Colour Temperature, Colour Rendering and R Values

Correlated Colour Temperature or CCT is used to determine how warm or cool the lighting effect is, the lower the value the warmer the colour temperature produced. Correlated Colour Temperature is specified in degrees Kelvin, the colour of the light is equated to the colour of light produced by a black body emitter when heated to the specified temperature. Flames demonstrate this effectively where a relatively cool flame such as a match or candle produces a warm colour of light and a hot flame such as a welding torch produces a white or blue colour of light. The sun produces a colour temperature of approximately 6000 Kelvin when viewed without atmospheric interference however when viewed from earth the geographical location, time of day and atmospheric effects can cause substantial shifts in this colour temperature.

Lighting is typically offered in several fixed colour temperature options in order to manage the number of LED types that are produced, although tunable white luminaires can produce a wide range of colour temperatures by mixing two or more colour temperatures. The most commonly available colour temperatures in the UK are 3000 Kelvin, 4000 Kelvin, 5000 Kelvin, 5700 kelvin and 6500 Kelvin. The following is intended to act as a guide to typical applications in which such colour temperatures might be commonly specified:

3000 Kelvin:
Most commonly used where a warm environment needs to be created, 3000 Kelvin is typically used in hospitality applications such as hotels and restaurants as well as some retail applications, particularly in fashion. It is also more commonly being used in external spaces where warmer colour temperatures reduce the impact of lighting on wildlife and the human circadian rhythm. It is generally not suitable for applications such as offices, academia or healthcare where cooler temperatures are needed. Retailers may wish to consider their product range and tailor their colour temperature selection to suit, a bakery or cheese retailer for example may enhance the product appearance through the use of warmer colour temperatures.

4000 Kelvin:
The most commonly used colour temperature in the UK, 4000 Kelvin is specified in BSEN 12464 as the default colour temperature for the majority of internal workspaces. Offices, academia and most healthcare installations, indoors industrial, manufacturing, logistics applications and many retailers commonly use 4000 Kelvin colour temperatures.

5000, 5700 and 6500 Kelvin:
The cooler colour temperatures are rarely used in the UK and tend to give a bluer colouration of light, some applications in healthcare or graphics may specify these higher colour temperatures to give a closer parallel to natural daylight, usually combined with a higher Colour Rendering Index. Some retailers may have a preference for higher colour temperatures, for example pharmaceutical, to create a clean and energetic environment.

More commonly R values are published in place of Colour Rendering Index. Colour Rendering Index is calculated as an average of the spectrum of light produced, therefore it is possible for a light source with a high Colour Rendering Index value to be very strong in some wavelengths and weak in others which may be problematic if specific colours need to be identified accurately. This can be especially problematic with high colour temperature luminaires such as 6500 Kelvin where the red content is lower and the blue content higher to produce the cooler colour temperature required. R values score the individual colour wavelengths allowing each part of the spectrum that the LED source produces to be fully understood. R9 is most commonly requested as this confirms the red content which is sometimes absent from cool white luminaires. The Ra value is the average of the score across each of the colour wavelengths. In the example diagram the source would still achieve an Ra value greater than 80 despite the very low red content, by using the R9 value in conjunction with Ra this can be easily identified: