Lighting Research & Technology

This paper investigated atmosphere perception as dynamic white lighting varied in illuminance or correlated colour temperature in different directions and with different functional forms of change. Fifteen Chinese observers evaluated 32 dynamic lighting stimuli. A 13-scale questionnaire was employed for each stimulus. Factor analysis was adopted to examine the gender difference and the effect on atmosphere perception. Two dimensions of atmosphere perception were obtained and identified as coziness and liveliness for both illuminance change and correlated colour temperature change. Gender differences exist mostly on the scales ‘informal-formal’ and ‘slow–fast’. Dynamic white lighting shows significant influence on atmosphere perception. For illuminance changes, high correlated colour temperature levels are useful to create liveliness and a formal atmosphere. Liveliness also increases when the light level rises. A linear change increases coziness most and produces the slowest perceived speed of change. For dynamic light variations in correlated colour temperature, changes under high illuminance level or changes from warmer light to cooler light create liveliness. Perceived evenness tends to decrease when the light level rises. Linear and quadratic changes offer the coziest atmosphere. Further studies are needed but the effect should be considered when designing dynamic lighting installations or when dimming for numerous applications.

Most studies on the effects of light exposure have been conducted with continuous light. The present study investigated the effects of intermittent light exposure on sleepiness, mood, electroencephalographic activity during sleep and performance the next morning. Fifteen volunteers were scheduled to come to the sleep laboratory to experience different lighting conditions: intermittent bright light, continuous bright light and continuous dim light. Subjective sleepiness and mood were assessed during light exposure, with electroencephalographic recording during sleep. After waking the next morning, participants filled out questionnaires and went through two cognitive tasks. The results revealed significantly lower ratings of sleepiness after intermittent light exposure, which is not different from the ratings in the continuous bright light condition, and an increase in vitality during later part of the evening and more beta activity during the first 90 minutes of sleep in the intermittent light condition, in comparison with the continuous dim light condition. However, both intermittent and continuous bright light exposure showed no difference from the continuous dim light condition in subjects' mood and cognitive functioning the next morning. The data indicated intermittent light during evening decreased sleepiness, had only minimal impact on mood in the evening, increased beta electroencephalographic activity during sleep, but had no significant influence on cognitive functioning the next morning.

Over the past decade, there has been a growing interest in lighting research on the effects of the recently discovered melanopsin receptor (also referred to as the intrinsically photosensitive retinal ganglion cell) and its impacts on health and vision. Presently, there is not a generally accepted metrology for dealing with the spectral response of the melanopsin receptor as applied to both lighting and vision research. A proposition to handle this issue from a vision science perspective has been presented in 2014 in the journal Trends in Neurosciences and from a more lighting perspective in 2017 in Lighting Research and Technology. These propositions are complex, and do not retain the CIE standard definition of a lumen. In this paper, we propose an approach based on effective watts and melanopic/photopic ratios that is both simpler and more closely aligned with CIE standard unit definitions. In addition, we include some practical examples of how such ratios are accessible now, and can be used for both lighting and vision research as well as applications.

Correlations between selected lighting quality metrics derived from the spectrum were computed and interpreted for a representative set of interior light source spectra. Metrics of brightness and the melanopic effect were strongly correlated with each other but not with the metrics of colour quality. Results can be used to establish a two-dimensional diagram with criterion values to ensure the general user acceptance of a lighting system. This diagram can be extended in the future by further metrics describing light intensity and spatial and temporal light distributions. Relationships between lighting quality metrics and colorimetric quantities as well as photoreceptor signal values are also shown.

