Incandescent

Genesis of incandescent bulbs

Incandescent means the production of light through heat. Essentially, anything hot enough to be seen by the naked eye is considered to be incandescent, or incandescing.

The most evocative examples of incandescence are the quietly flickering candle flame, the gently swaying oil lantern, or storm lantern. A more technically advanced favourite of the outdoor lover, is the pressurised lamp, commonly known as the Tilley lamp. Although these examples have fared well with the test of time some forms of incandescent lighting have been relegated to the past.

An example is the extremely high-powered incandescent light source generated by using a hot flame on calcium carbonate. Calcium carbonated was originally known as quicklime. This convenient and high-powered light was used in early theatres and was known as limelight, echoing the quicklime component. With the advent of reliable high-powered electric lights the much more dangerous limelight was phased out but the term affectionately persisted. To this day, it still means to be famous and in full public view even if only briefly, hinting at its earlier theatrical heritage.

The alternative, and arguably safer incandescent process, is heat produced by electric consumption. Electricity is passed through a resistive filament which then heats as current is passed through it. Interestingly, a tungsten filament has a positive resistance characteristic which allows for a natural regulation, it's an inherent property. Consequently, an incandescent bulb can be safely plugged directly into the mains or other power supply, assuming that it was designed to work in that particular set up or environment. This adds to the cheapness and simplicity in manufacture of filament bulbs. Incandescent bulbs are simple to produce and the technology has been refined since its invention in 1835.

A positive resistance characteristic means the hotter the filament gets, the higher the resistance presented to the electricity flowing through it. This allows the bulb to inherently control its current flow and consequently the power consumed. If the current increases then the filament will get hotter. The additional heat increases the resistance of the filament. As the voltage is normally stable, the current must decrease proportionally until the trinity of heat, resistance and current reach a happy equilibrium. As power is directly related to the product of resistance and current squared, this inherent regulatory characteristic is a very useful attribute. This property is termed as, “self regulation.” A reduction in voltage will still result in a reduction of power consumed and consequently the light output.

So filament bulbs have positive resistance characteristic. Unlike other forms of lighting such as Compact Fluorescent Lamps(CFL), which have a negative resistance characteristic, thus need to be carefully and externally regulated. A negative resistance means that once energised the resistance decreases with temperature. Essentially, the hotter the substance gets the lower the resistance presented to the current flow resulting in more current flow and its associated increase in temperature. This positive cycle results in the sky rocketing of both temperature and current with the consequential plummeting of resistance, usually resulting in either hitting an externally defined boundary or failure. The technical term for such a scenario is called, “thermal runaway.” In the case of fluorescent lights the externally defined boundary which regulates power is called the ballast. This power regulation could be in the bulb body but it is nearly always away from where the light is actually generated in the bulb, thus adding unwanted bulk.

Miscellaneous incandescent bulbs

The most ubiquitous example of incandescent lighting is the tungsten filament light bulb, otherwise known as the incandescent bulb or sometimes the tungsten bulb. The light bulb's filament has a long history of development from the primitive, carbon in the form of paper, through composite alloys, to finally double wound tungsten. Incandescent light bulbs usually come in two main types, the standard tungsten bulb and the halogen bulb. Actually both the varieties have tungsten filament and both are incandescent. The halogen light bulb produces a whiter light with a better conversion of electricity to the desired visible light. So halogen light bulbs have a better luminous efficacy than the tungsten bulb. A halogen achieves this by burning much hotter.

Essentially electricity is passed through a resistive metal which has to get hot enough to virtually glow white hot. This presents a number of problems but the most notable is that most metals melt before the 1200°C the filament needs to reach. The melting point of tungsten is 4500°C, this makes a good choice for a filament but at those temperatures tungsten is quite reactive with the normal gasses in the earth's atmosphere. Oxygen is one of the biggest culprits, so it is necessary to remove any reactive gasses. Early bulb designs used a vacuum and extracted the maximum amount of air out of the bulb which allowed the filaments to last longer.

Tungsten may not melt but it does evaporate or sublime, which means it goes straight from a solid to a gas when given sufficient energy. The energy used to produce the heat for incandescence will give some molecules of the tungsten wire sufficient energy to evaporate. If there is a complete vacuum, the expelled tungsten will be unopposed in its journey from the filament and consequently coat the bulb's internal glass surface. Unfortunately, this process reduces the light output as well as reducing the life of the tungsten element. Taken from the effect on the early bulbs, this process is still called carbonisation and has the appearance of very fine black dusting like soot. This inevitable occlusion of the glass is solely the tungsten evaporating and condensing and has nothing to do with carbon. A workaround is to use a larger glass bulb. The larger surface area of the bulb means that the dulling effect is diluted as the same tungsten residue is distributed more thinly over a wider area.

So in a total vacuum, there would be nothing to stop the tungsten filament evaporating and being distributed elsewhere. The greater the temperature of the filament, the more effective the conversion of electricity to usable light. Sadly the higher the temperature the greater the speed of evaporation, of the material comprising the element. To reduce this process, a non reactive gas such as argon is added to the bulb. When an inert gas is added, the molecules comprising the gas impede the path of the evaporated tungsten. Essentially the tungsten molecule gets blown off the filament and hits a weighty molecule of gas checking its progress and giving it the opportunity to condense back onto the filament, rather than the surrounding glass envelope. Also as the tungsten reaches saturation in the surrounding gas, it makes further evaporation more difficult thus extending the working life of the bulb.

Gasses commonly used are nitrogen and some noble gasses. Nitrogen, which is a diatomic gas and comprises roughly 78% of the atmosphere, is very cheap and easy to obtain. The noble gasses, which all sit in a specific location in the periodic table, such as argon, krypton and xenon are monatomic but extremely non-reactive. Monatomic meaning their gas molecules are comprised of a single atom as opposed to diatomic meaning molecules are comprised of two atoms. The Nobel gasses do occur naturally in the atmosphere with argon being the 3rd most common gas at 1% of its composition. Similarly, krypton and xenon also occur naturally but in very tiny quantities. Tungsten is a weighty metal particle with an atomic weight of 183.85 and so the heavier the gas used to check its path the better. Xenon has an atomic weight of 131.29 and so it would be the best choice if it were not for its rarity and expense. Consequently the mixture of gas used in an incandescent bulb is often reflected by its cost. Most tungsten filaments bulbs are predominantly argon and nitrogen filled. xenon and krypton filled incandescent bulbs are often seen in quality flashlight or car bulbs.

As the gas is used to block the path of the tungsten it would be beneficial if there was more of it. The way to achieve this is to pressurise the gas. Pressurising gas to any significant pressure is difficult with a large bulb but a smaller bulb is much easier to pressurise as it is stronger and the effective expansion energy in the gas is smaller.

Xenon filled halogen headlamp bulb

Still the higher the temperature the more efficient the conversion to visible light. Estimates suggest only 5% of energy is emitted as visible light and the rest is radiated as invisible infrared light and heat. So, to allow the element to burn at a higher temperature a method to help recycle the tungsten is required. This recycling process is what is accomplished within a Halogen bulb.

Read about the genesis of the Halogen bulb

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