Quang Vinh Trinh

COLOUR PREFERENCE OF HUMAN SKIN TONES UNDER DIFFERENT CORRELATED COLOUR TEMPERATURES OF A MULTICHANNEL HYBRID LED-LUMINAIRE

The beauty of human skin tones is a very important requirement for the luminaires in the lighting industry of cinemas, hospitals, cosmetics, shops, museums, galleries as well as in other human activities. Independent of the different demands of relaxation, entertainment, communication, business, cultural and art activities, human skin tones should be illuminated best according to their colour properties under both natural and artificial light sources. The research questions are whether the highest colour fidelity property of a light source for human skin tones can satisfy the light source users or is there another aspect of colour quality that should be added? What is the role of correlated colour temperature concerning this aspect of artificial light sources? In this work, these issues are investigated by means of visual experiments under a programmable hybrid LED-luminaire. Subjects had to scale the colour preference of their own skin tones visually. Analysing the visual results, the compromise between colour fidelity and colour saturation enhancement are discussed.

Colour preference, naturalness, vividness and colour quality metrics, Part 4: Experiments with still life arrangements at different correlated colour temperatures

Subjective colour preference, naturalness and vividness assessments of two different colourful still life arrangements viewed in a real room were analysed and modelled with the aid of the Rf colour fidelity metric combined with a chroma difference metric. Coloured objects were illuminated by a four-channel LED light engine with 36 different spectra at four correlated colour temperatures and nine object oversaturation levels. Results imply a significant dependence of the subjective judgments on correlated colour temperature.

Analysis of accelerated LED degradation by statistical methods

In the fast developing LED industry, new generations of LED packages are reaching the market in small time spans. In contrast to that some luminaires, especially in street lighting, should be long lasting and uses the same package over many years. That is one reason why standards are developed to be able to provide lifetime values for the customers. The standard method of calculating the lifetime of an LED package uses the mean values of the luminous flux of a certain number of samples. In a time interval (for example 1000 hours) these values are plotted and fitted by a function, in most cases the exponential function of TM-21 [1] is applied. Our results of LED aging show that often there is no such simple function that is able to describe all the degradation curve. Especially at different conditions (temperature, forward current) the course differ and are not always smooth. Another method to investigate the LED aging is based on statistics. This has been successfully applied in the semiconductor technology (e. g. transistors). In that case every LED package itself gives information, because distributions are the main component of the analysis. The Weibull statistic is tested to the data at different conditions. In order to get a failure rate, the value of a certain percentage of the radiant flux has been taken to define this parameter. The acceleration in the aging test is driven by temperature and forward currents. Both parameters have other effects to the different parts of the LED package. Finally, the acceleration is investigated by the Arrhenius method.