Tran Quoc Khanh

LED Lighting: Technology and Perception

Promoting the design, application and evaluation of visually and electrically effective LED light sources and luminaires for general indoor lighting as well as outdoor and vehicle lighting, this book combines the knowledge of LED lighting technology with human perceptual aspects for lighting scientists and engineers. After an introduction to the human visual system and current radiometry, photometry and color science, the basics of LED chip and phosphor technology are described followed by specific issues of LED radiometry and the optical, thermal and electric modeling of LEDs. This is supplem

Colour preference varies with lighting application

Numerous efforts have been made to investigate how to characterize the effect of light sources on colour preference. This paper describes a psychophysical experiment to compare the colour preference evaluation among three sources for different lighting applications. Three lighting applications – restaurant, retail display, and supermarket – were created in two side-by-side full-scaled rooms. It was found that the colour preference varied with the lighting applications. Although none of the existing single-value measures can predict the preference for all three applications, a single measure is necessary for general consumer use given a careful definition of general lighting. In addition, lighting application specific measures could be useful for lighting professionals and experts. To provide detailed information, a colour distortion icon, with improved test colour samples will be useful.

Colour preference, naturalness, vividness and colour quality metrics, Part 1: Experiments in a room:

In Part I of this work, observers scaled colour preference, naturalness and vividness visually on interval scales (0–100) labelled by semantic categories (e.g. ‘moderate’, ‘good’ and ‘very good’) in the context of office lighting. Five customary light sources without object saturation effect illuminated a table with coloured objects in a real room. The observers’ assessments were predicted by recent colour quality indices and selected pairs of indices combined linearly. Criterion values of the indices for ‘good’ colour preference and vividness were determined to provide a usable acceptance limit for the spectral design and evaluation of light sources. To predict colour preference, correlated colour temperature turned out to be useful. In Part 2 of this work, another experiment with the same method but using multi-LED spectra with more object saturation will be analysed and the two datasets will be merged.

Intercultural observer preference for perceived illumination chromaticity for different coloured object scenes

The preference for the chromaticity of high illuminance, high colour rendering LED illumination of different scenes was investigated for Chinese and European observers. An experiment about the preference for perceived illumination chromaticity was conducted with the same multi-LED light engine, same viewing booth and same coloured objects transported between Germany and China. Observer preference for perceived illumination chromaticity was significantly influenced by correlated colour temperature (2700 K–6500 K), object scene colour (red, blue or mixed), cultural background (Chinese or European origin, living in Germany or in China) and gender (men, women). The results can be used to choose an appropriate correlated colour temperature (e.g. warm white or cool white) for a high-quality lighting product to illuminate different object scenes (e.g. red or blue objects) so as to achieve good user acceptance in the global market.

Colour preference, naturalness, vividness and colour quality metrics, Part 2: Experiments in a viewing booth and analysis of the combined dataset

In Part 2 of this work, observers scaled colour preference, naturalness and vividness visually on interval scales (0–100) labelled by semantic categories (e.g. ‘moderate’, ‘good’ and ‘very good’) in the context of food lighting using the same questionnaire as in Part 1. Seven multi-LED light sources with more or less object saturation effect illuminated a viewing booth with coloured food objects. The two datasets (Part 1: room + Part 2: viewing booth) were merged and the observers’ assessments were predicted by recent colour quality indices and CIELAB chroma differences. Linear combinations of selected pairs or triads of descriptors were used to predict the merged dataset. Criterion values to achieve ‘good’ preference, naturalness and vividness level were determined.

Optimized flash light-emitting diode spectra for mobile phone cameras

Flash light-emitting diodes (LEDs) of modern mobile phones lack red and cyan spectral parts, however, the color gamut of their respective displays has increased in recent years. The influence of this discrepancy on the color reproduction of smart phones is investigated in this paper. Based on the CIECAM02 color appearance model, a metric is introduced to judge color reproduction of mobile phones under flash LED illumination. An evaluation method is established to compare the visual appearance of a scene under various surrounding illuminations with the reproduction of that scene. To facilitate a comparison with measurements, the evaluation method is based on the raw data of two test cameras and a Digital ColorChecker SG. To reduce the color shift between perception and reproduction, optimized flash LED spectra are presented. A single-LED and a double-LED concept with adjustable color temperature are derived from these results. Additionally, the common characteristics of flash LED spectra giving good results is investigated, identifying the spectral parts with the most influence on camera color reproduction and showing the spectral parts not contributing or even resulting in poor color reproduction. Finally the efficiency of optimized flash LED spectra is compared to standard flash LEDs.

