LEDs experiments in Paris 1. Colour rendering 2. Kruithofs rule F. Viénot, E. Mahler, A. Rambaud, M.-L. Durand, C. Boust, J.-J. Ezrati, CRCC, Muséum national dhistoire naturelle CIE, TC1-69, Stockholm, June 2008

Colour rendering - Illumination LED clusters RGB LEDs RGB RGB+Amber LEDs RGBA two-phosphor cold white LED + Red LED WR two-phosphor cold white LED + two-phosphor warm white LED + Amber + RGB LEDs WWARGB Control light : Solux filtered tungsten-halogen lamp WWARGBWRRGBARGB Solux

Colour rendering - Discrimination Distribution of errors RGB Solux

Colour rendering - Appearance Distribution of colourfulness ratings Distribution of hues Colourfulness rating RGB Solux WWRGBA RGB <- Red <- Yellow <- Green <- Blue <- Red Red -> Yellow -> Green -> Blue -> Red ->

Colour rendering - Conclusion Colour rendering –either Colour fidelity, colour discrimination –or Colourfulness enhancement Different tasks and different visual coding Results of thiese experiments can be predicted from CIELAB or CIECAM02 Agreement with Rea Mark S., Freyssinier-Nova Jean P., 2008, Color Rendering: A Tale of Two Metrics, COLOR research and application, 33:

Kruithofs rule - Background Kruithof, A. A.: Tubular Luminescence Lamps for General Illumination, Philips Technical Review 6, pp (1941). Kruithof has examined the illuminance limits between which users find the illumination pleasant. Kruithofs diagram (Fig. 10) shows the accepted combination of illuminance and colour temperature. The author states that, globally, a low and high illumination should correspond to a low and high colour temperature, respectively ( my translation ). Illuminance (log scale) Correlated colour temperature

Re-examination of Kruithofs rule Objectives Since LED illumination is very flexible in terms of luminous intensity and colour temperature, we have planned an experiment to re-examine this rule, questioning the interpretation and the physiological basis of the pleasantness sensation. Collaborators –Marie-Lucie Durand, Paris-Orsay University –Elodie Mahler, SNCF, France

Re-examination of Kruithofs rule Methods Light booth equipped with LEDs (cool white, warm white, orange, amber, green, cyan, blue) Illuminance : 150, 300 and 600 lx Correlated colour temperature : 2700, 4000, 6500 K Optimised CRI > 86

Re-examination of Kruithofs rule Tasks Three type of tasks –Performance (acuity, contrast threshold, reading) –Color perception (hue appearance, colour preference, cognitive colours) –Subjective feeling (pleasantness, mood, …) Methods (depending upon the task) –Scaling, ranking, thresholds, category scaling, semantic analysis, ANOVA 30 young observers

Re-examination of Kruithofs rule Performance Example of low-contrast acuity Illuminations Y in cd.m -2 low-contrast acuity (log)

Re-examination of Kruithofs rule Colour appearance Example of location of binary hues

Re-examination of Kruithofs rule Sensation scale Example of lightness sensation Illuminations Y in cd.m -2 Perceived lightness scale