A screen protector will affect your result; the Personal Vision Tracker is a high precision optical device with a 0.25 diopter resolution. A screen protector with a thickness of around 1/100 of an inch (or 0.25mm) can change the result up to 0.25 diopters. We do compensate the results for users with screen protectors, however, screen protector thickness varies significantly. This can make it impossible to completely compensate for the offset introduced by your screen protector. If you are using a screen protector on your smartphone, your results may be off by 0.25 to 0.5 diopters. We strongly recommend removing your screen protector and cleaning the screen before taking a test.