screen a few inches in front of your eyes.”
The popularity of Google Cardboard, and the upcoming commercial releases of the Oculus Rift, HTC Vive, and other modern head-mounted displays (HMDs) have raised interest in virtual reality and VR devices in parts of the population who have never been exposed to, or had reason to care about, VR before. Together with the fact that VR, as a medium, is fundamentally different from other media with which it often gets lumped in, such as 3D cinema or 3D TV, this leads to a number of common misunderstandings and frequently-asked questions. Therefore, I am planning to write a series of articles addressing these questions one at a time.
First up: How is it possible to see anything on a screen that is only a few inches in front of one’s face?
Short answer: In HMDs, there are lenses between the screens (or screen halves) and the viewer’s eyes to solve exactly this problem. These lenses project the screens out to a distance where they can be viewed comfortably.
Now for the long answer.
As everybody knows, it is very hard or impossible to focus on things that are very close to one’s eyes. Pick up a piece of paper with some text on it, hold it one inch in front of one of your eyes, and try to read it. Unless you are severely near-sighted, it won’t work (that, by the way, is what near-sighted means: being able to focus on things that are very close). To understand why it doesn’t work, we need to understand how the eye focuses on objects at different distances. Figure 1 shows a (simplified) eye, looking at an object at three different distances.
In each setup, some light rays emitted or reflected from a point on the object enter the eye through the pupil and the lens at the front, and hit the retina in the back, where the light is detected by light-sensitive cells (rods and cones). To create a sharp image, all light entering the eye from a single point on the object must be focused on a single point on the retina. That is the job of the lens: it bends the incoming divergent rays of light such that they converge on a point on the retina.
The central observation is that the amount by which light rays from an object diverge at the eye’s lens depends on the distance from the object to the eye. If the object is close, the rays diverge at a large angle (see Figure 1, left); if the object is at a medium distance, the rays diverge less (see Figure 1, center); if the object is infinitely far away, the rays are parallel and do not diverge at all. In other words: the closer an object is to the eye, the more the eye’s lens has to bend the incoming light rays to form a sharp image on the retina.
The human eye (and the eyes of all mammals, birds, and reptiles) achieves this by changing the shape of the lens, a process called accommodation. To focus on near-by objects, a ring of muscle around the lens (the ciliary muscle) contracts to make the lens thicker and rounder; to focus on far-away objects, the same muscle relaxes, and the lens flattens out, being pulled on by elastic ligaments connecting it to the rest of the eye.
But there is a limit to how much the ciliary muscle can compress the lens, and therefore there is a minimum distance on which the eye can focus. Any objects closer than that distance appear blurry. It is around 4 inches (10cm) for children and young adults with normal vision, and increases with age.
What this means is that, for most people, it would be impossible to focus on the screens of HMDs (unless the display is ridiculously large (source)). And even if it were possible, looking at very close objects for a long time can cause eye strain, as the ciliary muscles have to continuously squeeze the eyes’ lenses.
And that’s where the lenses in HMDs enter the picture. The left part of Figure 2 shows an eye looking at a very close object, in this case an HMD’s screen. The light rays from a point on the screen diverge so much that the eye’s lens can not converge them to a single point on the retina, even at maximum accommodation, meaning that the screen will appear blurry.
The right part of Figure 2 shows a screen at the same distance, but in this case, there is a lens between the screen and the eye. Just as the eye’s lens itself, this intermediate lens bends light rays, and reduces the divergence angle of light from the screen to a point where the eye can focus it, even at less accommodation.
What does this look like from the eye’s point of view? We established earlier that the amount of divergence of light rays at the eye is directly related to the distance to the object from which the light is emitted. If we extend the light rays between the intermediate lens and the eye backwards (the fainter lines on the right of Figure 2), we find that they all intersect in a single point behind the real screen. To the eye, it looks exactly like a screen that is farther away and larger (the grey horizontal line on the right of Figure 2). This is called a virtual image.
And that’s why viewers can comfortably focus on the screens in HMDs: they are not looking at the real screens, which are too close, but at virtual images of the real screens, which are at a much larger distance. How much larger depends on the model of HMD: in the Oculus Rift DK1, for example, the virtual screens were infinitely far away; in the Oculus Rift DK2, they are about 4.5 feet (1.4m) away.
Note: The above discussion assumes ideal lenses. The lenses used in modern HMDs are not ideal, and while that does not affect the core principle of virtual screens, there are side effects, specifically geometric distortion and chromatic aberration, which we will discuss in another post.