The doctor will see you now…

Lux magazine’s resident technical boffin takes the fear out of lighting technology by answering the questions you are afraid to ask

What is the wavelength of light?

To discuss this we will need to talk aboutlight as a set of wavelengths.

Visiblelight is energy that the eye can detect. The energy is delivered, in simplistic terms, as a wave and the wavelength is the length of the particular wave. The diagram below shows the different colours and wavelengths and also our neighbours in the visible light ‘spectrum’ called infrared (IR) and ultraviolet (UV).

These neighbours are not visible to the eye but are well known to people who frequent tanning salons or use a remote control for their TV (most of us, certainly for the latter).

Anyway, it is important to understand these wavelengths because they are part of more traditional light sources. Fluorescent lamps, for instance, use UV light to excite phosphors that convert the light to longer wavelengths in the visible part of the spectrum.

In incandescent lamps, which are wires heated to glow at different temperatures, most of the light is emitted in the IR part of the spectrum, which is highly inefficient.

What is chromaticity?

Chromaticity is easiest to explain by looking at a colour chart.

The CIE 1931 colour chart shows pure colours around the outside (de ned by their wavelengths) and then mixes of colour within the region. The chromaticity in this ‘colour space’ is denoted by x and y. The higher the value of x the further to the right you are in colour space and the higher the value of y the higher up you are in the chart.

As can be seen, all visible colours exist in the colour space and therefore this is a useful way of describing colours of light, whether they are pure or mixed up.

Around the edge of the colour chart are the ‘pure’ or saturated colours such as those emitted by lasers, low-pressure sodium lamps or coloured LEDs. As you go towards the centre, the light becomes more mixed up.

What is white light and what does colour temperature really mean?

White light is something that the eye perceives as being within a certain chromaticity region. To explain this better we can look at the colour, or chromaticity, chart for the CIE 1931 colour space.

A special combination of this mixing creates something that the eye (and brain) reads as white.

Just like people’s teeth, there are many definitions of white – if the colour is yellow, we describe this as ‘warmer’, if it is more blue, the colour is described as ‘colder’ – just like a cosy warm fire and a cold wintry moonlit night. For a treatise on teeth whitening, please talk to your local dentist.

Because white light is so special, it has its own descriptive value called the (correlated) colour temperature, CCT or Tc (K) which is shown in the colour chart. The white light region is denoted by the dark lines and numbering from 1500 to 10000K and then the symbol , denoting infinity – or very big.

The descriptor of colour temperature or CCT comes from the fact that a special object (known as a black body) when heated to different temperatures will emit light with a chromaticity that moves along the black line, called the black-body locus or Plankian locus, as discussed in last month’s edition of Lux.

The hotter the body becomes, the higher the colour temperature. What is strange but true is that the higher the colour temperature is, the colder the colour of the light appears to us humans. The warm light of a halogen is around 3000K whereas daylight is around 6500K and beyond. This is the very cold region where LEDs used to live (and still do in certain parts of the world).

This strange anomaly is simply because our brains are used to feeling warm around the colour temperature of candlelight and relight with CCT around 2000K and feel cold in moonlight which is at a higher (colder) colour temperature.

I have seen that LEDs can emit light at many different colours but how do they emit white light?

LEDs are made up of semiconductor diodes that emit light when electric current passes through them, which, in turn, is caused by a voltage being applied across them. The voltage applied is generally related to the wavelength of the light being emitted from the LED because of the diode structure.

A shorter wavelength like blue will have a high voltage and the smallest voltages will be in the red and infra-red regions.

LEDs prefer to emit light in narrow bands of colour. The diagram above shows a red, green and blue LED all emitting together. The hump on the left hand side is from a blue LED, the one in the middle is green and the one to the right is red. When all three LEDs are shone together, they can provide a mixed light chromaticity that sits in the white region of the colour space.

By varying the strength of the RGB colour combination many different colours (or chromaticities) can be achieved. This is one way in which LEDs can make white light.

The other way which is most common in the industry is to produce fixed white light from a single LED by coupling the light from the LED into a wavelength-converting material. These materials are generally phosphors and are usually orange/yellow in colour.

That is why when you look at an LED fixture or package it will appear to have an orange or yellow material. This is not the LED diode but the phosphor that converts some of the blue light into light of a longer wavelength.

The hump on the left hand side of the diagram below is again blue but now we see a broader hump caused by the some of the blue light changing into green, yellow, orange and red light.

The doctor will see you now…

There’s no getting away from it, lighting is a technical subject. But you don’t need a physics degree to understand it all – you need the doctor. Dr Gareth Jones, that is, one of the UK’s top experts. Send your questions to thedoctor@archive.luxmagazine.co.uk. Next!

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