Evaluating brightness and glare perception of self-luminous stimuli.

Hermans Stijn – KU Leuven

1. Motivation, specific objective

The fundamental goal of Colour Appearance Models (CAM) is to look for correlates between the measured optical spectral data of both the stimulus and its surrounding and the corresponding perceptual attributes, such as Brightness, Colourfulness and Hue (absolute colour attributes) and Lightness, Chroma and Saturation (relative colour attributes).
Very recently a new colour appearance model, CAM18sl, has been developed for the evaluation of self-luminous stimuli surrounded by a self-luminous neutral background. Modelling these kind of stimuli becomes very important when dealing with issues such as the brightness of LEDs, OLEDs, mobile phones, luminaires, and advertisement bill boards at different times of the day.
Light sources can also cause glare and visual discomfort. CIE defined discomfort glare as: “glare that causes discomfort without necessarily impairing the vision of objects”. In 1995, the CIE proposed the Unified Glare Rating (UGR) for the assessment of discomfort glare in interior lighting. In this UGR formula, the luminance levels of the light sources and background are included; together with the solid angle and the Guth position index of each source. In this paper, the application of CAM18sl is discussed and it is investigated if brightness perception as predicted by CAM18sl can be correlated to the UGR value.

2. Results

Concerning brightness, it is clear that the brightness of stimuli increases with their luminance if the luminance level of the background is kept constant. However, stimuli with the same luminance value, but more saturated in colour are also perceived brighter. This effect is called the Helmholtz-Kohlraush effect and has already been successfully modelled by the CAM15u model for unrelated stimuli. Also, the average brightness perception of a fixed stimulus decreases when the background luminance increases.
Based on this set of data, CAM18sl, a new colour appearance model for self-luminous stimuli, was developed. It combines the approach used in CAM15u (coloured stimuli and a dark background) with the successful implementation of the effect of the background on the brightness by using the Michaelis-Menten compression and adaptation formula. The input of CAM18sl is limited to the absolute spectral radiance of the stimulus and background. The brightness scale of CAM18sl is expressed in “bright”. One bright corresponds to the apparent brightness of a 10° spectral equal-energy self-luminous stimulus having a CIE 1964 10° luminance of 100 cd/m2 and surrounded by a dark background.
CAM18sl has been applied to the virtual luminaire and background and the brightness has been calculated for each combination. In the same way, the UGR for direct view has been calculated too. Comparing both values has revealed a very high correlation (coefficient of determination R2 = 0.99).