Colour temperature & rendering explained
Author: Glenn Harper
Published: June 20, 2014
Colour temperature and colour rendering are terms you’ll see when buying lights, but what exactly do they mean? These specifications will help you choose exactly the lighting you need. They are rooted in physics, but can be simply expressed:
- Colour temperature refers to the overall colour of white light. It tells us whether a lamp or light fitting has a warm bias (i.e. red or yellow) or a cool one (i.e. blue).
- Colour rendering relates to the underlying colours in any light source. White light is a mixture of many colours, which are not individually visible. To accurately show the colour of any object, that colour must be hiding in the light.
A useful example – the sodium street lamp
Have you ever noticed how, under traditional street lighting, it’s near-impossible to see most colours? This is an exaggerated example of poor colour rendering. It means the colour you’re trying to identify is not contained in the light.
The street light—despite its lowly colour performance—still has an overall colour. Its strong yellow hue gives it an estimated 1800K colour temperature (yellower than any household bulb).
Kelvin colour temperatures
Remember that kelvin colour temperatures are counter-intuitive: higher temperatures mean cooler colours (e.g. 2700K is warm and 6500K is cool).
Lighting technologies have specific colour properties, which may help you make good buying choices:
- Incandescent light is always warm in colour temperature (e.g. 2700K) and excellent for colour rendering, containing all colours of the visible spectrum. It is, nonetheless, relatively poor for displaying violet or blue colours, which are muted by its warm bias.
- Halogen light is always warm in colour temperature (e.g. 3000K), and excellent for colour rendering. It is better balanced than incandescent light, with stronger radiation of cooler blue and green colours despite its warm hue.
- Fluorescent lights are made in all colour temperatures, achieved by varied use of phosphors. Colour rendering is inferior to filament lighting. However, the ability to combine cool colour temperatures with high-quality colour rendering allows some fluorescent lamps to imitate daylight. The Sylvania T8 S.A.D. Fluorescent Tube is a great example of this.
- LED lights are also made in various colour temperatures. Again, colour rendering is inferior to filament bulbs, though it is of a high enough standard for most purposes. The best colour rendering in LED technology is prohibitively expensive compared to fluorescent equivalents. Dimmable LEDs have the advantage of maintaining their colour at all brightness levels, which is not true of filament lamps.
Below are two spectral distribution charts. Very simply, you can deduce from the smooth diagonal of the incandescent bulb that its colour rendering is more predictable than fluorescent lighting (LED is similarly disadvantaged). This benefit is offset by the strong red bias, which subdues violet and blue colours and is controllable in modern technologies.
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CRI (colour rendering index) ratings indicate the quality of colour rendering. They measure how accurately a light can render eight colour patches against expected results. The scores represent percentages (e.g. CRI 80 is 80% averaged accuracy). Although this specification is lenient and limited in scope, it gives some indication of quality of light. It says nothing about colour temperature or bias.
It is widely accepted that the human response to warm lighting is relaxation, whereas cooler lighting makes us more alert and focused. Thus, warm lights are used in homes and hospitality settings, whilst cool lights are found in work places and schools.
In the following examples, we’ll demonstrate alternate lighting choices in related applications:
In the dining area of a restaurant, a light such as the Graypants Moon 14 Pendant is bound to create an impression. Here, you’d want to fit a warm white lamp to emphasise the colour of the shade and create a relaxing mood.
Back in the kitchen, cool white LED light panels will help staff to stay focused and alert. Cooler light also appears brighter to humans, and in the case of LEDs is slightly more energy efficient.
In a furniture shop, you’d need warm light to emulate residential lighting. High-quality colour rendering is important to ensure vivid, accurate colours. The Kosmo Halogen Display Light is ideal. The Gamma TTS 3001 LED Downlight does not quite match halogen for colour rendering, but is far cheaper to run as overall lighting. LED lighting has a tendency to mute red and aqua colours.
A cool white light might be used in a shop such as a fish mongers. The aim is to emphasise the colour of the product. The Wilson 38W LED Recessed Display Light is a good option in this instance.
An LED fitting like the Manet Picture Light might have a cool white output, which is good for stressing blue colours in art.
The Searchlight Striplight Picture Light uses an incandescent lamp. This will accent red and yellow hues and flatter skin tones in portraits. Excellent colour rendering ensures nuances of colour are retained.
For more lighting information, advice and ideas take a look at our Lighting Advice section.