Fluorescent lights are used in many different situations for lighting purposes. In recent years, small-sized bulb-shaped fluorescent lamps have been developed generally comparable in size with normal incandescent lamps, and demand for bulb-shaped fluorescent lamps which can be substituted for normal incandescent lamps is increasing. Fluorescent light bulbs have become popular among consumers for a variety of reasons. They generate less heat than incandescent bulbs, typically have a longer operating life than fluorescent bulbs, and typically are less expensive to operate than incandescent bulbs. In addition, fluorescent bulbs provide soft, even lighting in comparison to the bright "hot spots" exhibited in most incandescent bulbs. A typical bulb-shaped fluorescent lamp accommodates a fluorescent tube and an electric lighting circuit inside an outer housing including a globe and a case. The inner surface of the wall of the tube is coated with a light-emitting substance, usually a fluorescent or phosphorescent metallic salt, such as calcium tongstate, zinc sulphide, or zinc cylocate. By suitable choice of the fluorescent substance, this light can be given any desired color. The tube is filled with mercury vapor at extremely low pressure. The electrons ejected from the incondensate electrodes will collide with the mercury atoms and cause these to emit radiation which consists, for the most part, of ultraviolet rays which are invisible. While fluorescent bulbs can come in a variety of sizes and shapes, the majority of bulbs are either 13/8 inches in diameter and either 48 inches, 72 inches, or 96 inches long.
