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.

Optics and Life: Strange Sight - The world in Ultra Violet

We are all familiar with rainbows, showing us the full spectrum of colour that we can see- red, orange, yellow, green, blue, indogo and violet, but the electromagnetic spectrum continues beyond both sides of the rainbow. Red is the longer wavelength (around 600nm), and longer we have infra-red (which is pretty much responsible for the radiated heat you feel from a fire or the Sun), microwaves and the longest - radio waves. Beyond violet we have ultra violet (UV), x-rays and gamma rays.

As you can see, the actual bit of the electromagnetic spectrum that we can see is very narrow. What would it be like if we could see beyond our limited range?

Well as a matter of fact, many organisms can. Indeed it is often an essential part of their lives. Pollinating insects such as bees can see into the Ultra Violet, and it is for visibility to bees that flowers have co-evolved their colours (along with the insects ability to discern them). But you might ask - if bees can see into the UV, then what do flowers look like to them? Well often they are very different indeed, here are a few examples:

This is the common dandlion - on the left is the normal visible light image, and on the right is the UV image. This is colour shifted so we can see it, but nevertheless shows us that there is a strong two tone image, with the bright part in the middle of the flower, telling the bees where the nectar is.

This one is an evening primrose. Again yellow to us, but the insects can see lines, almost like landing strips on the runway, pointing to the pollen and nectar in the center.

UV photography does require special equipment. Firstly, you need to be able to cut out the visible light using filters, and then you need detectors that are capable of imaging the UV light, and you also need lenses that can focus the light. More information can be found here:

http://photographyoftheinvisibleworld.blogspot.com/

Other organisms can see into the Infra-red. This is particularly useful, because water does not absorb infra red light so easily as other wavelengths, and so the fish can see further.

Some can even see different polarizations of light - again many bees and insects. This is particularly useful, as it allows them to see what direction they are going in, and possibly even see predators underwater.