Both of our refraction devices (the Personal Vision Tracker and VisionCheck) are built in a very similar way. The inner-working of each of the EyeQue refractive devices uses a technique called "Inverse Shack Hartmann" (US Patent #8783871). To give you a quick peek, there are three lenses built into each of the inside of the devices. The bottom lens lets light in from your smartphone screen and then refracts it into the green and red lenses are towards the top of the device; which is what you see. Then, when you move the lines closer or further apart, the light inside bends depending and you will see the lines overlap and form a solid yellow line even though the lines never really overlap. When you see the yellow line it is the point of correction that you need; the light is bending to the correct spot of your eye. We then measure the distance between the lines on the screen, put it through our algorithm and create the measurement of your eye for that step. It's repeated 8 more times to measure the cylinder and axis based on the meridians of your eye. If you want more information, please contact our support team or take a look at the support section of the website.