A vision defect is a mismatch between the eye’s focal distance the range at which it can actually bring objects into focus and the distance of the object it’s trying to focus on. Essentially, the new display simulates an image at the correct focal distance somewhere between the display and the viewer’s eye. 

The difficulty with this approach is that simulating a single pixel in the virtual image requires multiple pixels of the physical display. The angle at which light should seem to arrive from the simulated image is sharper than the angle at which light would arrive from the same image displayed on the screen. So the physical pixels projecting light to the right side of the pupil have to be offset to the left, and the pixels projecting light to the left side of the pupil have to be offset to the right. 

The use of multiple on-screen pixels to simulate a single virtual pixel would drastically reduce the image resolution. But this problem turns out to be very similar to a problem that Wetzstein, Raskar, and colleagues solved in their 3-D displays, which also had to project different images at different angles. 

The researchers discovered that there is, in fact, a great deal of redundancy between the images required to simulate different viewing angles. The algorithm that computes the image to be displayed onscreen can exploit that redundancy, allowing individual screen pixels to participate simultaneously in the projection of different viewing angles. The MIT and Berkeley researchers were able to adapt that algorithm to the problem of vision correction, so the new display incurs only a modest loss in resolution. 

In the researchers’ prototype, however, display pixels do have to be masked from the parts of the pupil for which they’re not intended. That requires that a transparency patterned with an array of pinholes be laid over the screen, blocking more than half the light it emits. 

But early versions of the 3-D display faced the same problem, and the MIT researchers solved it by instead using two liquid-crystal displays (LCDs) in parallel. Carefully tailoring the images displayed on the LCDs to each other allows the system to mask perspectives while letting much more light pass through. Wetzstein envisions that commercial versions of a vision-correcting screen would use the same technique. 

Indeed, he says, the same screens could both display 3-D content and correct for vision defects, all glasses-free. They could also reproduce another Camera Culture project, which diagnoses vision defects. So the same device could, in effect, determine the user’s prescription and automatically correct for it.