LED Colour Temperature

LED colour temperature simply means how Warm or Cool a light source appears.

Light sources with a Warm appearance are:

• The rays of light from a golden sunset.
• Light from a coal or wood fire.
• A dimmed filament lamp.
• Flames from bonfire or gaslight.
• Light which appears tinged with orange or yellow.

Examples of a Cool appearance are:

• A flash of lightning in a thunderstorm.
• Light passing through ice.
• Light reflected off snow.
• Clouds at midday.
• Moonlight in a clear sky.
• Light which appears tinged with blue.

In the UK and northern Europe, we associate Warm light with cosiness, being at home, comfort and relaxing. Cool light is more associated with work, sport, alertness and efficiency.

How is colour temperature measured? Colour temperature is measured in degrees Kelvin. These are the same as degrees Celsius except that zero degrees Kelvin starts at minus 273C. For example, the temperature of melting ice is 0C or 273K. A filament lamp of 2700K has a physical temperature of 2,427C.

What can cause confusion is that the larger the number in degrees Kelvin, the Cooler is the colour temperature. For example, 6000K is Cooler in appearance than 3000K.

To be technically accurate, colour temperature is called Co-related Colour Temperature. This is usually shortened to CCT. It’s called co-related because it is not a real, physical temperature. A cloud is not really as hot as 6000 degrees!

It all depends on the application. One of the most important considerations is the colour of the room surfaces and furniture. Red, yellow, cream coloured paint, wood, copper and brass all look beautiful under Warm light. However, you might find that blue, purple and some greys look a little flat.

At home, you will most probably want to use 2700K and 3000K LED light sources. Some LEDs are 2500K but below this, they can look yellow/orange rather than Warm white.

Conversely, Cool light can bring out the subtle differences in blue, purple and stainless steel type finishes. White surfaces will appear clean and “crisp”.

• Living rooms, dining rooms, bedrooms and hallways should use Warm LEDs.
• Bathrooms and kitchens often use Cool light like 4000K. Link to Ledinaire Panel, Ledinaire Downlights and LED Tubes.
• Home offices or studios are best with an intermediate colour temperature such as 3500K.

We recommend that you do not mix different colour temperatures in a room. This will ensure that all the light sources look the same.

Most commercial, office and industrial applications use 4000K or more.

Outdoors, it is preferable to use 3000K because it is more bat and wildlife friendly. 

Note that many floodlight installations use Cooler light sources such as 5700K. This is because TV cameras usually requires higher CCTs

A growing area of interest from clients is how colour temperature affects your alertness and wellbeing. Briefly, Cool sources of light, and this includes mobile phones and tablets, increase alertness and wakefulness but for the same reason can delay you sleeping at night.

This is because your body is attuned to the amount of blue light and this helps your body tell the time and stay aligned with your natural 24-hour cycle of sleeping and waking.

Cooler sources contain more blue light and so they help you wake up in the morning.

By contrast, Warm light sources can help you relax and sleep easily.

Some lamps and light fixtures can change from Cool to Warm depending on the time of day. These are often known as tunable light sources. This feature can be especially useful in care homes, hospitals, health centres or where a space is used for different activities.

You can find an introduction to this topic by following this link: https://www.interact-lighting.com/global/iot-insights/human-centric-lighting.

Related to colour temperature are the topics of colour rendering and colour fidelity. These terms describe how faithfully or accurately a light source reproduces colours compared with “natural” sources such as daylight, sunlight and old-style tungsten filament lamps. These are often thought to show objects and paints in their “True colours”.

However, even daylight changes from early morning to midday to sunset.

The general way of comparing light sources is by using a measure called the Colour Rendering Index, CRI. In simple terms, it divides the visible spectrum from blue at one end through yellow and green to red at the other end into 8 or 15 sections. The percentage “closeness” of the test light source achieved in each section is compared with the reference source. This is usually a laboratory filament lamp but sometimes a standardised version of daylight at 6500K is used. All the test values are then averaged to produce a single figure known as the CRI.

Indoors, you need to have a CRI of 80 or above. For most outdoor and industrial locations, a CRI of 70, or more, is sufficient. These values are enshrined in workplace legislation such as BS EN 12464-1. CRI is sometimes written as Ra, e.g. Ra 70.