Understanding Colour Correlated Temperature (CCT) in Lighting Design

Before energy-efficient LED lighting became mainstream, choosing a light bulb was quite straightforward. A 40-watt bulb not giving you enough light? Swap it for a 60-watt bulb instead and get more light.

Today the much-improved LED technology has allowed for a wider range of possibilities in terms of light specification and selection criteria for each space being designed. One criteria we select now is the colour temperature of the light source, known as the CCT, or Correlated Colour Temperature.

CCT is a number, measured in degrees Kelvin, that helps to describe the relative warmth or coolness of a light source. Most light sources will typically range from 2700K (warm, incandescent colour) to 5000K or higher (crisp, white daylight colour).

For most residential applications, 2700K and 3000K are preferred colour temperature options as they both create a nice, warm appearance that creates an inviting and relaxing environment.

For retail or commercial applications, 4000K is a popular colour option as it provides for a crisper, more energetic shade of white. For industrial or task-oriented applications, 5000K or even 6500K is a preferred colour option, as these colour temperatures best match natural daylight.

The difference between CRI and CCT explained

In an earlier post we explained what CRI is (how accurately a light source illuminates colours of an object). So we now know CCT and CRI measure two different aspects of colour. CCT tells us the colour of the light emitted by the light bulb, and is immediately visible to the casual observer by looking directly at the light source.

Why CCT is more fundamental and important than CRI

The explanation above should have made clear that CRI requires a colour temperature value in order to determine what we are comparing colour appearance against.

CRI is certainly an important metric that helps explain colour quality, but it is almost meaningless when used alone without a colour temperature value. Given a light bulb’s 95 CRI rating, you might be impressed and conclude that it must be very accurate. But accurate when compared to what? Incandescent bulb light colour (2700K), natural sunlight (5000K) or natural daylight (6500K)?

Think first about the colour temperature requirements for your application, and then worry about CRI after. Are you looking to replicate the light of natural daylight? Pick a high colour temperature value (5000K or higher), and then CRI value next to determine how accurate it is. A 2700K light bulb with 95 CRI, even with a high CRI rating, will not come even close to replicating natural daylight due to its colour temperature being way off.

Last example, in your quest to replicate natural daylight, let’s say you find a 6500K bulb but with a low CRI. In this case, the colour of light emitted by the bulb might look the same as natural daylight (due to the colour temperature value), but as soon as the light lands on any surface with colour, you will find that the colours do not appear the same as under natural daylight (due to the low CRI value).

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