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Fluorescents 101

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Vince:
We'll see this time the basics of fluorescent lamps! This article's probably gonna be huge LOL

HOW THEY WORK

The fluorescent tube is a low-pressure, low-intensity discharge lamp. The typical pressure inside a fluorescent is around 0.3% of the atmospheric pressure! Hence the term "vacuum loss"  ;D

Unlike HID lamps, a fluorescent tube will rarely draw more than 1A (there are a few exceptions though, which we'll see later).

Phosphors are very important in fluorescent tubes. While it is optional in a mercury lamp (a clear lamp still gives a usable amount of light), a fluorescent tube with no phosphor has a quite low lumen output (it does emit light, but that light is not worth using, both in terms of quantity and quality).

A fluorescent tube must first emit ultraviolet light in order to excite a coat a phosphor that will give the white light we are looking for. To emit UVs, the fluorescent needs a given mix of different gases. Historically we've used argon and mercury vapour. Newer energy-saving lamps and many T8s and T5s will also have krypton mixed with the argon. The role of the argon and/or krypton is to create an ideal atmosphere for a low intensity discharge to be maintained, because creating an 8ft long discharge in free air needs hundreds of thousands of volts! In a slimline the discharge runs at only 150V!



Here you see how a fluorescent tube is made.

The electrode is an essential part of the fluorescent, because it provides electrons to the discharge. That electrode is made of a filament, whose construction will depend of different parameters. A cathode shield and a mercury release bottle may be present.

The filament of the electrode is coated with special chemicals, usually alkali oxides, that are called "emissive substance", or "emissive material". These chemicals have the property to emit electrons in the low-pressure gaseous atmosphere when heated. Back in the 30s, this leaded to the idea of electrically preheating the electrodes to ionize the gas inside the fluorescent, then applying a high voltage to start the arc. This is the exact principle of the preheat fluorescent tube .

[Not completed yet]

merc:
Why LOL? It's a good basics for those interested in this kind of light source.

--- Quote from: Vince on November 21, 2010, 07:29:29 PM ---Phosphors are very important in fluorescent tubes. While it is optional in a mercury lamp (a clear lamp still gives a usable amount of light), a fluorescent tube with no phosphor has a quite low lumen output (it does emit light, but that light is not worth using, both in terms of quantity and quality).
[Not completed yet]

--- End quote ---
Without the phosphor and with a different glass type (quartz) they would also be like germicidal lamps, dangerous to eyes and skin. Or, is the gas mixture different and the effects less harmful?

Mike:
From what I understand, the glass tubes blocks virtually all the harmful rays so a clear fluorescent would be harmless, but the quartz tubes allows all the harmful rays to pass through.

merc:
Yes, virtually. :)
That's why MV (both clear and coated) make plastic refactors tanned (yellowed). It's not sun or heat because fixtures running HPS lamps remain clear/transparent.

I've seen a few fluorescent fixtures pretty yellowed as well but here I've no comparison to establish if that was caused by residual UV from tubes or by other factors.

Mike:
The glass envelope of the MV lamp blocks most of the UV light. HPS fixtures do get yellowed refactors too. Not as fast as MV because MV emits more UV and runs hotter than HPS. Yeah I've seen some older fluorescent diffusers yellow too.

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