First of all, look at the mask the appearance of the production process, the details do how can reflect the company's production capacity and emphasis. In addition, you can look into the incandescent light is too weak (the lighter, does not necessarily make him a very sensitive reaction, LED flashlight can not let dimmer mask black). Facing the incandescent when he will turn black, then you can adjust the mask side of the knob to adjust the darkness of the LCD screen to observe the darkness of the LCD uniform changes the entire screen (the inferior LCD screen he is not the whole screen evenly change black, but there will be a black line in the middle of the LCD screen as moving with the knob to adjust up and down or left and right). In addition, you can observe the LCD screen edge welded to see if it will find in the middle of the LCD screen will be black, the edge of the LCD screen will be much brighter phenomenon, this phenomenon is the poor quality of the LCD screen LCD screen with a production . Serve to protect the poor quality LCD screen on the eye, but will hurt the vision of the eye, causing severe phenomenon nearsighted or astigmatism. Please choose carefully-darkening mask.