TECHNICAL SPECS

A MEMORY COLOUR RENDITION METRIC, RM

Kevin Smeta aLight and Lighting Laboratory, Catholic University College Ghent, Belgium – KU Leuven,
Postdoctoral Fellow of Research Council Flanders

Kevin Smeta aLight and Lighting Laboratory, Catholic University College Ghent, Belgium – KU Leuven, Postdoctoral Fellow of Research Council Flanders The impact of the light source spectrum on the colour appearance of objects is traditionally quantified by the colour rendering index Ra (CRI) developed by the International Commission on Illumination (CIE). Colour rendition is assessed as fidelity with a CIE reference illuminant: the smaller the colour differences between a set of coloured cards illuminated by the test source and its CIE reference illuminant, the higher the fidelity (resemblance) and thus the higher the Ra value. Although such a colour difference based metric is required for many professional applications – colour reproduction, printing and quality control – several visual studies(1-7) with fluorescent sources and light-emitting diodes have shown that it is generally not well suited to predict the more subjective aspects of colour quality such as visual appreciation or naturalness, because the CIE reference illuminant need not be the most optimal source (4, 8). In light of this failure of the CIE Ra index several alternative metric have been proposed over the past years. Although most of them are based on some sort of comparison with a reference illuminant, an alternative approach would be to directly reference to the ideal chromaticities of familiar objects.

Such an approach should also correspond more closely to the way people judge the colour quality of light sources in everyday life as nobody walks around with a reference source to be able to judge the colour quality of the lighting in a room. Colours of object often look “wrong” or “distorted” when they are not what we expect or want them to be (9, 10). Based on the straightforward assumption that the colour quality of a light source increases when object colours are rendered more closely to what is expected, a memory colour quality metric was developed that is able to assess the colour quality of a white light source in terms visual appreciation: the MCRI or memory colour rendition index, Rm (MCRI) (11, 12). The memory colour rendition index has been validated in two recent scientific publications (12, 13) using observer ratings from several published visual experiments (3-5, 7, 14-18). The memory colour rendition index Rm was found to correlate highly (r = 0.88, p < 0.0001) with the visual appreciation of white light sources. In addition, the MCRI was found to be significantly better at it than several other published colour rendition and colour quality metrics. The results are illustrated in Figure 1.

Figure 1. Performance of several colour rendition and colour quality metrics.

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References

  1. Bodrogi P, Csuti P, Hotváth P, Schanda J. Why does the CIE Colour Rendering Index fail for White RGB LED Light Sources? CIE Expert Symposium on LED Light Sources: Physical Measurement and Visual and Photobiological Assessment; June 7-8; Tokyo, Japan2004. p. 24-7.
  2. CIE177-2007. TC 1-62: Colour Rendering of White LED Light Sources. Vienna, Austria: CIE, 2007.
  3. Jost-Boissard S, Fontoynont M, Blanc-Gonnet J. Thurstone scalings for attractiveness of fruit and vegetables under nine 3000K and eight 4000K light sources. (personal communication). 2009.
  4. Jost-Boissard S, Fontoynont M, Blanc-Gonnet J. Perceived lighting quality of LED sources for the presentation of fruit and vegetables. Journal of Modern Optics. 2009;56(13):1420-32.

WHAT IS COLOUR?

The “unexpected Martini philosophy” for lighting designers

HD RETINA LED® is the first light source COB (chip on board) developed and patented by a light fixtures manufacturer: Martini

MAIN FEATURES:

It’s a pleasant light rendering the true colors of the objects. Through an innovative technology, the light source gives back the chromatic information which are inside the human memory (Figure 1).

It is a light source which enhances the colors collected by the human mind through different experiences. For this reason it is a light source suitable for several applications, in particular for all those situations where it is needed to light up, all together, several objects having different colors, but getting a perfect and balanced colors saturation.

It comes from a revolutionary patented light spectrum which saturates – at the same time and with a perfect balance – either warm and cold colors.

Colors are not made with reference to the “old” metrics related to fluorescent and halogen light source, but they are made in such a way to reproduce – as much accurately as possible – the human eye memory and perception, based on familiar objects (Figure 2).

It gets a score based on the MCRI Index (memory colour rendering index), a particular index considering not only the colors saturation, but also colors memory perceptions made by the human mind.

The perfect saturation either of warm and cold colors is surrounded by a “true white” – keeping its natural and clear identity, which gives and enhances such an “unexpected and pleasant” visual comfort.