R STAHL : Always the right colour with LEDs

Basics

28.06.2021

Always the right colour with LEDs

Rico Schulz

For many years, the focus has been on the high efficiacy of LEDs, which presented the primary reason for their application. In the meantime, optimization of this light source technology has moved forward. The focus in all application areas - including the industry - is more and more on exact colour rendering and on the development of the spectra that are required. Today, high-quality LEDs fulfil even the most sophisticated requirements. The essential criteria and the state of the art are presented in this article.

LED luminaires and human colour perception

Type and quality of a light source have an effect on the way a colour is seen by a human being. Laboratories, printing companies or drawing tables are only obvious examples for workplaces where light - electromagnetic radiation in a spectrum the human eye can perceive - is very important.

In order to determine the average human colour perception and to make it describable, the Commission Internationale de l'Éclairage (CIE, International Commission for lighting) carried out a representative study with test persons who were able see colours (not colour-blind) as early as 1931. The test persons had to alter the brightness of a red, green and blue luminaire to adjust the mixed light impression on a white area to a number of colour charts. Based on the results, the CIE defines a standard observer colour space which assigns values for red, green and blue to the hues that are perceived by the human eye.

Reference charts still play a major role when the colour fidelity of light sources is qualified. It is shown as a so-called colour rendering factor alue R_a, which is sometimes also called CRI (Colour Rendering Index). Today, there are eight standard test samples available as test objects pursuant to DIN 6169. These are mostly in pastel colour, i.e. non saturated colours. As LED lighting becomes more and more popular, the introduction of six other colour charts is intended - especially such colour charts with a high colour saturation, as with stronger colours, the weakness of LEDs to display red, which originally was characteristic for LEDs, can be easily shown. However, a supplement to the standard has not yet been resolved.

The colour rendering quality that has to be achieved in working spaces can be found in EN 12464. The minimum requirements on illuminance and colour rendering for different scenarios is defined in tables. The ideal value is the value in daylight, R_a=100. In interior rooms in which people work or stay for longer periods of time, lamps with a colour rendering index greater than 80 have to be used pursuant to the standard. At fixed workstations that are used all the time, an even more reliable recognition of all colours should be ensured by suitable measures.

There is a trend towards higher color rendering values, which can be attributed to further technological development of the conversion material.

Rico Schulz, Product Manager Lighting Technology

Colour-fast, even with a limited spectrum

LEDs virtually are monochromatic semiconductor components which emit red, green or blue light with a high colour saturation in a very small spectral range. However, observers only see coloured objects in the correct colour when these objects are subjected to a bright light in a wide spectrum which covers the wavelengths of different colours. If some spectra are missing, these light sources for example have a yellow or a red tinge: The colour wavelengths that are not included in the light that is emitted can only be seen incorrectly in different hues.

Classic incandescent lamps emit light like the sun, in a continuous colour spectrum which includes the complete visible range up to infrared. This is why lamps with filaments render colour very good, R_a=100; they are on one level with natural daylight.

In the current state of the art, the colour rendering of LEDs is always below 100. The wavelengths of the different semiconductor materials that are used in the different LEDs are near the curved outer edges of the two-dimensional CIExy colour space. A locus farther inside the chart cannot be achieved. In order to expand the wave spectrum of LEDs and to enable the desired white light emission, a conversion substance has to be added. The most common way is to coat the semiconductor of a blue LED with a yellow conversion substance which consists of a mixture of inorganic materials. These powders, which are also called phosphors, have been significantly further developed during the past years. By combining an LED with a modern luminescent substance, a verybroadband white light with light rendering values of more than 90 can be emitted by now.

Appropriate selecton of the white light variant

However, it is not only the colour rendering value that is important for white light, but also its so-called colour temperature. The concept of the colour temperature, specified in Kelvin, is derived from the thermal radiation of an ideal-typical black body that does not reflect visible radiation. The nominal colour temperature corresponds to the physical temperature of the black body. Human beings regard white light with high Kelvin, measured pursuant to this rule, as being colder, low Kelvin values are regarded as being warmer. Depending on the phosphor characteristics, LEDs may today emit warm light which is similar to the light of candles or incandescent lamps and also colder light, which is the preferred choice for workplaces and workshops.

Nowadays, targeted decisions for white light sources with a certain colour temperature are often made, or even for lighting solutions that emit white light with different colour temperatures, as required. Such 'tunable white' makes use of the fact that suitable lighting design may positively influence the attention, motivation and mood of the employees. Varying the white light temperature is one of numerous aspects of 'human centric lighting', which plays an ever more important role at workplaces as well. In case of customary products for industrial applications that are based on LEDs, the selectable colour temperature is often limited. Many luminaires are available in only one or two different colour temperatures. For the so-called neutral white, EN 12464 defines colour temperatures ranging from 3300 K to 5300 K, calls temperatures below 3300 K warm white and such that have more than 5300 K cold white. Luminaires producers often use the same plain text designations differently. The best orientation thus is the colour temperature given in Kelvin, which has to be specified for every product.

Standard colour temperature for R. STAHL LED luminaires for example is 5000 K. R. STAHL generally offers three colour temperatures for all products - besides neutral white with 5000 K, it offers a cold white of 6500 K and a warmer variant with 4000 K. The colour rendering index R_a always is 80 or more and thus complies with the requirements on interior rooms specified in EN 12464-1.

Quality features of LED light colours

Due to production-related reasons, LEDs are subject to variations in quality and thus have to be sorted. So-called bins are defined for selection. There are bins for different criteria, which include luminous flux or forward voltage and also the colour locus in the CIExy colour space. As emissions of white LEDs are not exactly on the black body curve - the so-called planckian locus T_c - there may be visible and more or less annoying colour differences between LEDs that have the same colour temperature. To get a uniform lighting behaviour, only LEDs from the same colour bin should be used in LED arrays.

Physicist David Lewis MacAdam defined areas in the CIExy diagram in which the deviations of colour locus become more and more obvious. The tolerance range when printed in the CIE 1931 diagram with only few deviations to a reference point is not evenly distributed around a certain point, but in the form of an ellipsis. Concentric circles with increasing diameter provide the numerical value for homogeneity - the so-called 1-step to 7-step MacAdam ellipses. This step size is specified in the production documentation and may serve as an important quality criterion for customers in their decision to apply LED lighting solutions. For practical reasons, however, sorting is often not carried out pursuant to the MacAdam ellipses. Instead, it is carried out in so-called ANSI bins, even though a reference is made to the MacAdam step sizes, as this sorting can be carried out more easily.

In the two-dimensional CIExy colour space (the missing third dimension contains the information on brightness), the spectral locus of monochromatic LEDs is close to the curved outer edges.

MacAdam ellipses mark areas around a reference point which are perceived in a similar colour; they are used for 'binning', i.e. the sorting of LEDs pursuant to the homogeneity of their spectral locus.

Bin designations for white LEDs pursuant to standard ANSI.

1. In the two-dimensional CIExy colour space (the missing third dimension contains the information on brightness), the spectral locus of monochromatic LEDs is close to the curved outer edges.
2. MacAdam ellipses mark areas around a reference point which are perceived in a similar colour; they are used for 'binning', i.e. the sorting of LEDs pursuant to the homogeneity of their spectral locus.
3. Bin designations for white LEDs pursuant to standard ANSI.

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Further Reading:

• Lighting Technology in hazardous areas
• Explosion-proof lighting for any application

• LED luminaires and human colour perception
• Colour-fast, even with a limited spectrum
• Appropriate selecton of the white light variant
• Quality features of LED light colours