Designing a colour discrimination test to assess colour rendering of LED sources. Elodie Mahler – MST Optique Physiologique, Optique de contact et Optométrie, Paris 11, Orsay. Françoise Viénot – Centre de Recherche sur la Conservation des Documents Graphiques, MNHN, Paris. Jean-Jacques Ezrati – Centre de Recherche et de Restauration des Musées de France, Paris. Alain Bricoune – Lumières de Paris, Paris.PurposeConclusion Materials and Methods Test: We manufactured 3 a desaturated Panel-like discrimination test with 32 caps equally distributed along a colour circle in CIELAB. L* = ±0.69 C* ab = ±2.16  E ab = 3.02 ± 0.55 CIELAB units between adjacent caps. Figure 1. The colour discrimination test: the “Cercle 32”, designed to assess colour rendering of LED light sources. Light sources: - RGB LED cluster (RGB). - RGB+Amber LED cluster (RGBA). - two-phosphor cold white LED+Red LED cluster (WR). - two-phosphor cold white LED and two-phosphor warm white LED +Amber +RGB LED cluster (WWARGB). Relative intensities of each LED in every cluster were set in order to optimise the CIE Colour Rendering Indices (CRI) 4. - Control light (Solux tungsten-halogen lamp). Table 1. Colour specification of the light emitted by the sources used in the experiment. Figure 2. Spectral power distribution of the light emitted by the sources used in the experiment, measured using a Minolta CS-1000 spectroradiometer. Observers: 40 observers having normal colour vision as tested with Nagel anomaloscope. Each observer was tested with the five types of illumination sequentially. The sequence of illumination was balanced between observers. - Test duration : 3 minutes - Time interval between two illuminations : 3 minutes Results Discrimination test: - 7 / 40 observers successfully passed the test under all illuminations / 40 observers failed under one or more illuminations. In order to quantify the impairment, we computed a discrimination Index based on the elevation of the average perceptible colour difference. The full colour path in L*a*b* was obtained by adding the 32 colour steps between two caps placed next to the other by the observer. The length of the correct colour path is of CIELAB units. For example, the length of an erroneous colour path with an inversion between cap 17 & 18 is of This makes an increase of the path of 5.6%. For RGB illumination, 46% of failures occur along the yellow axis, namely among greenish-blue shades and among purple shades, instead of 25% for the unbiased distribution. Figure 3. Examples of fails under each type of illumination, selected among observers plots. They are representative of the most frequent errors. Colour Rendering Index: We calculated the general and the special colour rendering indices according to the Test-Colour Method as recommended by CIE. In order to compare the CRI with the visual discrimination test results, we also computed a CRI based on the C32 test colour samples. Figure 4. Indices describing the discrimination efficiency and the CRI of the light sources. We performed statistical tests about the number of erroneous tests (χ²), and about the average path (  ), and found that, at the risk of 5%, the five light tests are not significantly different. RGB LED illumination yields poor discrimination and poor colour rendering. The most impaired colours by RGB LED light lie along the yellow axis direction, and are located in two areas of the hue circle, that is to say greenish-blue and purple shades. A supplement of light, as in the WWARGB LED cluster, should be offered in the missing parts of the spectrum to achieve the same success as the continuous spectrum light. The colour discrimination is correlated with the colour rendering. An improved CRI based on uniformly distributed test samples would make appear the deficiency axis and also provide colour discrimination indices. Discussion LED technology offers light sources with narrow spectrum in the visible range. Now, several LED arrangements allow the production of white light¹. To assess the quality of colour rendering of LED white light sources, we recently designed an experiment with real colour surfaces, light sources and observers. 57 colour normal observers performed the desatured Panel DD15 test from Lanthony illuminated with various LED clusters, and with a control continuous spectrum light². Compared with the control illumination, all LED clusters impaired colour discrimination, with a severe impairment for RGB LED cluster around greenish- blue and purple shades. However, the steps between adjacent caps of the DD15, that is designed to highlight confusion lines, are not constant in L*a*b*. The steps along the yellow axis are small. In order to avoid biased conclusions, we have designed a colour discrimination test aimed at evaluating lighting quality, to reveal low discrimination efficiency in any region of the hue circle. 1. CIE Expert Symposium on LED Light-sources, Physical Measurement and Visual and Photobiological Assessment, Tokyo, Viénot F, Discriminating colours under LED illumination, AIC Grenade, Leprêtre G., private manufacture of the tray, Paris CIE, Method of Measuring and Specifying Colour Rendering Properties of Light Sources, Publication CIE 13.3, References Figure 5. Position of 32 caps colours in L*a*b* when illuminated by the sources used in the experiment and by the Planckian radiator at the same colour temperature. Figure 6. Number of erroneous choices of neighbouring cap, for each cap, including all examinations. The distribution of errors in the chromaticity circle is different between RGB LED illumination and Solux illumination. However, all illuminations have the same pattern of distribution approximately. Figure 7. Correlation between particular C32 CRIs and apparent steps between adjacent caps in L*a*b* of the C32 caps under RGB illumination The correlation between colour rendering and colour discriminating is fine. It is higher involving C32 or CIE CRI and average path increase than in any other combination of indices. Compared with the errors under control illumination, the errors under RGB LED light are often larger. Thus, rather than the number of erroneous tests, we look for an explanation of the path increase. Figure 5 shows that the chromaticity circle drawn for the C32 test is apparently widened for RGB LED illumination. Its average step between adjacent caps is of 3.86 ± In opposition, it is of 2.84 ± 0.52 for Solux illumination. RGB illumination changes the specification of colours. It pushes to the extremity of an ellipse the greenish-blue and purple shades. Besides, the steps at the apices of the ellipse are smaller, although the average step increases. The discrimination efficiency of RGB illumination is reduced precisely for these falsely saturated colours (fig. 6). Position in L*a*b* of caps when illuminated with the light sources used in the experiment