Optics vs. Digital Displays for Low Vision
Review and comments by:
Henry A. Greene, OD, FAAO
Co-founder, President, OCUTECH, Inc.
I expect that you’ll think that as a manufacturer of optical bioptic telescopes writing about Digital Head-Mounted Displays, I’m just carping on what might be considered our competition. But, as noted in this paper, HMDs are not yet compelling competition to optical bioptics. That’s not to say that head-mounted digital displays for low vision won’t someday offer compelling vision rehab opportunities. They likely will. There are, after all, limits to what conventional optics can do. It’s just that they’re not there yet. And it’s due largely to the limits of the technology currently available to HMD manufacturers.
4 June 2021
In a recent paper in Optometry and Vision Science, the journal of the American Academy of Optometry, entitled “Measuring Virtual Reality Headset Resolution and Field of View: Implications for Vision Care Applications ” by Lynn, et al, from Harvard Medical School and the Schepens Eye Research Institute, researchers evaluated the Virtual Reality (VR) Head-Mounted Display performance characteristics of devices promoted for use by the visually impaired.
This is the first study to evaluate the technical characteristics of VR systems for use in eyecare settings. The study measured a sample of VR headsets to evaluate eye-to-screen distance and the other physical and optical characteristics needed to calculate minimum angular resolution in logMAR and field of view in determining their feasibility for vision applications.
Minimum angular resolution
The studies found the minimum angular resolution in logMAR of the six different devices they tested ranged from 0.28 (20/39) to 0.79 (20/123). These were measured using the device’s native (non-digitally magnified) image size. When digitally magnifying the image, the logMAR resolutions will be reduced by the magnification factor. For example, when using the device with the highest display resolution (0.28) at 2x digital magnification, resolution would be reduced to 0.56 (20/78) and at 4x, a commonly prescribed telescope power, resolution would be 1.12 (20/156). By comparison logMAR resolution of the equivalent power optical telescopes is -0.2 (20/10) for both 2.2x and 4.0x powers.
Field of View
The fields of view of the headsets were calculated using the lens focal distance and the visible areas of the display. To verify the calculations, fields of view were also empirically measured using a Goldmann perimeter. The manufacturer’s claimed field of view did not match that measured by the researchers. The marketed field of view of VR low vision devices, which all presently use the Samsung Gear VR headset, ranges from 70° to 101°. The researchers found the field of view without magnification was actually 78° and therefore the average field of view when used as a 4x visual aid is only 19°, much closer to that of equivalent power optical telescopes.
Conclusions as a Low Vision Aid
The researchers evaluated these VR devices for use for visual acuity testing, amblyopia, as well as low vision treatment. They report that the minimum angular resolution in logMAR of current VR technology is insufficient [to] provide ample resolution when digitally enlarging the image [for use as a low vision aid]. The fields of view during movie watching or gaming is substantially less than that reported by manufacturers.
Reviewer’s take on this.
Why all the hoopla and patient interest?
Marketing! HMD manufacturers have great emotive testimonials: they can make the blind see; he could see her eyes for the first time in years; she can read, play piano, play cards; they can do things they never thought they’d be able to do again! But one cannot ignore the social acceptability issues of a device that hides the user’s eyes—both for the user, for their companions, and for the general public they may encounter. Since distance vision is a social sense, users seek, among other goals, to make eye contact, read body language, and feel socially connected. Devices as socially disruptive as present HMDs will certainly undermine this interpersonal functional goal.
Registered with the FDA
HMD manufacturers claim that they are registered with the FDA. Registered is not the same as approved by the FDA which the FDA does not do!. Fill out the form, pay your money and your device can be registered too! It’s false credibility. Every manufacturer of any type of medical device is required by law to be registered with the FDA. The FDA is firm in its position that registering a product with the agency does not confer the FDA’s approval nor prove its efficacy. What confers efficacy? Independent clinical trials, not testimonials or marketing.
New Isn’t Automatically Better
Of course, new can be appealing and exciting. But new isn’t automatically better just because it’s new. New is better when it’s clinically proven and well received by the patient. Tried and true as optical bioptics are, they may not be as exciting as HMDs, but you won’t hear that from long-term users. Bioptic users will tell you that their devices have changed their lives. Ocutech’s prescription return rate is less than 0.1%. Instead, we receive products to be cleaned and refurbished almost daily. That’s because they are used. Many are returned to have the carrier lens prescriptions updated. Patients wouldn’t bother spending the money if their bioptics weren’t helpful.
Clinical Data and Implications
What questions should we be asking? Which patients might benefit from HMDs? Which diagnoses might respond best? What range of visual acuities is most appropriate? What display settings are most helpful? While the HMDs are capable of very high magnification, what is the highest magnification that a user can tolerate? It’s usually no higher than 6 or 7x. So, what’s the benefit of claiming higher powers? What is the abandonment rate? HMD manufacturers don’t answer any of these questions.
Why do most HMDs completely surround the user’s eyes? It’s because the display isn’t bright enough to be visible outdoors without enclosing the user’s eyes. Try looking at the screen on your phone or your digital camera outdoors. Can’t see it, can you? HMDs have the same problem.
What is the breadth of functionality of HMDs? Can users drive with HMDs? No. Can users walk with HMDs? Only if they are lifted out of the way or have their magnification readjusted to zero. So, where’s the benefit then? Can users wear them all day? No, they only run for a few hours. Are they comfortable to wear? No, they’re much heavier than optical bioptics, and you may perspire underneath them. Are they easy and intuitive to use? To be determined.
What about optical bioptics? Lightweight, great outside, drive with them (where legal and appropriate), walk with them, and socialize with them. They don’t enclose the user. They are indicated for central vision loss with best-corrected acuities of 20/300 or better. The clinical data and methods are well established. And, they’re easy to use—all you do is look. Do they work for everybody? Nothing works for everybody! They’ve been around a long time, haven’t they? What’s wrong with that? It means they work and are well accepted. Low vision specialists prescribe them all the time. They wouldn’t continue to do that if their patients weren’t successful with them.
The direct to patient sales model
Another important point is that HMD firms frequently use salespersons to demonstrate and sell their products directly to the consumer. No matter how sympathetic, supportive, and engaged these reps are, they are pursuing a one-time sale, not a long-term relationship. As such, they have little or no investment in the customer after the sale is made. They are not eye care professionals who provide ongoing care for their patients and as a result are personally invested in their patient’s long-term support and the appropriateness of the devices they recommend.
In summary, as the authors of the paper conclude…
“…current virtual reality technologies have limitations that should be carefully evaluated and considered before their implementation in clinical settings. When marketing these devices as low vision aids, approaches for reporting field of view should be consistent with industry standards based on existing telescopic aids, allowing informed comparison by clinicians and consumers.”