Eye care vision chart principle: It makes use of psychological principles of space perception, so that a 2-dimensional figure can be viewed as a 3-dimensional extension. Prolonged visual exposure to close objects causes visual fatigue. It is recommended to gaze at the chart for 25 seconds to relief visual fatigue.Soft green desktop background color is recommended by ophthalmologist to effectively alleviate eye fatigue.For android system version 2.2 or above , you can install this application to SD card (App 2 SD, APP2SD, APP TO SD) .***Important***In order to keep the app 100% free, you will receive the following