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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 tube. 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.
Unlike fluorescent lamps, the arc tube of HID lamps is filled with gas at a very high rate of pressure (up to 50 psi). This allows the electrical arc created by the electrodes to operate in the visible part of the spectrum, producing usable light without the addition of phosphors.
Like fluorescents, HID lamps require a ballast to control the electrical current in the arc tube. Certain HID lamps also require an ignitor, which produces a
high voltage to pulse the arc tube, allowing the arc to strike.
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 10 minutes, during which time the lamp will flicker until the metal inside fully vaporizes and the lamp reaches is full operating temperature.
New HID lamps require a “burn in” period of approximately 100 hours before the lamp will reach its true specified color. Until this process is completed, lamps can be unstable and vary in color.
As HID lamps age, chemical changes occur that cause color shifting. The shifts vary depending on the lamp type. Standard probe start lamps tend to shift
color about twice as much as pulse start lamps. Results vary from lamp to lamp. It is recommended to conduct a group relamping once it is determined that
a lamp or group of lamps is changing color or failing. This way, the area being illuminated will maintain an even balance of color and light.
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