A memory colour quality metric for white light sources
Highlights
► A novel metric to assess the colour quality of light sources based on memory colours. ► Memory colours are those colours associated with familiar objects in long-term memory. ► The better the similarity with the memory colour, the better the colour quality. ► The memory colour quality index Rm correlates highly with visual appreciation.
Introduction
Lighting accounts for 19% of the total global electricity consumption and this percentage is still rising [1]. The highly energy-inefficient light source, the standard incandescent bulb, consumes about 30% of the electric-lighting energy budget and is typically used for residential lighting [1]. Typically, incandescent bulbs return only 5–10% of the supplied energy in the form of light. Several countries have passed regulations that will phase-out the use of the incandescent bulb in general lighting in favor of more energy-efficient light technologies, such as compact fluorescent lamps (CFLs) and Light-Emitting Diodes (LEDs). The primary focus is on energy efficiency [2], [3]. Linear fluorescent light sources (LFL) and compact fluorescent sources account for about 45% of the electric-lighting energy consumption [1]. These more energy efficient sources consume between one fifth and one quarter of the electricity of an incandescent bulb. Another energy-efficient type of lighting is Solid State Lighting (SSL). Currently, luminous efficacies of white phosphor LEDs are exceeding those of compact fluorescent sources and fluorescent tubes [4], [5] and continue to rise. Although some hazardous materials can potentially be released to the environment from both fluorescent type lamps (mercury) and SSL lamps (copper, nickel, lead, arsenic [6]), life-cycle analyses have shown that this risk is outweighed by their increased energy-efficiency [2]. In addition to the potential economic and environmental benefits associated with the increased energy-efficiency, solid state lighting has also some technically and commercially interesting properties such as the compactness of the light source, the long lifetime, the tunability of the spectrum to create and/or enhance the mood of a room [7], [8], [9].
However, no matter how energy-efficient they may be, light sources that poorly render the colours of the objects they illuminate, i.e. they exhibit poor colour quality, are often unacceptable in general lighting. Colour quality is therefore one of the key challenges for SSL to become acceptable for general lighting applications. The colour quality of a light source can be defined as the psychological judgment – conscious or unconscious – of the effect of the light source's spectral power distribution on the colour appearance of objects illuminated by it in terms of one or more of several aspects, such as visual appreciation (aesthetic judgment concerning the attractiveness or preference level for the object colours illuminated by the source), fidelity (faithfulness of the object colours with respect to a reference illuminant), naturalness (how natural do the object colours look when illuminated by the source), vividness (a measure often found to correlate with increased saturation of the object colours), colour harmony (aesthetic judgment of the relationship between the different object colours), visual clarity (a clear distinction between surface colours of objects under illumination [10]) or feeling of contrast, colour discrimination (the ability to detect colour differences). As colour quality has many different aspects, no single colour quality metric can provide a complete description [11]. Nonetheless, a metric that is able to assess visual appreciation would be highly relevant to the acceptance of SSL in general lighting as consumers, lighting designers and architects are often interested in how good objects or the lit environment would look. Unfortunately, the only standardized method to evaluate colour quality – the Colour Rendering Index Ra [12] developed by the International Commission on Illumination (CIE) – has been shown to correlate poorly with visual appreciation (and other aspects) of many white light sources. The metric especially fails to correlate well for non-smooth – exhibiting multiple peaks and troughs – and narrowband sources such as tri-band fluorescent lamps [13], [14] and Light-Emitting Diodes [15], [16], [17]. The CIE colour rendering index Ra assesses the colour quality of a light source in terms of its fidelity with a CIE reference illuminant: the smaller the colour differences between a set of coloured cards illuminated by the test source and by a CIE reference illuminant, the higher the fidelity (resemblance) and thus the higher the Ra value. Although such a colour difference based metric is required in many professional applications – colour reproduction, printing and quality control – it is clearly not suited to assess a more subjective aspect of colour quality such as visual appreciation or naturalness, as has been shown in many visual studies [15], [16], [17], [18], [19], [20], [21]. The main reasons for the poor correlation are probably the choice of reference illuminant in combination with the fact that all deviations from that reference illuminant are penalized. A Planckian radiator or daylight phase of the same correlated colour temperature as the test source need not be the optimum source to assess visual appreciation. Indeed, several studies have shown that the effect on the colour appearance of the light from many sources is visually more preferred than that of the CIE reference illuminants [16], [20].
It is clear that assessing the colour quality of a light source based on a comparison with a reference illuminant is difficult. Either one has to know which deviations should or should not be penalized or one has to know which illuminant can be considered perfect. In fact, for both of these approaches the knowledge of what objects ideally look like is required. But once this is known, the need for an illuminant as a reference becomes obsolete because referencing can then take place directly to the ideal chromaticities of the 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 in their pockets to be able to judge the colour quality of the lighting in a room. Colours often just look “wrong” or “distorted”, especially when they are not what we expect or want them to be [22], [23]. Therefore, it is reasonable to assume that people are, consciously or subconsciously, judging the colour appearance of (familiar) objects against the colours they mentally associate with those objects, either in memory or in preference.
Based on the straightforward assumption that colour quality might increase when object colours are rendered more closely to what is expected, the authors propose a memory colour quality metric that is able to assess the colour quality of a white light source in terms visual appreciation. This paper will review the work done by the authors to develop and validate the memory colour quality metric. It will shortly illustrate some of the key differences between the metric and the CIE colour rendering index. In addition, all equations and data necessary to calculate an estimate of the visual appreciation of light source with reference to the memory colours of a set of familiar objects will be systematically presented.
Section snippets
Development of a colour quality metric based on memory colours
Memory colours and preferred colours have long been of interest for their potential use as reference to evaluate the colour appearance of objects. In the sixties and seventies, Judd and Thornton, respectively proposed a flattery [24] and a colour preference index [25] – based on the work of Sanders [26] and Newhall [27] – to assess the colour quality of a light source. However, they did not refer to the memory colour chromaticity of familiar objects directly, but instead calculated a preferred
Validation of the memory colour quality index
Although the memory colour quality metric references directly to the memory colours of familiar objects while taking the unequal tolerances for hue and chroma differences implicitly into account, there was no guarantee that it would actually perform any better than the CIE colour rendering index, Judd's flattery index, Thornton's preference index or any of the other metrics that have been proposed in an attempt to quantify the colour quality of a light source. The metric has been validated in a
Conclusion
Memory colours were successfully incorporated into a metric to assess the colour appearance of objects, and hence colour quality of a white light source. The new memory colour quality metric not only takes the chromaticity of the memory colour of an object into consideration, but also the psychological response to a deviation from the memory colour. This response is described by similarity functions derived from the colour appearance ratings of real familiar objects obtained in a series of
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