 This is done only on a certain percentage of the lamp, such as 210 degrees or 180 degrees, so that the remaining lamp is NOT coated.After this coating has dried or has been treated to ensure it will stick to the surface of the glass (using heat, for example) the lamp is coated on the inside with the phosphor blend as usual.Anywhere from 3 to 5 different chemicals are typically used in a blend, with the actual proportions and chemicals closely guarded as trade secrets.