What are Brewsters Fringes? Ever wondered why your windows sometimes display an oily rainbow effect when viewed in certain lights? Well wonder no more. What you’re seeing is the optical phenomenon known as “Brewster’s Fringe’s”. So what are Brewsters Fringes? In this short piece, we will explain what Brewster’s Fringes are, what causes them and why you need not worry about their effect on the performance of your windows.