Quickfire Question: How do LEDs work?

LEDs, or Light Emitting Diodes are very common devices used in a wide variety of applications from some street signage, power indicators, transmitters in remote controls and even LED torches. They are very efficient devices, which much like sodium lamps, convert most of the current passing through them into light, with very little loss as heat, but how do they work?

All LEDs are made from semiconductor materials - materials which have conducting properties somewhere between insulators (like glass) and conductors (like metals). Semiconductors can be carefully constructed to perform a variety of applications, such as diodes, which only allow current to pass through in one direction, to transistors - which either allow current to pass, or stop it, depending on the voltage at a "gate", solar cells and much more elaborate structures ranging from logic circuits all the way up to computer chips.

Like the previously mentioned diodes, LEDs only allow current to pass through in one direction, and when the current passes through, light is emitted. There are a couple of ways that LEDs can be constructed, I will concentrate on the simplest.

Semiconductors can be "doped" with other materials, which can either donate electrons (n-type semiconductors), or can accept electrons (p-type semiconductors). The former have extra electrons which can flow through the semiconductor from the negative to the positive terminal, and the latter have "holes" which are like positively charged electrons, that flow from the positive to the negative terminal. When the two meet they can recombine, and release energy in the form of light.

By varying the dopants, we can manipulate the wavelength of light that is emitted. Other methods of varying the colour are more elaborate, and involve the use of quantum tunneling, different sorts of junctions, and even adding additional materials. As we can see, the spectrum of conventional LEDs tends to be very pure:

producing a very limited spread of colour in each LED, however the addition of other chemicals such as phosphors, can "down convert" high frequency light such as blue, and re-emit that light in a broader spread of wavelengths. This is a common method of producing white LEDs.

More recently another type of LED has been developed, known as an Organic LED, or OLED. In place of inorganic materials such as Indium and Gallium, OLEDs use carbon based chemicals (hence organic) that emit light. The semiconductor properties of these materials are similar, though the emission is somewhat different, having a much broader spectrum. Some of the details and issues surrounding OLEDs will be covered in a later post.

Quickfire Question: How do incandescent (filament) bulbs work?

We are all familiar with incandescent bulbs, which have been until relatively recently been the most popular sort of bulb.

A voltage is placed across a metal filament held in an inert gas like argon, neon or nitrogen, in order to stop the gas from reacting with the filament and allowing the bulb to live longer. The filament has a high resistance to the current flowing through it, and this heats the filament, causing the atoms in the filament to vibrate. As the atoms vibrate, they then radiate energy in the form of light.

An important point that can be made here, is that all vibrating atoms and molecules will radiate somewhere on the electromagnetic spectrum. The hotter they are, the faster they vibrate, and thus the higher the energy (and frequency) the photons are that they emit.

A problem with this sort of lamp, is that they are very inefficient. Because the light is generated by heating, large amounts of energy is lost in unwanted heat. Also as the filament is heated, it slowly evaporates over time and eventually breaks, leading to a relatively short lifetime. It is mainly for these reasons that there is a move to using energy saving bulbs, which both have a longer life, and also produce the same amount of light for less energy input.

Quickfire Question: Why are street lamps amber?

We are familiar with the colour of many street lamps, the amber glow of the sodium vapour:

But how do these lamps work, why are they the colour that they are, and why do we use them?

(1) How do Sodium Lamps Work

Inside the tube, there is a small amount of sodium metal, with a little neon and argon gas in there. A voltage is applied across the tube, and this excites the outer electrons in the neon and argon gas warming the sodium and also being responsible for the faint red glow that you can see before the light turns on fully. Eventually the sodium vaporizes and due to the voltage, its outer electrons too are excited. When those electrons spontaneously drop back into their ground state, they emit light at a very particular wavelength - the amber light that we see.

(2) Why are Sodium Lamps that colour?

Only one electron is excited by the electrical discharge, and only with enough energy to jump up one energy level, so when they drop back to the ground state they can only emit a very pure colour of light. The wavelength of sodium light is 589.3nm. (actually there is a little more going on, meaning there are two lines very close to one another at 589.0 and 589.6nm). This is known as monochromatic light, and it is also the reason it is hard to pick out any colour of objects lit by a sodium lamp alone.

(3) Why do we use sodium lamps?

There are a number of reasons, and I will outline a few: Sodium lamps are very efficient, because most of the energy is turned straight into light, unlike incandescent filament lamps, which turn a lot of the energy into heat. That wavelength is also pretty close to the optimal response of the human eye:

This graph (called the scotopic response curve) shows how the eye responds (side axis) to different wavelengths of light (on the bottom axis). We can see the sodium lamp is pretty close to the middle. That means that a lower level of light is needed to see things clearly.

They also use no mercury or dangerous metals, and so are easily disposed of, keeping the cost down even further.