subject: NETRA : A Thermometer For Visual Performance [print this page] NETRA : A Thermometer For Visual Performance
This month, a new device out of MIT Media Lab's Camera Culture group utilizes one of the most rapidly proliferating technologies in the developing world mobile phones to make the diagnostic process of refractive errors of the eye, including nearsightedness, farsightedness, astigmatism, and age-related vision loss, both affordable and accessible to underprivileged populations. NETRA (Near-Eye Tool for Refractive Assessment) uses a phone's LCD display and intelligent software to administer a quick, effective and accurate vision test that patients can perform at home. NETRA means "eye" in Sanskrit, and stands for Near Eye Tool for Refractive Assessment.
Devices using the Shack-Hartmann sensor shine a laser into the eye of the patient, are quite expensive, and require a trained professional operator. The NETRA system uses the dual of a Shack-Hartman sensor, and replaces the laser with simple user interaction. This significantly reduces the cost of the device and makes it appropriate for self-evaluation, while still providing comparable data. As of now, the NETRA can only be used with a high-resolution cell phone display, but Raskar said he hoped that "though not everyone can buy such a phone, a village shopkeeper may have one and it can be used to provide eye care to everyone in the village". NETRA current prototype cost less than USD 2.00 using laser-cut, hand-assembled parts. Since the device essentially made of plastic, this cost will rapidly reduce to just a few cents if mass produced.
The micro-lens array in front of the LCD essentially creates a 4D display. A traditional 3D display presents a slightly-different view to a person's left eye and their right eye. With NETRA, we present different views through different parts of the same eye. If the eye is free from aberrations, it focuses clearly and these disparate views overlap perfectly. But far and near-sighted users have to press a few buttons to make that happen. The numbers of tweaks it takes for them to hit that ideal alignment reveals the correction they need to focus clearly. NETRA relies on a programmable high resolution display to generate patterns at arbitrary depths. The resolution of cheap LCDs found on popular phones recently took a giant leap. For instance, while current LCD monitors support about 90 Dots-Per-Inch (DPI), the Nexus One and the new iPhone 4G supports 250 and 326 DPI respectively. The display resolution defines the resolution of the NETRA device in diopters, and for it to be usable as an optometry solution, reasonably high resolution displays are required. The resolution is 0.4 diopters using the Nexus One device (focal length 30mm, pinhole pitch 3mm, lens-eye distance 15mm). The Apple iPhone 4G, with the new Retina Display should achieve a resolution of approximately 0.28 diopters (focal length 30mm, hole pitch 3mm, lens-eye distance 15mm).
For measuring eye correction, the average absolute errors from the known prescriptions were under 0.5 diopter ( = 0.2) for both cylindrical and spherical powers. The average absolute error of our estimates of the cylindrical axis was under 6 degrees. Optometrists typically prescribe in multiples of 0.25 diopter, and 10 degrees axis.
At its most basic application, the NETRA test consists of a small plastic device clipped onto the phone's screen. The patient looks into a tiny lens, presses the phone's arrow keys to make a set of parallel green and red lines overlap, bringing view into sharp focus, and repeats this eight times for each eye as the lines appear at different angles. Software loaded onto the phone then provides the precise prescription data. It may sound laborious, but the entire process takes under two minutes.
What makes NETRA particularly promising is its reliance on a cheap and increasingly widespread device. With more than half of all households in the developing world owning a cellular phone, growing at six times the adoption rate of the developed world according to UN statistics, and nearly one billion people worldwide in need of corrective eyewear, NETRA offers a brilliant marriage of need and convenience.
