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What you should know about HBLEDs (Part 1)

14 Jul 2014  | Ed Rodriguez

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Gaseous discharge lamps

Gaseous discharge covers neon, low-pressure sodium, fluorescent, and high intensity discharge (HID lamps). Within the HID category, there are mercury vapour, metal halide, induction, and high pressure sodium types.

For the sake of simplicity, we'll bypass a discussion of neon (known best for signs) and low pressure sodium (which has fallen out of favour because of its extremely poor colour rendering (it makes everything look brown). All gaseous discharge lamps operate on the principle that a controlled arc-over must be created across the length of the gas-filled tube. This ignition is facilitated by a ballast (which may be of either the autotransformer or electronic type) to create a momentarily high starting voltage. (Consider a gaseous-discharge lamp to be a fancy version of a sparkplug in a vacuum bottle).

The fluorescent lamp principles were discovered in the 1800s but not refined into commercially useful form until the 1930s and, early on, by GE. The fluorescent lamp is noted for its exceptional energy efficiency and long life. Generally having five to six times more lumens per watt than an incandescent, the fluorescent also can last 10 to 15 times longer. Unlike an incandescent, fluorescent lifetime is degraded by very frequent on/off switching and the attendant physical deterioration of the electrodes caused by that switching. A simplified explanation is that every time it is switched on, a microscopic amount of material gets knocked off the electrode.

Finally, fluorescent lamp lumen output is quite sensitive to temperature and even requires a higher ignition voltage to come on at all when temperatures drop substantially. But despite their shortcomings, the popular 4ft fluorescent tube remains an extraordinarily cost-effective, energy efficient light source for larger indoor areas.

The high-pressure mercury vapour lamp was an extension of the basic fluorescent lamp. A much smaller arc, under high pressure and higher temperature, allowed a much more compact, higher power lamp to be created. Fifty years ago, the mercury vapour streetlight became commonplace, in spite of its less than desirable bluish tint. Like the fluorescent, it provided over five times the energy efficiency and 15 times the life of an equivalent incandescent bulb. Consequently, it's no surprise that many municipalities and factories moved towards these lamps.

But, unlike fluorescents, mercury vapour lamps have a five-minute or more warm-up period and when turned off, must first cool down before being receptive to turn-on again.

That delay, called the re-strike time, is caused by the very hot gas requiring a far higher ignition voltage than the ballast has been designed for. After some cooling, the required ignition voltage drops to where the lamp can operate.

The inconvenience caused by long warm-up and re-strike delays has dogged the HID industry for decades and has prevented these extraordinarily efficient, compact, long-life lamps from being used in far more places. The mercury vapour lamp also has carried the baggage of having that bluish tint, which has been an issue when good colour rendering has been desired.


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