Colour temperature & rendering explained

Colour Temperature

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).

 

Technologies

 

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.

 

&nbsp Incandescent light spectral distribution &nbsp Fluorescent light spectral distribution

CRI Ratings

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.

 

Colour choices

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:

 

Restaurant lighting

In the dining area of a restaurant, a light such as the Edit Springsteen Ceiling Pendant is bound to create an impression. Here, you’d want to fit a warm white lamp to emphasise 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.

 

Retail lighting

In a furniture shop, you’d need warm light to emulate residential lighting. High-quality colour rendering is important to ensure vivid, accurate colours. Try using LED Spot and Track lights to focus lighting at your displays, alongside warm floor lamps to replicate the customers home.

 

edit floor lamp

 

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 Flash recessed display light is a good option in this instance.

 

Picture lights

Picture lights have improved greatly since the original halogen options. LED fittings are now both better value, but also better for the photo or artwork it is adorning. They give off minimal UV radiation compared to their halogen and incandescent predecessors, therefore they emit no harmful toxins and are safe to use for many years.

 

edit hudson picture light

 

For more lighting information, advice and ideas take a look at our Lighting Advice section.

 

andrew-author-bio

Andrew Evangelidis Head of Buying

Andrew is an experienced buying professional who takes an entrepreneurial approach to identify new lighting solutions and ensure Lyco have first-to-market ranges for our customers. Having previously worked for well known brands such as Wickes, Carphone Warehouse and Toys R Us, Andrew has now turned his hand to sourcing commercial lighting and ensure our customers receive top brand quality products at marketing leading prices. He manages a team of commercial and decorative buyers who travel the world finding new products that our customers don’t even know they need yet.

Lighting your picture properly

Picture lighting is important to most people whether they’re a homeowner with a favourite water-colour, a gallery owner with a priceless Rembrandt or even a hospital art coordinator looking to make the corridors a pleasure to walk down with fetching scenes. Picture lights provide a convenient solution to displaying artwork but choosing the right light fitting to get the best out of your artwork can be testing. Below we look at two of the most important factors to consider when it comes to lighting your pictures properly: size and colour temperature.

What size of picture light should I choose?

Rule of thumb

The most common rule of thumb is ‘choose a picture light that is half the size of the picture, excluding frame’, but if you dig around you’ll confusingly find one or two others. In truth, with smaller artworks you’ll often be able to choose lights that are as little as a third of the picture width. The opposite is true of extra large artworks (e.g. over 36” width), where the ideal is two-thirds.

Don’t go too small

This sliding scale of requirements comes from the varying distance between the edge of the light and the picture edge. With a large picture, there is a greater risk of creating a vignetting effect and leaving large corner areas underlit. In general though, the half-picture-size rule is a decent guideline.

Don’t go too large

A light fitting that is too wide for the picture is equally undesirable. Remember that light will spread when reflected off a surface. By limiting the amount of light that falls outside the picture, you are focusing attention on the artwork. This is especially important with dark pictures, where brightly lit surroundings will immediately draw the eye away from the subject.

How important in picture lighting is colour temperature?

Most people enjoy art without paying great heed to the lighting, but lighting directly affects the colour of art. Warm white light emphasises warm colours, and cool emphasises cool. It makes sense, then, to pick a light colour that enhances the dominant colours of your picture.

Shades and technologies

Each lighting technology has different colour properties, as follows:

  • Incandescent: warm white light with a typical 2700-2800K colour temperature. Less than ideal for blue or violet-coloured pictures, but excellent for emphasising reds, oranges, and yellows (flattering to skin tones in portrait pictures).
  • Halogen: warm white light with typical 2800-3200K colour temperature. Stronger radiation at shorter wavelengths (violet and blue) makes halogen appear whiter and brighter than incandescent lighting. It is better balanced than incandescent for artworks containing warm and cool colours. Both technologies offer superb colour rendering, though with a warm overall hue.
  • LED and low-energy fluorescent: manufactured with warm or cool colour temperatures (i.e. anything from 2700K to 6500K). A cool colour temperature lends a natural overall appearance, not unlike window light. These technologies are widely used in art museums (the Louvre Museum in Paris recently switched to LED). However, filament bulbs are inherently more reliable for reproducing individual colours and nuance. Vastly reduced running and maintenance costs may tip the balance!

Lighting Vincent

Below we compare the effect of warm and cool lighting on a Vincent van Gogh self-portrait. You’ll notice the blue areas are emphasised by cool light, whilst the complexion, hair, and palette are more vibrant under warm light. You’d probably base your picture lighting decision on the focus of the painting, which is the artist himself. You may well choose to sacrifice some of the blueness to avoid a pallid complexion. The decision is ultimately subjective.

Vincent under warm white light Vincent under cool white light
Vincent under warm white light Vincent under cool white light

Image courtesy of National Gallery of Art, Washington

Feel free to view our full range of picture lights. Alternatively if you want to more inspiration and advice, take a look at our Lighting Advice section.

andrew-author-bio

Andrew Evangelidis Head of Buying

Andrew is an experienced buying professional who takes an entrepreneurial approach to identify new lighting solutions and ensure Lyco have first-to-market ranges for our customers. Having previously worked for well known brands such as Wickes, Carphone Warehouse and Toys R Us, Andrew has now turned his hand to sourcing commercial lighting and ensure our customers receive top brand quality products at marketing leading prices. He manages a team of commercial and decorative buyers who travel the world finding new products that our customers don’t even know they need yet.