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Colour Temperature Explained

Background of Colour Temperature

Colour Temperature is a new concept that to be quite honest was of no real relevance when we just had filament light bulbs. This was because the light was created by heating the filament with electricity and it glowed white hot which actually produced a light with a warm white colour and this was all we had. Now with the introduction of LED bulbs there is far more variation and choice as the LED lamps can be made with any colour temperature.

Explanation of Colour Temperature

Colour Temperature is measured in the unit Kelvin and is the standard method for measuring the colour of light emitted from a lamp. The reason why this colour measurement is actually using a temperature unit the Kelvin is because it is correlated by observing the colour of a piece of steel when heated to the prescribed temperature. The piece of steel will glow a different colour depending on the temperature it is heated to, varying from red hot through yellow to amber then white and finally a blueish white. The colours of relevance to light bulbs start from 2700K which is a yellow warm white colour, then increasing to a fairly white colour at around 3500K which is similar to a halogen bulb. As the temperature is increased further to 5500K then the colour is similar to the light outside on a sunny day and then finally to 6500K which is the light colour on an overcast day. The aspect which is then confusing to start with is that a colour temperature of 2500K would be specified as a very warm white colour but as the temperature then increases to 4200K then this is designated as cool white even though the temperature has increased! So, you just need to remember that the cooler light colours have a higher Kelvin measurement.

Colour Temperature Choices

The selection of light colour is ultimately personal choice but there are some factors which should be considered. A warm white light colour is far softer on the eye and creates a warm ambiance and accordingly is normally the choice for interior domestic spaces. Additionally the warm white colour will not show up every detail on a surface and is more flattering when peoples' faces are seen as and fine wrinkles or blemishes will not be so prominent and visual skin toning will look better. In contrast the cool white light is much better for visual clarity and therefore is the normal choice for office environments and work environments where vision is of paramount importance. At the top end of the scale there is daylight colour which is the best for vision and accordingly is normally chosen by people who are reading and artists because it shows colours as they would be seen outdoors. The fact that daylight white gives improved vision means that it is the natural choice for the partially sighted and care homes.

Colour Temperature Table

Colour Temperature Kelvin Code* Types of lamps and normal uses
Very Warm White 2700K 827 Traditional Filament Bulbs. Warm white CFL's and LED's for ambient interiors
Warm White 3000K 837 Halogen Lamps and warm white CFL's and LEDs for interior spaces
White 3500K 835 Fluorescent tubes and white CFL's and LEDs for work areas
Cool White 4000K 840 Fluorescent tubes and cool white CFLs/LEDs made for work environments
Natural White 4500K 840 Fluorescent tubes and natural white CFLs/LEDs made for clinical and high technology work environments
Daylight 6000K 860 Fluorescent tubes and daylight CFLs/LEDs for environments requiring high visual clarity

Colour Rendering Index and the 3 Digit Code

*Key: The Colour Rendering Index is a measure of the quality of the light from the perspective of how faithfully it will illuminate objects showing their true colours. It is measured as a percentage with 100% for a light source which illuminates objects so they can be seen with their true colours. The CRI is then combined with the colour temperature of the light source to produce a 3 digit code which is often used by manufacturers to classify their lamps. The first digit is the CRI with 8 representing 80% and 6 representing 60% etc. The last 2 digits are the colour temperature in hundreds of Kelvin so 6000K would be represented by 60. So for example 840 means the lamp has a CRI of 80% and a colour temperature of 4000K

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