Incandescence, Tungesten, Quartz, and Halogen

[gafftaper]

Recently in a thread on the future of incandescent lamps, I tried to explain these terms in a way that was quick and easy. It was also late at night and I was in the middle of tech week for Cabaret. In the process I oversimplified and didn't get some of the information right. Rather than edit and bury the topic in another thread. It thought it was a topic worthy of it's own thread so here we go.

Incandescence: When an object is heated to the point that it glows, it is producing light by incandescence. A modern incandescent lamp has a metal filament through which electricity is passed. Due to electrical resistance in the filament, the metal heats up and begins to glow.

The Filament: In 1879, Thomas Edison invented the "light bulb". His first filament was made of partially charred bamboo... it only lasted a few seconds before it burned up. It took 28 years of experimenting until the perfect filament material was discovered, Tungsten. Tungsten, and it's alloys, provide the maximum combination of both light emission and durability. (Useless fact: the other wires inside the lamp that hold the filament in place are typically molybdenum).

The "Glass" Envelope: The outer bulb part of a lamp is called the envelope. Some household lamps are still made of glass. However the high temperature of theatrical lighting is too much for typical glass. Our lamps are made of a much stronger fused silica material called quartz. Quartz doesn't soften until it reaches 1600 degrees Celsius... it's very tough stuff. Touching the lamp with your bare fingers can leave enough oil that it will boil the surface of the lamp and cause it to fail very rapidly. Some people keep a pair of thin cotton gloves just for changing lamps, sometimes you can use the plastic or foam material the lamp is packed in to hold it. If you do touch the lamp clean it with alcohol. I keep a box of the small medical alcohol wipes they use when giving you an injection near bye. They are only a few dollars for 100 individually wrapped pads.

Gases inside the Lamp: Edison knew that normal oxygen rich atmospheric gas caused the rapid destruction of the filament, so his experimental lamps were usually vacuum sealed. In 1913, it was discovered that filling the lamp with a inert gases not found in typical atmospheric gas could actually prolong the life of the filament. Inert gases like argon and Krypton don't react to the tungsten and cause deterioration. (Useless fact: Philips HPL's use Krypton in them.)
Later in time it was discovered that adding a Halogen to the gas mix had a unique property. As the lamp heats up, tungsten molecules are driven off of the filament by the intense heat. In a non-halogen lamp, these molecules eventually bump into the envelope and are stuck there since it is much cooler than the rest of the lamp. Over time, a dark build up on the envelope occurs and eventually the filament weakens and fails. In a halogen lamp, the Tungsten molecules combine with the halogen to make a form of salt. This new molecule floats around the lamp and once it comes back near the filament, the heat breaks the molecule apart and the tungsten is deposited back on the filament. This prolongs the life of the lamp significantly. This cycle doesn't work when lamps are dimmed, so it's good idea to run your Halogen lamps on high for a few minutes at the end of the show. So that the process can kick in and the tungsten molecules can be relocated back to the filament.

[squigish]

Do you know which halogen gas is typically used in Tungesten-Halogen lamps?

[Lightingguy32]

Tungsten Halogen lamps tend to use a mixture of Krypton-Xenon gas for the halogen cycle. How ever, most lamps are filled with the cheaper Krypton only or Argon Only blend.

[gafftaper]

A quick internet search found a great long article located here. Some of the highlights:

-Bromine is probably the most common Halogen used.

-Argon is cheap and used in common applications. Krypton and Xenon are used in more high end lamps. They are more effective at slowing down the initial evaporation of tungsten from the filament.

-The first commercial Halogen bulbs were introduced in 1959.

-A tungsten filament can reach temperatures over 3600 degrees Fahrenheit and can produce temperatures on the envelope over 1600 degrees F. (DON'T TOUCH!)

[ship]

gee, what ever happened to iodine?

[gafftaper]

Originally Posted by ship

gee, what ever happened to iodine?

From what I've read Iodine is rarely used these days except in some long life lamps. Reason? Iodine requires a bulb wall temperature 90 degrees Fahrenheit higher than Bromine does for the halogen cycle to work.

If anyone still wants to know more I found another good reference page from Sylvania.

[Kelite]

Thank you for your thoughtful posting, gafftaper. This sharing of information is what makes the online forum so valuable.

Thanks again-

[ship]

Nice post. Not sure I agree with all of it - think you are confusing low voltage lamps and fairly standard lines of them for line voltage or ANSI low voltage especially when run in series which also doesn't seem to be effected by dimming. Line voltage lamps and those in series are not effected by dimming where the halogen cycle is concerned with other low voltage lamps.

Soft Lime Glass.

Bit straying off topic when you get into bags, cotton gloves and denatured alcohol or isopropyl alcohol pads in the Quartz glass area. (As if I have any standing by way of this off topic concept.)

Bamboo don't conduct. explain.

(Inert gases like argon and Krypton don't react to the tungsten and cause deterioration. (Useless fact: Philips HPL's use Krypton in them.) ) The purpose of using innert gasses, including also xenon than is what?

Deteration is from the halogen gasses - note gasses not a single gas, is also linked to low voltage lighting in attacking the coolest parts of the lamp when not fully heated.

"so it's good idea to run your Halogen lamps on high for a few minutes at the end of the show." Would have to see what reference you found this info from, I'm thinking a wives tale. Once on the glass envelope, most likely it's not coming off. Have to better describe the Halogen Cycle in discounting dimming effects on this to appriciate this doubt I have.

Still well done and hopefully really useful in educating tech people on lamps - as a start.

[gafftaper]

As always I give a nod to Ship who knows a whole lot more about this than I do because he's deals with it every day. I'm not an expert... I'm just a guy who is good at taking what I've read and putting it in a way that is easy for others to learn.

A good deal of that info came from the relatively new book "Automated Lighting"... as I was reading the book at the time I wrote the post. Although I had several other sources like that Sylvania webpage.

As for the Bamboo filament. Apparently Edison took thin strips of Bamboo and then partially burned them... making a sort of Bamboo charcoal strip.

[ship]

no knod needed, you are working on putting it down in a way that conveys concepts better than I could hope. Just questions on my part. Hmm, automated lighting, on the shelf one of these days I'll get to reading it.