Appearance

The dictionary translates appearance as ‘the look or occurrence of something or someone’. In the scientific domain of optics and lighting, the term ‘appearance’ is the umbrella term for all properties related to the perception of light sources (self-luminous colours) and materials (related colours). The final purpose of appearance related research is to find a correlation between visual perception and essential characteristics of appearance (hue, saturation, glare, gloss…) on the one hand, and measurable physical quantities (such as luminance and spectrum) on the other hand. The international commission on lighting CIE (Commission Internationale de l’Eclairage) has defined the research on the correlation between physiological (in this case visual) subjective impression and object physical parameters as ‘soft metrology’. This area of metrological research tries to formulate an answer to the question: Is it possible to measure what we perceive?

In the broad ‘apperance’ research area, the Light & Lighting Laboratory focuses on three topics: colour, gloss, and glare.

On colour, research is conducted on colour perception, colour appearance, subjective colour assessment and the influence of different (environmental) parameters (spectrum, colour, brightness of the illumination, background, nature of the stimulus…) on the perception and evaluation of colour. The study on colour research focuses on the colour rendering properties of light source, the difference between 2° and 10° stimuli, the difference between reflective and luminous colours, the deviation between colour matching with broadband and small band light sources, and colour harmony.

Gloss is usually measured with a gloss meter. This device determines the degree of specular (mirror) reflection on a sample compared to the specular reflection on a reflection standard, in a predefined measurement geometry (20°, 50°, or 85° geometry). Besides the degree of mirror reflection, the gloss is visually dependent on several other parameters such as the distinction of the reflected image, the contrast between the reflected light and the background. It is not surprising that the correlation between the optical characterization of gloss using a gloss meter and the visual perception of gloss is low. The Light & Lighting Laboratory focuses on the development of alternative measurement procedures, with a better correlation between measured results and the visual perception.

Glare issues of luminaires are in Europe usually expressed by the Unified Glare Rating (UGR). The UGR was developed based on uniform light sources, with as consequence that glare of non-uniform (e.g. LED) lighting is not accurately described. Important parameters which cause glare are the luminance (measure of brightness), the position of the light source in the field of view of the observer, and the background luminance. In a test- and demonstration room, there parameters are individually investigated, with the aim to develop a new glare index for both uniform and non-uniform light sources.