Colour rendering index

The appearance of coloured objects is affected by the interaction between the colours. The effect a light source has on the appearance of coloured objects is described by its colour rendering properties. These are grouped into grades based on the “general colour rendering index” CRI.

The colour rendering index indicates how closely the colour of an object matches its appearance under the relevant light source. To determine the CRI values of light sources, fifteen defined test colours commonly found in the environment are each illuminated under the reference light source (CRI = 100) and then under the source being evaluated. The greater the difference in the appearance of the test colours rendered, the poorer the colour rendering properties of the light source under examination.

 

Light and colour define the atmosphere of a room and influence our mood and sense of human well-being by their perceived “warmth” or “coldness”. Guaranteeing correct colour perception under artificial light forms is a very important part of the lighting designer‘s task.

The appearance of coloured objects is affected by the interaction between the colours - i.e. the spectral reflectance of the objects we see and the spectral composition of the light illuminating them. In everyday life, we come across surface colours which can differ in appearance depending on how they are illuminated but which we recognize for what they are thanks to stored visual experience independent of lighting type.

For instance, we have a stored impression of the colour of human skin in daylight. Where artificial lighting lacks a particular spectral colour or exaggerates certain colours in its spectrum (as is the case with fluorescent lamps with CRI 80), skin seen under it may appear a different colour but will still look “natural” because of empirical compensation.

An extreme and most obvious example would be the ultra-violet lighting: it makes white extremely bright, the teeth glossy and the skin tone appears extremely tan. Of course, the extreme effect is obvious and the eye therefore knows that the colours are artificially shifted.

The effect a light source has on the appearance of coloured objects is described by its colour rendering properties. These are grouped into grades based on the “general colour rendering index” CRI. The colour rendering index indicates how closely the colour of an object matches its appearance under the relevant light source.

To determine the CRI values of light sources, fifteen defined test colours commonly found in the environment are each illuminated under the reference light source (CRI = 100) and then under the source being evaluated. The greater the difference in the appearance of the test colours rendered, the poorer the colour rendering properties of the light source under examination. In theory the CRI can go below zero, but such a result is discarded as the colour rendering of such source provides no useful data.

Under a light source with a CRI of a 100 all the colours have the same optimal appearance as under the reference light source. The lower the CRI index, the poorer the rendering of the surface colours of the illuminated objects. In practical use CRI is an important aspect when choosing light sources. Those designated standard are cheap, but their CRI can reach only 60 or even less. The standards defined in EN 12 464-1 demand CRI of at least 80 for living spaces, inferior light sources are to be used only in corridors or storage spaces where colour rendering is of much less importance.

In several industrial sectors the demand for correct colour rendering is even higher, requiring light sources of CRI above 90. This is especially important in printing presses where correct colour assessment is vital, but can be as important in retail or in shop windows to correctly show the potential customers the colour of clothing, for example. For such shops the correct colour rendering is important also in the cabins where the customers try the clothes on. Wrong illumination there can lead to lower sales, with customers not being able to see the colour correctly. For LQS purposes, highest marks are awarded for CRI at or over 90.



Na stiahnutie

Please, fill correctly all fields!
Send