LCD backlights serve as the illumination source for LCDs. LCD backlights frequently employ fluorescent lamps. Fluorescent lamps consist of a phosphor coated glass cylinder with cathodes at each end. The cylinder is filled with argon gas and mercury. When the proper current is supplied to the cathodes, an arc is formed in the gas. The arc produces ultraviolet light (UV), which excites the phosphor on the glass. The excited phosphor produces visible light which is usually white. The color pixels in an LCD are produced by color filters at each subpixel.