The NETRA Auto-Refractor

Re-engineering Refractions: the problem of accommodation and how virtual reality plays a role.

Refraction in the health space is the process of collecting data to issue a prescription for glasses or contact lenses. Most engineers relate eye refraction to optical measurements of the patient’s eye and the posterior compensation for the sharpest vision. This is, however, a very, very simplistic description of the problem.

Refraction is about psychology, not optics. The most correct eyeglass prescription has very little to do with the pure optical needs of the person. Measuring instruments that rely solely on the opto-physical realm of the problem bring just rough, barely acceptable measurements to the table.

Sure, it all starts with optical compensation but, for machines, it quickly becomes a physiological nightmare. When physiology is finally addressed, psychology comes into play. In the end, if you don’t solve it all, the patient is likely to not accept and not wear the prescription over time.

The most common physiological backlash to refraction is a beautiful feature of the human eye (for correctness: vertebrate eye): accommodation. The mechanism in which the eye can change its focus from far fields to near reading ranges in a fraction of a second.

In order to capture measurements for the perfect optical compensation, the eye must be focusing as far as it can. A machine might claim to capture the most accurate optical readings, maybe with precision down to the tenth of a diopter, but if the eye is focusing at the wrong depth, the results will be completely off. Technically, the machine can still say that it is accurate. It is correctly measuring the wrong state of the eye.

When the machine finally manages to get the eye focusing far, it must now overcome the involuntary and constant focusing changes. Accommodation is used by our navigational systems to generate and validate depth information. How to measure something that keeps changing? If you take several readings, what is the most accurate one?

Along the years engineers have either ignored or went completely crazy about this question: how to stop accommodation. Yes. There are dilating drops that force the eye look far, but who wants to take those drops these days?

At EyeNetra, we spent more than one year doing research and side-by-side validations solely on accommodation control systems. Hardest part is not to develop, but to integrate into refractive measuring devices. That experience led us to rethink old habits and wrong conclusions most people in the field still believe.

The most common misunderstanding is that open-field designs work. They don’t. Open-field makes the patient look far into the real world. As far as it can. It does work well on myopes, but not with hyperopes. Hyperopes need to be focusing beyond infinity (wait.. what?). Yes.

Hyperopes can focus beyond. At least in the optical sense. Obviously, no object in this reality can be placed “beyond infinity”. The only option is to use lenses to “fog” the eye. The most common technique to relax the human eye (fogging) is the main reason why Subjective Refraction is still the gold standard.

Fogging is a closed loop technique: Blur the eye, measure its refraction, and optimize the blur and measure it again. Repeat until you have compensated for the most positive sphero-cylindrical errors. Due to accommodation, fogging and subjective refraction are tricky procedures and require an experienced doctor to be performed.

We needed a simpler version. No cyls. We couldn't afford that amount of lenses into a simple device like NETRA. And static spheres. We knew we can't allow the patient to adjust the fogging by himself. It is recipe for errors.

No cyls, means that we need to rely on software to control which axis the patient is focusing at. And static spheres means that the two eyes will be fogged by the same amount. Far from ideal, but software needs to solve that as well.

In the end, we chose a Virtual Reality binocular design (the first and only in the market) with a built-in fogging technique. Not because it looks cool and is very trendy to say the least. But because it is the best eye relaxation method we found. Virtual reality is key to compensate for the lack of the ideal optical fogging procedures.

A controlled, binocular, multi-spectral, time-dependent pseudo-reality simulator does the a better job than most extremely expensive machines out there.

And better than all? You can still play with it. :)