How HID BULBS/LAMPS AND BALLASTS Work- Metal Halide, High Pressure Sodium, & Mercury Vapor

A lot of apprentices (and journeymen) have trouble understanding what makes HID lighting any different than regular lighting so troubleshooting them can be a bit confusing. Let's talk about it!

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HID lamps are easy to understand once you wrap your head around the idea that the light inside of one is produced from an arc inside of a tube. There are no metal wires inside of an HID lamp like you'd find in a standard incandescent. In fact this is the philosophy behind fluorescent tubes, neon signs, and compact-fluorescent as well. It's just an arc being struck in a tube, then the arc mixes with certain chemicals to form an arc-stream to produce visible light.

All HID lamps have a few things in common. 1) They use an arc to illuminate rather than a piece of metal. 2) They all use ballasts. 3) They are all used in relatively the same places - sports arenas/stadiums, street lighting, and parking lot lighting.

How these bulbs differ is where things get interesting:

Metal Halide
Metal Halide lamps use halide salts on the inside of the arc tube. Halide salts such as fluoride, bromide, chloride, and iodide are deposited around the inside of the inner arc-tube and when an arc is stricken through the tube, the halide salts start to vaporize into the arc stream. Each halide salt has a certain color profile that it adds to the finished light output, and a specific temperature has to be maintained in order to keep the lamp at the same color consistency. This is achieved by painting the pinched edges of the arc-tube white to reflect light back into the chamber.

Mercury Vapor
Rather than having any secondary elements in the arc tube to control the light color, a mercury vapor relies solely on the blue light emitted from vaporized mercury (which all 3 of these lamps have). What makes mercury vapor the most recognizable is the white coating around the inside of the lamp's outer envelope. Much like a fluorescent lamp, a mercury vapor lamp utilizes phosphorescence to control the color. Phosphor glows when introduced to UV light, so this coating helps to make the color of the glowing mercury less blue, and more white.

High-Pressure Sodium
HPS lamps are pretty well known for their yellowish/orange color output. This lamp operates similarly to the MH and MV lamps, however, the substances it uses to achieve its color and efficiency are hotter and require more pressure to ignite. Mercury is still inside of the inner arc-tube, however, there is also sodium which is a very hot burning substance. The arc-tubes of HPS lamps are very skinny and made from an opaque ceramic rather than quartz or glass like a MH or MV lamp would be.

CONTINUE READING FULL ARTICLE AT: https://www.electricianu.com/podcast/episode-27-hid-lamps-and-ballasts-metal-halide-high-pressure-sodium-mercury-vapor

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