In this study, two psychophysical experiments, one on colour preference and the other on colour discrimination, were conducted. To investigate the colour preference for blue jeans, 27 subjects with normal colour vision were asked to rate their visual appreciation of seven pairs of jeans with a colour gradient pattern. Nine LEDs, with uniformly sampled correlated colour temperature (CCT) values ranging from 2500 K to 6500 K, were used to illuminate the jeans. These lights produced a constant illuminance of approximately 200 lux, and their colour rendering indexes were between 79 and 91. In addition, using a Farnsworth-Munsell (FM)-100 Hue Test, the blue-region colour discrimination of 42 observers was assessed for five LEDs of the same type but with different CCTs (2500 K–6500 K, 1000 K interval). The results indicate that there is an optimum CCT of 5500 K for jeans, at which observers were found to exhibit the greatest capability for colour discrimination and the highest rating for colour preference. Interestingly, a significant gender difference was found in this study, which had not been observed in our previous work with quite similar experimental settings but different experimental objects. The findings of this study should provide a deeper understanding for the lighting design of shopping malls for jeans.

Fourier analysis is applied to the profile of illuminance around the curved surface of a small cylinder, due to one, two and three point sources of light and due to an extended source. This reveals only one odd harmonic, whose amplitude is equal to half that of the illumination vector. Lighting can also generate a diminishing series of even harmonics. The lighting at any point in open space can be expressed in terms of the harmonics of illuminance around three mutually perpendicular cylinders. Visual discomfort is often associated with the presence of even harmonics. The photometry of cylindrical and semi-cylindrical illuminance, and of cylindrical harmonics, is discussed.

The subjective impressions of pedestrians are necessary in order to decide on the appropriate colour of light to be used for street lighting. Therefore, a pilot project aimed to compare subjective evaluations of the sidewalk illumination under two street lighting installations, realised by LEDs of 3000 K (warm white) and 4000 K (neutral white), was recently conducted in Belgrade. Both installations had comparable sidewalk illuminances as well as other relevant photometric parameters. The evaluation was done through a questionnaire. A group of 139 (61 male and 78 female) respondents, all of them university students, was asked to grade both lighting installations for the sidewalk light intensity, the appearance of human faces, the colour of light and the colour rendering as well as the overall impression. According to the median values, the 3000 K LED installation was considered better than the 4000 K installation for all aspects assessed as well as the overall impression. Although the survey results convincingly showed a preference for 3000 K LEDs for this comparison, additional research is needed using a more representative sample of people and a wider range of locations before a definite conclusion can be reached.

In today's world, light-emitting diodes are quickly replacing conventional light sources, both indoors and outdoors. Being a solid-state device, adverse ambient conditions such as high temperature and humidity lead to its poor performances, light quality and longevity. In outdoor applications, exposure to high ambient temperature during daytime, even when the lamp power is off, causes the lamp performance to deteriorate. In tropical countries, the environmental temperature in most of the days is higher than 25℃ and sometimes reaches 50℃. Here, the humidity varies between 30% and 80% and frequently over 80%. Thus, the reliability of the light emitting diodes is a question that needs a thorough study. In line with this, in this study, an effort has been made through an experiment. Here, commercially available surface-mounted device and chip-on-board-based light-emitting diodes are tested through an environmental chamber, and their measured illuminance values were observed by varying the relative humidity. Then, the temperature ageing or stress was set at 60℃ on the above types of light-emitting diodes at power off condition for 6000 hours and after that its luminous flux and spectral distribution were observed. The degradation observed under these conditions may guide designers and manufacturers to upgrade their lighting systems to make them fit for tropical conditions.

Two different types of commercial organic LED panels were aged with ultraviolet light. One type was aged in an over-filled and open circuit condition with broadband ultraviolet radiation, and the other in an under-filled and closed circuit condition with spectrally dispersed light. Both the luminance values and the spectral power distributions of the panels were measured for both types. The results show that the light output and the spectrum of the panel decay faster when the panels are exposed to ultraviolet radiation when compared to natural ageing. For the panels aged using the under-filled method, the wavelength that reduces the output most is between 320 nm and 340 nm, and the ultraviolet exposure with wavelengths longer than 350 nm did not accelerate the ageing. The ultraviolet radiation exposure increased the ageing by a factor of eight as compared to natural ageing, when the used ultraviolet levels corresponded to one year of exposure on a clear summer day condition. Based on the results, the under-filled method is a valid option for accelerated ageing, especially for studying simultaneously accelerated and natural ageing.