Windshield glass contains a black enamel band (called the frit) around the periphery that is baked into the glass. This black band includes a border of dots. The band has an etched surface to enable adhesive to bond to the glass. When car manufacturers install the windshield, they bond the windshield to the vehicle with adhesive placed on the etched part of the black enamel glass area (inside surface). The outside of the enamel band shades the adhesive from the Sun and protects it from ultraviolet radiation. The band also serves a cosmetic purpose by covering up the adhesive and gives the windshield edge "a more finished look."
The black dots blend the border into the glass

Rather than a straight black edge, the dot border blends the solid band into the clear glass. Some windshields also use the dots as a third sun visor to block the Sun behind the rearview mirror where most visors don't reach.
Source(s):
http://www.wonderquest.com/windshield-dots.htm


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