Colour preference, naturalness, vividness and colour quality metrics, Part 3: Experiments with makeup products and analysis of the complete warm white dataset

Light source colour quality is a complex phenomenon with several visual and cognitive components. Subjective colour preference, naturalness and vividness assessments of reddish cosmetic products in a viewing booth lit by light sources with a correlated colour temperature of 3200 K and to 550 lx were analysed and modelled by a new combined colour quality formula – a linear combination of a colour fidelity index and a measure of chroma change. Results of the warm white light sources described in all three parts of this work were merged. They were modelled by optimising the parameters of the same formula. The formula supports multi-LED light sources with moderately accentuated local spectral maxima to enhance object chroma over a moderate range.

Effect of chromatic mechanisms on the detection of mesopic incremental targets at different eccentricities.

Spectral sensitivity functions for the threshold detection of mesopic incremental targets were compared for different target eccentricities (10, 20, and 30°) and for different mesopic backgrounds (0.1, 0.5 and 1.0 cd m−2). Relative responsivities of achromatic mechanisms (L + M and rods) and chromatic mechanisms (S and |L–M|) were estimated for each eccentricity and background. Chromatic mechanisms contribute significantly to detection but their effect is lower at 30°. A new contrast metric (CCHC2) is introduced to account for the selective adaptation of the photoreceptors and the effects of the chromatic mechanisms i.e. broadening of the range of spectral sensitivity with multiple local maxima and yellow sub‐additivity of detection performance. The CCHC2 metric is compared with the achromatic contrast metric of the MOVE model (CMOVE). For the same target, CCHC2 generally predicts a higher visibility level than CMOVE. However, in accordance with visual observations, for grey or yellowish incremental targets appearing at the eccentricities of 20 and 30°, the visibility predicted by CCHC2 is less than the visibility predicted by CMOVE.

Measuring, simulating and optimizing current LED phosphor systems to enhance the visual quality of lighting

Lighting engineering aspects of light emitting diodes (LEDs) with down converting phosphors (phosphor converted LEDs or PC-LEDs) were investigated by building sample PC-LEDs, measuring their spectral emission physically and simulating them with a software. This article describes the influence of the phosphors’ chemical, physical and technological properties on some well-established and widely used measures of visual lighting quality including luminous efficacy and the color rendering index. The dependence of these measures on YAG and LuAG phosphor particle size and activator concentration in the PC-LEDs is dealt with. Light sources of superior lighting quality can be obtained by mixing several phosphors. Hence, the mixture of a green and red phosphor system was also studied for a set of real PC-LED light sources and their simulations. Two green and two red phosphors are considered in this study. To predict and optimize the lighting quality of PC-LEDs with a reasonable accuracy, a usable input shall be provided for the simulation software assuming the understanding of the emission mechanisms. To validate our simulation results, the PC-LEDs’ measured spectral emission characteristics were compared with their simulated counterparts. The measurement and simulation of spectral power distributions contribute to the understanding of new phosphor mixtures. The present simulation method turned out to be usable to optimize the target parameters of the PC-LED (luminous efficacy, white tone quality and color quality of the illuminated colored objects).

Psychologische Blendung bei Halogen- und Xenonscheinwerfern.

Die psychologische Blendung ist neben der guten Ausleuchtung der Fahrbahn ein entscheidender Aspekt bei der Beurteilung von Frontscheinwerfern im alltaglichen Strasenverkehr. Das Fachgebiet Lichttechnik der Technischen Universitat Darmstadt leistet bereits seit mehreren Jahrzehnten auf diesem Gebiet einen entscheidenden Beitrag. Erstmals wurde nun der Einfluss der Lichtquelle, des Adaptationsspektrums und der Scheinwerferoptik auf die psychologische Blendung unter realen Bedingungen in einer Testreihe fur Halogen- und Xenonscheinwerfer untersucht. Die Ergebnisse werden mit Berechnungen anhand eines Verfahrens verglichen, das auf einer spektralen Empfindlichkeitsfunktion fur die psychologische Blendung basiert.