Each Android smartphone has different types of hardware; their specifications are not universal to each other smartphone on the market. Over the past year, EyeQue has been testing many Android-based smartphones and has come up with a universal calibration for Android devices. The primary factor that EyeQue calibrates the Personal Vision Tracker to is the pixel per inch density (PPI), or the space between the pixels, of the smartphone’s screen. Our software and optical teams have devised a universal calibration to work with Android smartphones that have a PPI greater than 250. That means no matter the model, manufacturer or type of Android smartphone you will be able to use the PVT as long as the PPI is higher than 250. After testing, we found that those smartphones with a PPI lower than 250 can have a result that is a full diopter or more off making it less accurate and unreliable. If you do not know your smartphone's PPI you can use this PPI calculator located here to determine the PPI of your screen. All you need to know if your screen size and the maximum resolution of your screen.