By Martin Krautter.

It doesn’t always have to be light art by a celebrated name: LED technology has created new ways of using colour temperature to subtly generate atmosphere and show off colours to brilliant perfection.

Whether it’s a trade fair stand, concept store, bar or museum – it’s imperative for the atmosphere of a space to reflect a brand’s individuality and for the materials and colours of the objects on display to be presented in the right light. Regardless of how something is designed and what material it’s made of, it won’t work without successful interaction between object, light and viewer. As demonstrated by the works of James Turrell or Olafur Eliasson, the colour of light can have an extreme impact on human perception.

But classic architectural lighting also uses light for creative accentuation by setting coloured surfaces aglow and making them stand out almost in relief. The light emphasises certain information within the space, deliberately steering customers’ and visitors’ attention towards whatever they’re meant to notice. Warm white light with a higher proportion of red, for instance, can underscore the tasteful atmosphere of a store fitted with warm, natural materials, whereas cool white light with a higher proportion of blue is ideal for creating a sense of freshness in an ice-cream parlour. And a bright red cashmere sweater, warmly lit, will stand out all the more if the rest of the shop is bathed in cool light.

Now that restaurant, retail and museum lighting mainly relies on LEDs, the composition of the light spectrum plays a key role. It influences which hue we attribute the light itself with and how we perceive the colours of the objects in our surroundings. Then there are biological aspects, such as the way our body clock is governed by the blue wavelengths in the spectrum. Above all, however, the spectral characteristics of modern LEDs can be customised for different applications or even be conveniently controlled by digital means. In short, the conditions for making subtle but effective use of light as a virtual design tool couldn’t be better.

Herren Fashion Henschel Heidelberg
Design concept by blocher partners for Henschel, Heidelberg.
© Joachim Grothus for blocher partners

Colour temperature: how can the visible spectrum be used to design atmosphere?

Colour temperature is the most perceptible aspect of the respective LED spectrum. The shade of white emitted can range in character from reddish and warm to bluish and cool and is expressed in kelvin (K). Contrary to everyday logic, the “warm” white of a bulb has a low colour temperature of about 2,700K, whereas the “cool” white of the midday sun has a high colour temperature of 6,500K. On the other hand, it seems entirely logical that spaces are perceived as cooler and larger in bluish light than in warm, reddish light.

Designers can either choose to immerse a setting in a uniform cool or warm atmosphere, or they can play with contrasts and use a noticeably different colour temperature to accentuate individual objects against a homogeneous background. In practice, white LEDs are mostly blue-emitting semiconductors with an additional phosphor layer which, excited by the blue, contribute the missing yellow and red wavelengths. By varying the composition and amount of the phosphor, the colour temperature can be influenced to produce effects that go beyond standard colours like warm white (2,700K), neutral white (4,000K) or daylight white (>5,500K).

Colour rendering: are there any standards for natural-looking colours?

A company restaurant with no natural light: using tunable white lighting to simulate the way daylight changes over the course of the day enhances the atmosphere.

Sometimes our perception of colours plays tricks on us: the tie or scarf that was such a good match with the colour of our favourite shirt in artificial light looks totally different in daylight. This basic problem of how faithfully a light source is able to render colours is something LEDs have in common with fluorescent lamps. In contrast to the continuous spectrum of the sun or an incandescent bulb, they both generate a spectrum which, when depicted as a curve, resembles a jagged mountain range.

But rather than perceiving light evenly across the visible range, the human eye only detects it patchily by means of three different types of photoreceptor cells known as cone cells, which are sensitive to red, green or blue depending on their photopigment. The brain uses the relative levels of activity in the three types of cone cells to define an impression of colour. As a result, light that looks white to us can nevertheless have gaps in its spectrum and renders the colours concerned less faithfully as a result. This effect is quantified by the CIE Ra or CRI (colour rendering index), which is actually independent of the colour temperature, even though warm-coloured LEDs often have a higher CRI as well. Halogen or daylight with a CRI of 100 serves as the reference source.

While values of 80 or higher are considered good, anything over 90 is rated very good – and is the minimum that should be considered for lighting in museums or exhibitions, for instance. But be careful: the CRI is only determined on the basis of eight test colours (defined by the DIN 6169 standard in Germany), all of them on the pastel side. They don’t even include a rich red, which only comes ninth in the series of colour samples tested. That’s why a separate value is sometimes stated for this red – the R9 value. In view of the many different spectra LEDs can have, the CRI doesn’t tell you as much as it used to. New standards are under discussion but have yet to establish themselves on a broad scale.

Custom colour temperatures: what light makes things look fresh and tasty?

The fact that different LEDs render colours differently can be used as a creative tool by gearing the light to the object colour of the surfaces or things to be illuminated. That can be done in a relatively unspecific way via the choice of colour temperature: neutral or cool light emphasises technical materials like steel, glass or concrete, whereas warm light shows natural materials like brick, leather or wood off to the greatest advantage. But there are more precise ways to go about it too: everybody’s familiar with the pink light at the meat counter that makes the mince look so much more appetising. In the past, this effect was achieved by means of special discharge lamps and filters. Today food retailers use LEDs with special colour temperatures for various product categories, from fruit and vegetables or bread all the way to fish and seafood. At the same time, it’s a fine line between optimal presentation and manipulation.

Below Black Body: Was macht Weiß weißer und Farben satter?

Das ging früher nur mit Vollwaschmittel – heute bieten die Hersteller unter Bezeichnungen wie »Crisp White« LEDs an, die Weißtöne strahlender und Farben satter als gewohnt erscheinen lassen. Die Zauberformel lautet »Below Black Body«, das heißt, der Farbort dieser LEDs liegt im CIE-Diagramm unterhalb des planckschen Kurvenzugs. Der Trick funktioniert besonders gut bei höheren Farbtemperaturen und wird gern im Handel mit Mode und Textilien eingesetzt, eignet sich aber grundsätzlich, wenn ein sehr buntes Sortiment frisch und knackig erscheinen soll. Ein weiterer Kunstgriff: LEDs mit erhöhtem Violettanteil, der die optischen Aufheller in Textilien aktiviert.

Below black body: what makes white whiter and colours richer?

In the past, making something whiter than white was only possible with laundry detergents, but nowadays manufacturers are selling LEDs with names like “Crisp White” that make shades of white look more radiant and colours appear richer than usual. “Below black body” is the magic formula, meaning that these LEDs’ colour point in the CIE diagram is below the Planckian locus. This trick works particularly well with higher colour temperatures and is often used in the fashion and textile trade, although it’s actually suitable for making any very colourful range of goods look fresh and crisp. Another ploy is to use LEDs with a higher amount of violet, which activates the optical brighteners in textiles.

Effects like warm dimming ortunable white are unthinkable without digital control. Contemporary systems like Casambi are programmed and operated via a smartphone or tablet. © Occhio

Intuitive control, live experience

Because of their dynamic component, the light qualities described above fall under interface designers’ remit too. Variable light sources obviously need user interfaces, which are however limited in their ability to depict colour nuances because of the display technology. So there’s certainly plenty of overlap between design and the spectral composition of light. And while this article is certainly no substitute for professional studies, it will hopefully demystify the odd buzzword and encourage creative consideration of the subject matter.

One thing is vital: any designer who wants to use special colour temperatures and LED spectra should check out and experiment with their effect on samples or a mockup first – preferably with the involvement of future users. Because at the end of the day, the effects are virtually impossible to simulate, often difficult to describe and, on top of everything else, are perceived differently by different people.


First published in the designreport edition 01/2019.

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