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David Dupont
David Dupont

Light has always had an undeniable attraction. It allows us to see, it helps plants grow through photosynthesis and it brightens up our days. Every year major cities organise festivals of light or “fête de la lumière” as the famous festival is called in Lyon, France. Light hypnotizes us, triggering  our imagination, just as it did  that of many painters and photographers before us. In this blog we will look at the science behind this incredible phenomenon and give you an overview of the innovations we have been through to be able to generate light on demand.

We will skip the invention of fire, candles and oil lamps, although these were also remarkable inventions for their time, and fast-forward to the first incandescent light bulb – the one that most of the readers have grown up with. We will explain why the transition to the fluorescent (energy-saving) light bulb and LED lighting was an important step and give some examples of what the future of lighting may hold.

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Light, or more specifically visible light, is in fact electromagnetic radiation with wavelengths between 390 and 700 nm. Every wavelength is observed as a particular colour by the human eye and a blend of all the colours is perceived as white light.

 

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But there is a lot more radiation floating around in the air than just the visible light. Radiation is actually nothing more than a form of energy. Radiation can be categorised from small to large wavelengths, which corresponds to a decrease in energy. It is very intuitive to associate this decrease in energy with how we perceive the different types of radiation in the real world. Gamma radiation can kill any form of life immediately, X-ray radiation is still very dangerous but can be used for short diagnostic purposes, UV radiation is safer, but can still burn our skin on a warm summer day, visible light can make plants grow, infrared light is felt as comfortable heat, microwave radiation can be used to heat up chicken soup and radio waves are used to listen to the radio!

 

 spectrum

The whole spectrum: visible light is only a very small part of the truth!

 

Incandescent light: the black-body radiation

Now that the concept of light is no longer a mystery, let’s move on to practical things. Incandescent light bulbs were the first commercial electrical light bulbs. They  emit a broad-spectrum white light by heating up a filament wire made from a heat resistant compound like tungsten or osmium. It is surprising to note that even radioactive thorium oxide was once used because of its very high melting point! The bulb is also filled with an inert gas to avoid oxidation of the wire. The very high heat causes the wire to emit some of its energy as electromagnetic radiation. This phenomenon is called black-body radiation and is also found in stars. The emitted radiation consists of visible light and infrared radiation, which has longer wavelengths and is felt by us as “heat”. Unfortunately this means that this type of light bulb generates a lot of heat as a side product, which is why they consume so much energy. For lights we can state that the energy we input is transformed into light and heat. The more the energy is turned into light, the higher the efficiency of the light bulb.

 

ENERGY = LIGHT + HEAT

Fluorescent light: the rainbow effect

 

Everyone knows the delightful feeling of touching a hot incandescent light bulb after a few hours…a mistake you won’t make twice!

This problem was partially resolved when the fluorescent light bulb made its appearance around 1980. This light bulb uses a different mechanism which emits much less infrared radiation and is therefore much more energy efficient. The light is no longer generated by heating up a material, but by a phenomenon called “fluorescence”, which transforms UV light into visible light of a certain colour with high efficiency. A blend of blue, green and red phosphors is used to achieve the desired white light. An electrical discharge bombards the mercury gas in the lamp with electrons, generating UV light that is then turned into visible light by the rare-earth phosphors coated on the inside of the light bulb.

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Inside a fluorescent lamp

 

This mechanism is much more energy efficient and has therefore gradually replaced incandescent light bulbs. In 2008 the European Union passed a law to phase out incandescent light bulbs by 2012. However, the problem with fluorescent lights is that they contain critical rare-earth elements and mercury, making their recycling essential. Without proper disposal and recycling of these lights, the mercury would pose a big threat to the environment. Most countries have therefore put in place collection mechanisms, and some companies like Solvay have started recycling the used phosphor powders to retrieve the critical and valuable rare earths from these powders. This is a good example of urban mining, with a closed product life cycle.

 

LEDs: the inversed solar panels

 

The newest generation of lighting solutions are so-called “light emitting diodes” or LEDs. These are even more energy efficient and barely generate heat. Even after hours of illumination time, a LED can be touched without any problem, which means that the conversion of energy to light is very efficient. An individual LED does not provide a lot of light, which is why large amounts of LEDs are combined to make light bulbs. LEDs can be thought of as inversed solar panels. While solar panels turn light into electricity, LEDs turn electricity into light using the opposite process. This technology is based on semiconductors (e.g. silicon), and many different colours of LEDs can be manufactured using a variety of different materials. The principle is that electricity is used to create charges in the semiconductor (diode), and when the positive and negative charges are recombined elsewhere in the material, energy is released in the form of light of a certain wavelength (colour).

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 LED rainbow

LEDs are still relatively expensive, but their very long lifetime makes up for this. An LED can have a life of  up to 20 years, while fluorescent and incandescent light bulbs have to be replaced much more often. LEDs haven’t replaced fluorescent light bulbs yet, but they have found widespread application in TVs, computer screens and household appliances. Unfortunately, certain LEDs require rare elements like indium or toxic elements like antimony, selenium or tellurium. OLEDs have therefore made an appearance and are used today in (expensive) TVs. OLEDs or “organic-LEDs”, don’t use the same toxic metals, can be made much thinner and can render more realistic colours. The  deep rendering of the colour black, especially, explains their use in high-end TVs.

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LED (left) vs OLED (right)

  The lighting market is clearly a huge and very dynamic market where new technologies and improvements are emerging every year. There is a bright  future  for  scientists or manufacturers who  can develop more efficient materials, better colours, longer lifetimes and lower ecological footprints to bring light to  all corners of the world.

 

Image credits

http://www.qu3.nl

http://www.worldwidetelescope.org/Interact/?Spectrum

http://home.howstuffworks.com/fluorescent-lamp2.htm

http://steeph-k.deviantart.com/art/LED-Rainbow-395233739

http://www.digitaltrends.com/wp-content/uploads/2013/08/led-lcd-vs-oled.jpg