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Assessing the Colour Quality of LED Sources XML Template (2014) [17.11.2014–9:44am] [1–26] //blrnas3.glyph.com/cenpro/ApplicationFiles/Journals/SAGE/3B2/LRTJ/Vol00000/140055/APPFile/SG-LRTJ140055.3d (LRT) [PREPRINTER stage] Lighting Res. Technol. 2014; 0: 1–26 Assessing the colour quality of LED sources: Naturalness, attractiveness, colourfulness and colour difference S Jost-Boissard PhDa, P Avouac MSca and M Fontoynont PhDb aLaboratoire Ge´ nie Civil et Baˆ timent, Ecole Nationale des Travaux Publics de l’Etat, Universite´ de Lyon, Lyon, France bDepartment of Energy and Environment, Danish Building Research Institute, Aalborg University, Aalborg, Denmark Received 10 June 2014; Revised 26 August 2014; Accepted 23 September 2014 The CIE General Colour Rendering Index is currently the criterion used to describe and measure the colour-rendering properties of light sources. But over the past years, there has been increasing evidence of its limitations particularly its ability to predict the perceived colour quality of light sources and especially some LEDs. In this paper, several aspects of perceived colour quality are investigated using a side-by-side paired comparison method, and the following criteria: naturalness of fruits and vegetables, colourfulness of the Macbeth Color Checker chart, visual appreciation (attractiveness/ preference) and colour difference estimations for both visual scenes. Forty-five observers with normal colour vision evaluated nine light sources at 3000 K, and 36 observers evaluated eight light sources at 4000 K. Our results indicate that perceived colour differences are better dealt by the CIECAM02 Uniform Colour Space. Naturalness is better described by fidelity indices even if they did not give perfect predictions for all differences between LED light sources. Colourfulness is well described by gamut-based indices and attractiveness was found to correlate best with gamut-based indices but also with a preference index or a memory index calculated without blue and purple hues. A very low correlation was found between appreciation and naturalness indicating that colour quality needs more than one metric to describe subjective aspects. 1. Introduction questions about conventional CIE colorim- etry and the calculation of the CIE General LEDs will soon become the major source of Colour Rendering Index (Ra).1 light due to attractive total cost of owner- Thirty years ago, the CIE introduced the ship, but their spectral properties are quite current method for measuring and specifying different from the light sources they are colour rendering. Since then, the Colour replacing. This leads to possible risks of Rendering Index has been widely used and is deterioration as well as improvement of currently the only internationally recognised lighting quality. Their different spectral indicator for measuring and specifying the power distributions (SPDs) also raise colour-rendering properties of light sources. However, over the past few years, there has Address for correspondence: Sophie Jost-Boissard, been increasing evidence of its shortcomings Laboratoire Ge´nie Civil et Baˆtiment, Ecole Nationale des and several attempts have been made to update Travaux Publics de l’Etat, Universite´de Lyon, Lyon, France. 2–6 E-mail: [email protected] the method. Since the emergence of LEDs, ß The Chartered Institution of Building Services Engineers 2014 10.1177/1477153514555882 XML Template (2014) [17.11.2014–9:44am] [1–26] //blrnas3.glyph.com/cenpro/ApplicationFiles/Journals/SAGE/3B2/LRTJ/Vol00000/140055/APPFile/SG-LRTJ140055.3d (LRT) [PREPRINTER stage] 2 Jost-Boissard et al. psychological experiments have revealed that metric are adequate to predict naturalness, the CIE colour-rendering metric correlates colourfulness, and preference when con- poorly with visual appreciation.7–12 In 2006, sidered together. Smet et al.23 in their after the publication of the CIE report on review of various visual studies found those colour rendering of white LED sources13 a components relevant for naturalness aspects Technical Committee (CIE TC1-69) was but not optimal for preference/attractiveness. formed to investigate the new methods for Contrary to Islam et al. who found that assessing the colour rendition of white light colourfulness of objects defines the natural- sources and new metrics have been pro- ness, Smet et al.24 found that naturalness posed.14–18 The term ‘rendition’ was employed and colourfulness are non-correlated and to mean the influence of light sources on colour that two separate factors are necessary to appearance of objects. It is different from describe vividness/colourfulness on the one ‘colour rendering’ defines by CIE hand and naturalness/fidelity on the other International Lighting Vocabulary as the hand and that preference/attractiveness ‘effect of an illuminant on the colour appear- could be considered as a combination of 25 ance of objects by conscious or subconscious both. Bodrogi et al. tested nine different comparison with their colour appearance under properties of colour quality and their results a reference illuminant’. (The term rendition