A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen, who developed the chart in 1862. Select a vertical surface you want to pin the chart on, step 6 meters away from it, direct the rear camera towards the surface and start the program. After the chart is loaded, you can change its position and size using the control panel or right on the screen with the help of drag-and-drop, rotation (with two fingers) or zooming in/out (with two fingers) gestures. If you want to change the vertical surface, tap the button with two round arrows. You can disable help texts or sound using the corresponding buttons.