Wavefront technology: coming to a smartphone near you

In the last few years there has been an increasing trend of Shack-Hartmann wavefront technology being incorporated into auto-refractors, such as the Zeiss iProfilerPlus and Topcon KR-1W. This is helping to give practitioners a more accurate impression of the refractive status of patients, proving especially useful for those with particular vision requirements.
                                                                                    Today we will look at both the basic principles behind Shack-Hartmann wavefront aberrometry and also a really exciting development in its use.

Shack-Hartmann (SHWA from now on!) was designed by Roland Shack and Ben Platt in the 1960s using an aperture array principle described in 1900 by  a physicist and astronomer, Johannes Franz Hartmann, based on the original principles of Scheiner's disc experiments (1619). The original aim of the project was to solve a problem with taking photographs through the Earth's atmosphere. Needless to say the scope of it's use has stretched far beyond its original remit.

The principle is as follows, a ray of light is shone in through an aperture array, the ray bounces out of the eye and through a video-plane where the vertical and horizontal displacement of the reflected ray (compared to the expected position) are used to calculate the optical lens required to correct the individual points and thus the refraction for each of these points. The clever part is in the capture array

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The need for a specialised capture array of lenses is easily understood as if a single lens (a la a fundus camera) was used only one retinal point would be recorded. The beauty of a multiple-lenses arrangement is that the data from each point can be combined to form a wavefront map. An ideal wavefront would be circular and uniform, most are not, you can see mine at the top of the article. To put this in perspective the image above is my right eye which refracts to +0.75/-0.25x80.

Usually these arrays are very expensive and the equipment they are used in are also more dear in comparrison to a standard auto-refractor. However, a new technique has been developed by MIT called Inverse Shack-Hartmann. This has been developed into a device called NETRA - Near Eye Tool for Refractive Assessment, interestingly it also means eye in Sanskrit.

As the name of the technique suggests this uses the opposite of the Shack-Hartmann sensor. So instead of using the reflected light from the eye it uses the absorbed/detected light. In order to capture this data the user indicates to the device when the points are aligned, much how a patient responds on a Lee or Hess screen. Amazingly the NETRA device can clip onto most smartphones and it only costs $1 (plus a smartphone).

This is revolutionary in terms of the affect it can have, especially in the third world setting. It's truly thrilling to see the smartphone revolution being used for effective medical/optometric interventions and it should open new diagnostic possibilities to those where cost of equipment is a significant barrier to entry.

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