Colour rendering and its importance in lighting

Any light source, whether natural or artificial, has a particular light spectrum.

When food looks much more greyish in the kitchen than in the supermarket, when the pretty coloured dress suddenly becomes lighter in the bedroom or when the face reflected in the mirror simply doesn’t look natural, this is probably due to poor colour rendering of the bulbs. On all modern light bulbs there is an index. This is a lighting quality criterion that must be taken into account when choosing a bulb.

Natural light, from the sun and the sky, has a visible spectrum (a radiation with a wavelength between 380 and 760 nanometres (nm)) in a continuous form). The combination of the various radiations that make up this spectrum forms, by definition, the so-called white light: it is the only one that allows the eye to appreciate with the greatest accuracy the colour of the objects and the most delicate nuances. The different coloured radiations composing natural light appear easily when they are refracted and reflected by drops of water, as in the rainbow.

What is colour rendering?

The colour rendering index is a characteristic of light bulbs, such as LED bulbs, halogen bulbs or low-energy bulbs. The index (abbreviated as Ra) contains information on the natural colour of objects illuminated by the artificial light of a bulb. This rendering is thus a criterion of the quality of a bulb and its light.

How is the colour rendering measured?

The colour rendering is measured with the Ra index using measuring instruments, such as a spectrometer. Natural daylight or sunlight is considered as the reference. His index is 100 Ra. That’s the maximum. All bulbs are evaluated against this reference for representing colours in natural daylight. The evaluation is carried out using 14 test colours according to DIN 6169: 8 primary reference colours (old pink, mustard yellow, yellow green, light green, turquoise blue, sky blue, violet and lilac) and 6 secondary reference colours (saturated red, saturated yellow, saturated green, saturated blue, flesh colour (pink) and dark green). The aim is to evaluate how the colours are represented by the different bulbs under artificial lighting in relation to the representation of colours in daylight. This provides an objective assessment.
Here are the 14 test colours according to DIN 6169

What do the values of the color rendering index mean?

An index of 100 Ra corresponds to a natural colour rendering. A color rendering index of >= 90 Ra indicates very good color rendering. A value >= 80 Ra indicates good colour rendering. Bulbs with an index of less than 80 Ra are not recommended for residential use, as they may distort the colours of food or clothing.

Which bulb offers the best color rendering?

With its continuous light spectrum, the incandescent bulb has a colour rendering of 100 Ra, high-voltage halogen bulbs can also reach a value of 100 Ra, and quality LED bulbs have a good or very good colour rendering.

Why does the colour rendering differ between bulbs?

The differences in colour rendering are explained by the differences in the light spectra of the bulbs. Only part of the electromagnetic radiation is visible to the human eye. This visible part of the wavelengths between 380 nm and 780 nm is called “light”. This light itself consists of several spectral colors. Natural daylight contains all electromagnetic wavelengths, i.e. all colours. Thus, only a bulb whose light contains all the spectral colors can faithfully reproduce the colors of the objects. If the light spectrum of a bulb has peaks in some colours, while others are not very present, the brightness is applied irregularly to the colour range. Some colours are thus exaggerated. And if a color of the spectrum is missing, the areas of this color appear grayish.

Where is colour particularly important?

A good rendering will be important, for example, where safety colours must be perfectly recognized, such as in industry. Doctors, architects and graphic designers are also professions for which accurate colour reproduction is essential. In supermarkets and shops in general, bulbs with a high rendering will be preferred to make fish and meat, for example, even more appetizing. This is also essential in clothing stores. Inside the house, a faithful reproduction of colours is preferable in the bedroom where you choose your clothes, in the bathroom near the mirror and of course in the dining room for meals. Although in private practice, colour rendering is not a question of turnover or vital, poor colour rendering quickly leads to discomfort. It is still more pleasant to be very keen to put on your beautiful red dress, to find yourself quite well in the mirror and to rejoice at the sight of an appetizing meal.