Both of our refraction devices (the Personal Vision Tracker and VisionCheck) use the same basic principle to determine your refractive error, which is a technique called the "Inverse Shack Hartmann" (US Patent #8783871). There are three lenses inside the device: a clear lens, a green lens, and a red lens. The clear lens takes the light from your smartphone screen and refracts it through the green and red lenses towards the top of the device. This produces the red and green lines that you are asked to overlap as part of the test procedure. As you move the lines closer or further apart, the light inside bends accordingly, and you eventually see the lines overlap to form a solid yellow line. In reality, the lines never physically overlap, it is the light being bent to the optimal spot on your retina (the back of your eye), which indicates your refractive error. The algorithm then takes the distance between the lines on the screen and produces a measurement of your eye for the specific orientation you tested. The same test is repeated 8 more times in different meridians (orientations) to measure the cylinder and axis needed to correct for any astigmatism you might have. If you would like more information, please contact our support team.