Saturday, 6 August 2011


I would go along with Henry Ford, "any colour you like as long as it's black" but unfortunately this doesn't really work with lights. 

Colour has something to do with the frequency of light, although it's not really that simple. 

We can see about one octave of light. If it were sound, this would be equivalent to being able to hear 12 keys (7 white and 5 black) of the 86 on a piano. The eye is a fairly precise instrument. 

I'm sure there are slight differences in each person's range, and different people are no doubt sensitive to different colours. The architect was talking about westerners seeing light differently to Japanese people because they have blue eyes, but this sounds like nihonjin-ron. I pointed out that my eyes are brown. 

The conventional theory of the way the eye works is that there are rods and cones. The rods are numerous and sensitive to low levels of light, while the cones are more concentrated around the middle of the retina, can pick up colour but are not so good as it gets dark. You can observe this as the colours are sucked away when it gets dark, and there's also a phenomena when you can see a dim star, but if you look at it directly, it vanishes. 

More recent research suggests that many cones tune into particular colours, so it seems very likely that the range of colours in the environment in which we develop will shape our colour perception.

A lot of animals can only see whether something is dark or light. Some can sense one colour, for example green. Very few can also sense red, and have the colour spectrum that we do. Apparently there is a correlation with hairless-faced primates. A possible connection is the ability to see face colour, which depends on the red blood running through the veins, and the importance of this in determining emotional states of fellow members of social groups. We now use language, and people have been using make up to cover up or enhance their face colour for a very long time, but the ability to distinguish puce from beige remains.

When it comes to the colour of lights, our first point of reference is the sun, which sends out radiation across a broad spectrum. We've been brought up on incandescant lights, which are like miniature suns in that they are not so discriminating about the frequency that comes out. As a results, and to their detriment, most of the radiation is not visible, and comes out as heat. 

LEDs start from exactly the opposite situation, emitting light at a very precise frequency, a function of the material the LED is made of. This makes them efficient as they are not emitting heat as invisible radiation. Also, because insects are attracted to light outside the spectrum visible to humans, they are not attracted to leds, making leds perfect for camping, and indeed for lights in or outside a house in a country with insects. LEDs are used for indoor plant growing, and researchers have been finding that different plants respond to different frequencies of light, corresponding to receptors in their DNA.

The first LEDs were red. I can still remember when a boy turned up to our school with a digital watch. You pressed a button on it, and the LEDs lit up the time for a few seconds. Green LEDs were invented a little later, then blue. First attempts at domestic LED lighting involved arrays of red green and blue LED, which combine into white light. Today this all goes on at a much smaller scale, with the colours being mixed up within individual LED chips, or broad spectrum blue LED substrates are mixed with green and red fluorescent materials. 

The problem is how to express this in numbers, so that you have a good idea of what you're getting when you buy a light. Gone have the glorious days of incandescents when you could have any colour you like, and in fact every colour, whether you liked it or not, and the wattage would tell you how much light came out. For a few years, different colours of fluorescants have been sold, under names like light-bulb coloured, cool light or warm light. Colour temperature is one measure of the colour of light, rather confusingly measured in kelvin, partly because it was Lord Kelvin who thought of it. Colour temperature represents the colour given off by a black body heated to that temperature. To add further confusion, low temperatures, under 3,000 are warm colours while high temperatures, over 5,000 are cool colours. If you consider that blue-hot and white-hot are hotter than red-hot, but red is a warmer colour, this makes some kind of sense. As things get hotter, they produce more light at higher frequencies. In terms of frequency, "warm" colours mean more light at the lower end of the spectrum, and "cool" colours have more light at the higher end of the spectrum. 

The question is not simply the colour of the light coming from the bulb, as if it was on a spectrum from red to indigo. We usually don't want light of a single colour, unless we're aiming for a dark room motif in our interior design. We want the light in the kitchen to shine on red, green and yellow peppers, and for each of them to come out strongly in their own colours. We want this for our kitchens and dining room tables in our houses, but supermarket owners want this much more for their shelves. There is a lot of electricity to be saved, especially in the refrigerators where inefficient light not only means higher bills for lighting, it also means more work for the heat pumps. 

Anyway, you need to know how much light is coming out of the light at each frequency of the spectrum. The machine-gun approach of the sun, or incandescent light means that there is an even light, so each frequency is rendered well. The measure of this is Colour Rendering Index (CRI), which is a number up to 100.

LEDs get a CRI of up to 98. Anything under 80 may not be ideal for kitchens or dining room tables.