Screen protector is the most common cell phone accessory you can find on the market. And most cell phone owners get a screen protector for their phone to protect the screen, especially with smartphones with touchscreens. There're thousands of manufacturers around the world producing all kinds of screen protectors with different pricetags. Then do you have any idea how to pick the high quality screen protectors from the bad ones?Actually there are several ways to test a screen protector. The first is to test its transparency. Screen protectors are designed to be attached on the screen of a cell phone, so it has to be transparent enough to allow users see through it. However, light would become weak after going through a medium because some of the lights would be lost during transmission. And due to the restriction of handcraft limit, the max transparency of the current optical glass is 97% or so. But a screen protector made of plastic materials cannot reach the transparency of 97%. Therefore, you should take a second thought when a seller pushes you a screen protector with transparency of 95% or higher. The best screen protectors on the market have the transparency between 92% to 95%.The second way to test a screen protector is to test its abrasion resistance. As the protector of a cell phone screen, it is supposed to resist the wear and abrasion from fingernails and other harsh things like keys maybe. Therefore, a screen protector with higher abrasion resistance would be better. However, it is not cool to judge a screen protector only by the abrasion resistance. The higher abrasion a screen protector could resist, the more rainbow glare would be. The best abrasion resistance would be between 3.5H to 4H.The third you should consider when choosing a screen protector would be the size and quality. It is better to get a screen protector that is specially designed for your cell phone. It would fit your cell phone better. For instance, if you own an iphone 4s, then it is better to get a specially designed iphone 4s screen protector or iphone 4 screen protector other than a universal one. and an excellent screen protector should be easy to attach and easy to remove, without any residuals on your cell phone's screens.Another thing you should know about screen protectors is that, some cell phones screens already have great abrasion resistance that no need to put another plastic layers to protect them. iPhone is a great example. The screen of iphone is made of high quality glass which is durable enough for everyday use, so some experts say there's no need to use any screen protectors. Well, that's really your choice here.