Optical bonding is the use of an optical-grade adhesive to glue a glass to the top surface of a display. The main goal of optical bonding is to improve the display performance under outdoor environments. This method eliminates the air gap between the cover glass and the display.  Moreover, anti-reflective coating is often used in optical bonding glass. The real problem for display readability in outdoor environments is not the displays brightness but its contrast. Contrast means the ratio of the white level to the black level; in other words, the contrast ratio of display means the difference of light intensity between the brightest white pixel and the darkest black pixel. The main purpose of optical bonding is to increase the displays contrast ratio by reducing the amount of reflected ambient light.