In this article, I want to discuss what seems often overlooked in AR Field of View (FOV) discussions, namely the view of the real world. It seems like for some headsets, the view of the real world is almost an afterthought. This article follows up on my blog entry “FOV Obsession.” In the process of writing FOV Obsession, I found the SPIE paper by Bernard Kress that deals with the real-world view. Figure 11 of the paper compares the view of the real world through a few VR and AR solutions.
For much of this article, I am going to be referencing figures in Kress’s paper, which I recommend reading. Kress is very generous in supporting the AR industry as a whole. Still, Kress works for Microsoft on Hololens, and his paper does have a bit of a “Hololens bias” (more on that later).
What does AR gain If It Blocks Out the Real-Word?
Paraphrasing a biblical expression, “For what profits a man if he gains the whole world but loses his own soul,” what does AR gain (over VR) if it blocks out the real-word? Ideally, AR should provide a view of the real world that is unfettered. After all, this is what differentiates AR from VR.
I’m particularly concerned with how some headsets seriously impair peripheral vision. Peripheral vision, while inferior in resolution, detects objects that may be of danger and keeps people from running into things.
Hololens 2 versus Magic Leap One Human Factors
Figure 11 seems mostly aimed at comparing Hololens 2 (HL2) to the Magic Leap One (ML1). I have included similar side by side views of the two headsets below. Not only does the HL2 support a much better view out, it also has significantly more eye relief, which further improves the downward view.
Magic Leaps One’s View Out, or Lack Thereof
The blocking out of the real world is something I faulted the ML1 with in “Magic Leap One – FOV and Tunnel Vision,” and also in “Magic Leap Review Part 1 – The Terrible View Through Diffraction Gratings”. In that post, I took pictures through a 3-D printed model of the ML1 that showed the view through both eyes (right).
Using both eyes the view is not even all or nothing as Kress’s figure 11 suggest, but rather, there is a region of binocular overlap where you end up with a double-image where one eye is seeing the frame and the other eye can see through. I also mapped the view through the ML1 onto the view through the eye (right).
How Much Light is Blocked and the Artifacts?
Kress’s figure 11 is more of an “all or nothing” concerning the real world view, but it does make an important point. We need to be concerned about how much light is blocked in addition to distortions and artifacts.
ML1, in the area around the display, blocks about 85% percent of the light (transmits only 15%), and with 6 layers of diffractive waveguides produces a lot of distortion and artifacts. The pictures (left) were taken through the ML1 and HL1 is from my blog article “Magic Leap Review Part 1 – The Terrible View Through Diffraction Gratings”. Note, the exposure was different for HL1 as it lets through more than twice the light as the ML1.
Mistakes in Kress’s Magic Leap One Figure
One thing that caught my eye, having mapped the ML1’s view onto human vision, was that Kress’s figure 11 did not show the view below the glasses. The view out is not that good, and you see something different in each eye, but it at least one can see their feet.
Kress’s “Fixed Foveated Region,” something Kress draws somewhat arbitrarily at 50 degrees (see “Human Vision and the ‘Fixed Foveated Region’” section of FOV Obsession). Unfortunately the rectangle showing ML1’s display FOV was drawn smaller, and thus inside the circle, relative to the way it was drawn on the Hololens 2 where it touches the circle.
The UploadVR figure (left) compares HL1 and 2 versus ML1 based on the spec’s from Microsoft and Magic Leap. Be advised that manufacturers are not consistent in how they measure FOV, and it varies with how the glasses are worn, the shape of various people’s heads, and other variables.
Is the Head or Just The Eye Tilting in the Figure?
Sometimes there are many things that “you just know” from years of working in the industry. One of these things is that human vision is biased toward looking down. The saying goes, “snakes and tigers were more of a threat to our ancient ancestors than birds.” But then as you look at human factors diagrams such as figure 8 from Kress (below left), he had a line that Says “relaxed line of sight” and 15-degrees. First of all, the angle of the line in the figure was wrong, but more importantly, is the issue of what is relaxed, the head, the eyes, or both?
Many older human factor studies have similar diagrams. I included (above right) a figure from the 1993 book, “The Measure of Man and Woman” by Tilley. Most of these studies are for human factors in workplaces where the display is a fixed computer monitor. What both Kress’s and Tilley’s figures do is conflate head movement with the eyesight. The Tilley figure for a relaxed line of sight line assumes that the head is tilted 15-degrees which is typical for a person sitting.
The eyes are not tilting 15 degrees down in addition to the head’s 15-degree tilt. With an HMD, when the head tilts down, then so does the display that is attached. In this case, it looks to me like Kress just copied from old human factors studies without pointing out the implications for HMDs.
Kress has an unconstrained range of 15-degrees up and 20-degrees down for a total of 35 degrees, where eyes will move without much effort or unconsciously. But as noted in my FOV Obsession article referencing the Haynes 2017 Ph.D. dissertation, even 30 degrees of constant movement will tire the eyes over long periods. Thus, a user interface that tends to force the user to constantly move their eyes more than 20 degrees from the center will tend to cause discomfort.
Related Ergonomic Information
Tilley’s figure above says that a 15 degree tilt down is the “No Fatigue” angle for the head when sitting. The neck muscles, in general, will become sore if the head is tilted up for a long period of time. And as said above, the eye motion is also biased toward looking down. Just about every ergonomic guide for computer monitors tells users to have the top of the monitor at or below the eye level.
The ML1 blocks most of the person’s peripheral vision in every direction, whereas HL2 only blocks the upward view. The upward view is not as critical because the human physiology of the forehead and eyelids tends to constrain the upward view. Typically when something is much above the horizontal, people usually tilt their heads rather than move their eyes.
It seems that in all the technical difficulty of building wearable see-through displays, designers are losing sight (both figuratively and literally) of what they are trying to do with “Augmented Reality.” If they go to the point where the real world view is significantly compromised and where it becomes unsafe to wear the headset except in a very controlled room, then they are not so much “augmenting” but replacing the real world. If all they want is to keep some of the real-world for say a game application, then it would seem they would be better off with pass-through (camera) AR.
Addendum: Eye Saccades and Fixation
As discussed in FOV Obsession, a human only has a tiny area of high-resolution vision at any instant of time. The human eye typical makes 3 to 5 rapid movements per second called saccades which take between 20 and 200 milliseconds to complete (depending, in part, on the distance), followed by a fixation when it does not move. While the eye is moving, vision is essentially blocked out. The human visual system is putting together what people perceive as a single high-resolution image from a series of mixed resolution fixation snapshots. The figure below shows a simulation of what the eye sees in a single fixation.
I want to thank Ron Padzensky for reviewing and making corrections to this article.