You will have noticed that light bulbs in different situations often have different colours. Restaurants tend to have warm, yellowish light, reminding people of a fireplace. More functional rooms such as offices or warehouses have colder, bluer light, a bit like light coming through a window.

There are lots of colours in between, and a way of describing them is by using the so-called 'colour temperature'.

This is a scientific scale to describe how "warm" or how "cool" the light source is. It is based on the color of light emitted by an incandescent source. As a piece of metal is heated, it changes color from reddish to orange to yellowish to white to bluish-white. The color of light emitted by an incandescent object depends only on the temperature. We can use this scale to describe the color of a light source by its "colour temperature."

When we say a lamp has a colour temperature of 3000K, it means a glowing metal at this temperature would produce light of the same color as the lamp. If the metal is heated to 4100K, it will produce whiter light. Direct sunlight corresponds to about 5300K, while daylight, which has blue from the sky mixed in, is typically 6000 Kelvins or above. A standard incandescent lamp has a filament at 2700K, and therefore a colour temperature of 2700K.

Bulbs are available for different situations with a range of colour temperatures. The bulb packaging often tells you the colour, in the 'designation code'.

Below is a table listing some colour temperatures with their descriptions


2700K designation code 827, "Warm White": Similar light to “normal” incandescent bulbs, giving a warm “cosy” feel

3000K designation code 830, "Warm White": The colour of most halogen lamps. Slightly whiter than ordinary incandescent lamps.

3500K designation code 835, "White": The standard colour for many fluorescent and compact fluorescent tubes.

4000K designation code 840, "Cool White": Gives a more clinical or “high tech” feel.

6000K designation code 860, "Daylight": Fluorescent or compact fluorescent lamps simulating natural daylight.

6500Kdesignation code 865, "Cool Daylight": Extremely “white” light used in specialist daylight lamps.

Your local supermarket will not have a very good range of bulbs if you're after a particular colour temperature, but all of the above are easily available online.

Areas where lots of work needs to be done need higher colour temperatures. Longer wavelength light gives a more relaxed environment.

Some lamps (e.g. LEDs - light emitting diodes)) have extremely harsh blue light, and a colour temperature way over 6500K. This is because of the way they are manufactured; the light emitting element can only emit light of a particular frequency or collection of frequencies.

LEDs are by far the most energy efficient lamps around, but there is some way to go before they will be welcome in the home (except for emergency lighting).

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