will suggest that six factors are needed to explain also be preferred within this manuscript). those nine properties. TC1-69 ended with the recommendation to In this paper, we wanted to assess key consider colour fidelity and other aspects of aspects of colour rendition quality which have colour quality separately. Two new TCs have been raised by members of the community. been launched: TC 1-90 which evaluates avail- We investigate with a psychovisual experi- able indices based on colour fidelity for assess- ment the objective aspect of colour fidelity ing the colour quality of white light sources and dealing with the estimation of colour differ- TC 1-91 which investigates new methods for ence and the subjective aspects related to evaluating colour quality excluding fidelity. natural rendering, appreciative viewing and Colour fidelity in comparison to a refer- degree of colourfulness. The performances of ence illuminant is one important aspect of 19 colour metrics have been evaluated colour quality but other aspects such as through correlation between metrics’ predic- preference and naturalness are also of inter- tions and perceived colour quality judgements est in everyday life. One question of interest by a panel of observers. is the number and nature of non-correlating indices required to obtain a reasonable good 2. The experimental set-up prediction of overall colour rendition. To this end, Guo and Houser19 compared, 2.1. The triple booth device computationally, several indices and recom- An experimental triple booth device was mended the use of at least two metrics, one developed. It consists of three identical reference-based and one area or volume- single booths (600 Â 300 Â 300 mm) placed based index. These two components were side by side (Figure 1). The walls and validated by Rea and Freyssinier6,20 who bottom of the booths are painted in recommend they be used jointly to predict medium grey matt paint (Munsell N5). discrimination, vividness and naturalness for They are lit by different light sources general illumination application. Dangol located in diffusing ceilings. The light et al. and Islam et al.21,22 also found that passes through a polycarbonate double dif- a reference-based metric and an area-based fuser which provides a homogeneous Lighting Res. Technol. 2014; 0: 1–26 XML Template (2014) [17.11.2014–9:44am] [1–26] //blrnas3.glyph.com/cenpro/ApplicationFiles/Journals/SAGE/3B2/LRTJ/Vol00000/140055/APPFile/SG-LRTJ140055.3d (LRT) [PREPRINTER stage] The colour quality of LEDs 3 Figure 1 Photograph of the triple booth experimental device illuminance distribution (difference less than À0.06 nm and the readings for the lumi- 5%) at the bottom of the booth. The central nance and the illuminance differed by Æ5%. and the left booths are equipped with five SPDs of the light sources were measured different types of LEDs (white, cyan, green, in the booths at the surface of a spectral amber and red), and the right booth is reflectance white standard provided illuminated with halogen or fluorescent by Gigahertz-Optik (BN-R986SQ2C) placed sources. The temperatures in the LEDs’ on the centre of the horizontal plane of booths were checked and stabilised during the booths, accounting for inter-reflections the experiment. The setup is located in a from booth surfaces and the absorption room with black wall with no daylight or of the white finish. The SPDs were ambient light. measured for quasi-perpendicular illumin- ation and a 458 viewing angle, which corres- pond to the geometry of the visual 2.2 Measurement instruments observation. A Minolta CL200 illuminance meter was used to measure horizontal illuminances. 2.3 Light sources under study The Specbos 1200 spectroradiometer In this study, we measure the psychophys- (Number series 319090), was used to meas- ical responses to the colour rendition of ure the spectral power distributions (SPDs) light sources. Two experiments based on the of the light sources and the spectral reflect- same protocol were done at two correlated ance of the fruits and vegetables. The colour temperatures (CCT), corresponding spectral range of measurement was 380 to to typical indoor lighting in France 780 nm with increments of 1 nm. The (3000 K and 4000 K). Some results of the spectroradiometer was calibrated by 3000 K experiment have been presented in SCIENTEC in France before the experi- Jost-Boissard et al.26 ment. The calibration results were per- For each CCT, we selected a light source formed with a source with spectral lines with a good Ra (reference sources) along with Helium OL-455-8 N892201147 calibrated by a number of LED clusters (test sources). the LNE (N8G0805291 B.N.M. COFRAC For both CCTs, the luminance and illu- N82-25). The wavelength verification pre- minance of the booths’ bottom surface was sented a deviation within À0.002 nm and set to approximately the same value, i.e. Lighting Res. Technol. 2014; 0: 1–26 XML Template (2014) [17.11.2014–9:44am] [1–26] //blrnas3.glyph.com/cenpro/ApplicationFiles/Journals/SAGE/3B2/LRTJ/Vol00000/140055/APPFile/SG-LRTJ140055.3d (LRT) [PREPRINTER stage] 4 Jost-Boissard et al.
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