Incandescent lighting has been around for a long time...a really, really long time by technological standards! With Edison's first successful tests being carried out in 1879. Whether or not Edison actually invented the first incandescent light bulb is a pretty hotly debated topic, as others, such as in 1854 by Heinrich Goebel, and of course in the mid 1850s the first prototypes being made by Joseph Swan. Edison however WAS the first to produce one which was commercially viable, both with a workable lumen output and a sufficiently long lifetime to make it useful. The aforementioned debate isn't something which I'm going into here, all we're going to talk about is the technology itself. It has to say something for a design when it's remained fundamentally unchanged since the early 1900s through into the 21st century, inefficient or not.
The earliest lamps employed carbon filaments in a vacuum. These lamps were hugely inefficient due to the fact that the filament temperature had to be kept very low due to the fact that carbon sublimes rapidly in a vacuum at higher temperatures. This meant that if the filament temperature was too high, the lamps would not only suffer from a short lifetime, but the lamp would have dimmed excessively throughout its lifetime due to evaporated carbon from the filament depositing itself on the inside of the bulb, blackening it and blocking light from the filament.
Around 1905, metal started to get used as a filament material, firstly tantalum, then pressed, drawn and finally coiled tungsten - which is what we are still using in incandescent lamps to this day.
Filling the bulb with a gas rather than having the filament run in a vacuum was a way of decreasing the rate of evaporation, as the gas (which generally is argon - xenon and krypton can also be used, but the high cost of these gases limits their use in lamps to those of higher power ratings) causes the evaporated filament material to be "bounced" back onto the surface of the filament. This extends the life of the lamp somewhat, but also has a negative effect on the efficacy, as it increases thermal loses from the filament.
Halogen incandescent lamps are an evolution of the above technology, where a halogen vapour (often iodine) is added to the mix. This has the effect of chemically "scrubbing" the evaporated filament material from the inner wall of the bulb, and depositing it back on the surface of the filament. This allows the filament to be run hotter, and in the conjunction with an outer envelope made from quartz, allows more compact lamps to be produced. In fact, they *have* to be smaller in general terms, as for the halogen cycle to operate correctly, the wall temperature of the bulb needs to kept at around 650°C. Any significant deviation from this temperature will reduce the effectiveness of the cycle, causing the lamp to blacken and burn out excessively quickly. The added bonus of halogen lamps, is that in increasing the operating temperatures, the relative spectral output is shifted towards the blue end of the spectrum, reducing the ratio of radiation in the infrared. This has the effect of increasing the ratio of visible light to infrared emission, increasing the luminous efficacy substantially. However, even the most efficient halogen incandescent lamps are far, far behind even the cheap branded CFLs.
Nothing however yet offers the convenience of incandescent lighting. Instant light with no* warm-up time, ability to turn the lamp on or off at will, hot or cold, a low purchase price, and the ability to run with no special control gear**.
* Well...There is warm up time...but I think fractions of a second as far as switching a light on are concerned are irrelevant!
**Low voltage
halogens being the exception, needing a simple step down transformer.
11th June 2023: Re-ordered page listing alphabetically to make it easier to navigate & made some code changes in the background.
