Yandan Lin

Model predicting discomfort glare caused by LED road lights

To model discomfort glare from LED road lighting, the effect of four key variables on perceived glare was explored. These variables were: the average glare source luminance (Lg), the background luminance (Lb), the solid angle of the glare source from the perspective of the viewer (ω) and the angle between the glare source and the line of sight (θ). Based on these four variables 72 different light conditions were simulated in a scaled experimental set-up. Participants were requested to judge the perceived discomfort glare of these light conditions using the deBoer rating scale. All four variables and some of their interactions had indeed a significant effect on the deBoer rating. Based on these findings, we developed a model, and tested its general applicability in various verification experiments, including laboratory conditions as well as real road conditions. This verification proved the validity of the model with a correlation between measured and predicted values as high as 0.87 and a residual deviation of about 1 unit on the deBoer rating scale. These results filled the gap in estimating discomfort glare of LED road lighting and clarified similarities of and differences in discomfort glare between LED and traditional light sources.

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.

Investigating methods for measuring face recognition under lamps of different spectral power distribution

Face recognition is one of the interpersonal judgements carried out by pedestrians. Road lighting should enhance the visual component of such judgements after dark. This paper presents experiments carried out using two different procedures, identification and perceived recognition, to investigate why earlier studies have led to inconsistent conclusions about the influence of spectral power distribution (SPD) on face recognition. For the identification procedure, two observation durations were employed (1 s and 3 s). The two procedures led to similar conclusions regarding recognition ability at different distances. Review of these and past results suggests that an effect of lamp (SPD) is more likely to be found when the task is difficult, i.e. small size, brief observation and correctly naming the target rather than simply picking a face from a sample.

Cross-cultural variation of memory colors of familiar objects

The effect of cross-regional or cross-cultural differences on color appearance ratings and memory colors of familiar objects was investigated in seven different countries/regions - Belgium, Hungary, Brazil, Colombia, Taiwan, China and Iran. In each region the familiar objects were presented on a calibrated monitor in over 100 different colors to a test panel of observers that were asked to rate the similarity of the presented object color with respect to what they thought the object looks like in reality (memory color). For each object and region the mean observer ratings were modeled by a bivariate Gaussian function. A statistical analysis showed significant (p < 0.001) differences between the region average observers and the global average observer obtained by pooling the data from all regions. However, the effect size of geographical region or culture was found to be small. In fact, the differences between the region average observers and the global average observer were found to of the same magnitude or smaller than the typical within region inter-observer variability. Thus, although statistical differences in color appearance ratings and memory between regions were found, regional impact is not likely to be of practical importance.

The influence of luminance, observation duration and procedure on the recognition of pedestrians’ faces

Past studies carried out to determine whether the spectral power distribution of a light source affects facial recognition have led to mixed results. One reason for this could be that different studies presented different levels of task difficulty, this being a function of target size, observation duration and type of procedure. This paper presents two facial recognition experiments carried out using matching and identification procedures to explore task difficulty as defined by observation duration and target luminance. It was confirmed that identification is more difficult than matching. A relationship between task difficulty (luminance and duration) and recognition probability was found and this allows the luminance to be determined for a given probability.

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.

The prediction of perceived colour differences by colour fidelity metrics

A psychophysical experiment was conducted to evaluate the performance of various colour spaces, colour difference formulae, colour matching functions and colour fidelity measures in predicting perceived colour differences. Ten observers evaluated the colour differences of 20 colour samples under 11 pairs of light sources. The results suggest that the colour differences calculated using the CIE 1964 colour matching functions in the CAM02-UCS colour space can predict the perceived colour differences. IES-Rf which used the 20 colour samples and the reference illuminants used in the experiment was highly correlated to the perceived colour difference. The importance of a uniform spectral sensitivity for colour fidelity measures is also identified.

Colour quality evaluation of natural objects based on the Feeling of Contrast Index

Past studies have shown that the Feeling of Contrast Index (FCI) correlates with preference in some circumstances, but the exact relationship is not clear and may differ among visual conditions and targets. This paper presents a psychophysical experiment in which 70 observers evaluated the colour quality of natural objects under different spectral power distributions with different values of FCI but the same correlated colour temperature (3000 and 5000 K). Results show that FCI holds a nearly perfect linear correlation with vividness (r = 0.976); unnaturalness associated with enhanced vividness rendered by excessively high FCI deteriorates preference. These effects are independent of correlated colour temperatures, but are dependent upon the visual targets used because of limitations in the derivation of FCI. Compared to other metrics, FCI can characterise vividness under a light source.

Preference for appearance of Chinese complexion under different lighting

The method for evaluating the colour rendering of light-emitting diode sources is controversial, especially for the appearance of human complexions. A psychophysical experiment was conducted using Chinese models to examine the effect of various illumination settings, characterized by two levels of preferred skin colour index and four levels of correlated colour temperature, on preference for the appearance of the Chinese complexion. Results showed that the preferred skin colour index was an effective indicator. Taking incomplete colour adaptation into account, the preferred skin colour index was recalculated and shown to be the best colour rendering index for the appearance of the Chinese complexion. This research examined the concept of preferred skin colour index for Chinese women. This can supplement other colour quality evaluation methods, and shows promise for commercial lighting applications.