Talk:Halogen lamp
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[edit] Questions
1. If a lamp is referred to as "halogen burner" does that mean it is incandescent or metal halide?
2. What's the temperature range of the filament? Of glass? Quartz?
See reference 1 in the article, which contains a much better description of halogen lamp operation than the entry, and some answers to your questions. (Redcliffe92 (talk) 14:55, 10 March 2008 (UTC))
[edit] New
I am unhappy about this Article, which has some inaccuracies, is not very comprehensive and has limited references (ref 2 seems unobtainable) I propose to offer the revision below, retaining the sections headed "Safety" and "Handling Precautions". I am unsure about "Halogen Spectra" as the picture is too indistinct to be readable. The spectra will be fairly similar to a black body at the filament colour temperature.
Any comments would be appreciated. (Redcliffe92 (talk) 15:45, 15 March 2008 (UTC))
PROPOSED REVISION Introduction A halogen lamp is an incandescent lamp wherein a tungsten filament is sealed into a compact transparent envelope filled with an inert gas, which may be argon, nitrogen, krypton or xenon or a mixture, plus a small amount of halogen such as iodine or bromine. The bulb stays clean throughout life. The halogen lamp can operate its filament at a higher temperature than in a standard gas filled lamp of similar wattage without loss of operating life. This gives it a higher efficacy (10-30%) . It also gives light of a higher colour temperature (whiter). Alternatively it may be designed to have perhaps twice the life with the same or slightly higher efficacy. Because of their smaller size halogen lamps can advantageously be used with optical systems that are more efficient.
Principle of operation The function of the halogen is to set up a reversible chemical reaction with the tungsten evaporating from the filament. . In ordinary incandescent lamps this tungsten is deposited on the bulb. The halogen cycle keeps the bulb clean and the light output remains almost constant throughout life. At moderate temperatures the halogen reacts with the evaporating tungsten, the halide formed being moved around in the inert gas filling. At some time it will reach higher temperature regions, where it dissociates, releasing tungsten and freeing the halogen to repeat the process. In order for the reaction to operate, the bulb temperature, all over, must be higher than in conventional incandescent lamps, necessitating the use of fused silica (quartz) or a high melting point glass (such as aluminosilicate). Quartz being very strong, the gas pressure can be higher, which reduces the rate of evaporation of the filament, permitting it to run a higher temperature (and so efficacy) for the same average life. The tungsten released in hotter regions does not generally deposit on the hotter parts of the filament where failure will eventually take place. Regeneration of the filament is possible with fluorine, but its chemical activity is so great that other parts of the lamp are attacked. .[1][2]
The first commercial lamps used elemental iodine, and were known as known as Quartz Iodine Lamps, launched by GE in 1959. [3] [4] Later bromine was found to have advantages, but was not used in elemental form., Certain carbon hydrogen bromine compounds give good results. [5] [6] . The first lamps used only tungsten for filament supports, but in some designs it has been possible to use molybdenum - an example being the molybdenum shield in the H4 twin filament headlight for the European Asymmetric Passing Beam.
High temperature filaments emit some energy in the UV region, and doped quartz or optical coatings can be used to block most of the UV, if this is harmful - where people or sensitive material are directly exposed to the lamps. Hard glass blocks UV and has been used extensively for the bulbs of car headlights. [7] Alternatively the halogen lamp can be mounted inside an outer bulb, similar to an ordinary incandescent lamp, which also reduces the risks from the high bulb temperature. Undoped quartz halogen lamps are used in some scientific, medical and dental instruments as a UV-B source.
For a fixed wattage and life, the efficacy of all incandescent lamps increases for lower supply voltages Halogen lamps made for 12 to 24 Volt operation have good light outputs, and the very compact filaments, possible because the voltages gradients are low, are particularly beneficial for optical control. The range of MR16 (50 mm diameter) reflector lamps of 20W to 50W were originally conceived for the projection of 8 mm film, but are now widely used for display lighting and in the home. More recently the wider beam versions are available designed for direct use on supply voltages of 120 or 230V. [8]
The MR16 series uses a multilayer heat transmitting, light reflecting film on the reflector. The idea of putting a light transmitting, heat reflecting film on the bulb has been the subject of research for many years. Lamps are now available using this IRC (Infra- red Reflective Coating) which are generally described as "energy saving" since they offer similar performance for about 30% lower wattage consumption. [9] [10].
Voltage, Light Output and Lifetime
Tungsten halogen lamps behave in a similar manor to Incandescent Lamps, but the light output is reported as proportional to voltage to the power 3 and the efficacy proportional to the power 1.3 [11] The normal relationship regarding life is that it is proportional to voltage to the -14. Halogen lamps are manufactured with a halogen dose to match the rate of tungsten evaporation at their design voltage. Increasing the applied voltage increases the rate of evaporation, so at some point there may be insufficient halogen and the lamp goes black, so over-volting is not generally recommended.. With a reduced voltage the evaporation is lower and there may be too much halogen, which can lead to abnormal failure. At very much lower voltages the bulb temperature may be too low to support the halogen cycle, but by this time the evaporation rate is too low for the bulb to blacken significantly. There are many situations where halogen lamps are dimmed successfully, but whether the very extended lives predicted by the formula are achieved is doubtful, and is in any case dependent on the lamp construction, the halogen additive used and whether dimming is normally expected for this type.
