Satco S3130 2100K 35-Watt Clear Medium Base ED17 High Pressure Sodium Lamp, 5.4 x 2.2 x 2.2 Inch

The operational concept behind HID lamps is very similar to that of fluorescent lamps. Electrodes are positioned at either end of a tube, whose chamber is filled with gas and metals such as mercury. An electrical charge passes from one electrode to the other. In fluorescent lamps, this charge creates ultraviolet (UV) light, which converts to visible light once it passes through the phosphors on the tube's interior. In an HID lamp, the electrical arc, gasses and metals are contained in what is known as the arc tub. The arc tube is made from either quartz (used in mercury vapor and metal halide lamps) or transparent ceramic (used in high-pressure sodium lamps because of their high temperature). All arc tubes are housed within a larger outer glass envelope. The biggest difference in HID lamps is the fact that they need a start-up time to reach their full brightness. This usually takes five to ten minutes during which time the lamp will flicker until the metal inside fully vaporizes and the lamp reaches it's full operating temperature.