Clearly the utilisation of daylight reduces the need for artificial light and thus should form an important part of an adaptive facade's control strategy for reducing building energy use. However, the introduction of natural light is not a guarantee of visual comfort. Physiologically, daylight can cause visual discomfort when distributed unevenly in a room, resulting in patterns of high contrast. Outdoor views can make an interior seem dark and gloomy, and direct sunlight can make a room too bright. Both of these examples can cause discomfort glare and in the worst cases disability glare. Such inadequacies lead to occupants closing blinds and switching on lights, resulting in the unnecessary use of electric lighting.
The Commission Internationale de l’Eclairage (CIE) defines glare as:
"visual conditions in which there is excessive contrast or an inappropriate distribution of light sources that disturbs the observer or limits the ability to distinguish details and objects."
Glare is quantified by a glare index, depending mainly on window illuminance and reflections within the room. Glare caused by a direct view of the sky is considered to be acceptable if the glare index at a particular point in the room, does not exceed the recommended level for the particular operation. There are various forms of glare indices available for the designer, these include: the British Glare Index, based on research by Hopkinson and Pertherbridge; and the CIE Glare Index proposed by Einhorn. In practice, the use of these indices in blind control is limited by the nature of the light sensors used, the many assumptions required and an analytical method that can not account for the subjective human responses often associated with visual comfort. Indeed an occupants decision on preferred blind angle often depends upon a trade off of perceptions.
Vision is the most developed of our senses and it can affect an individual’s mood and cognition. It is not adequate to simply provide adequate illumination levels to satisfy the multidimensional nature of visual comfort. Daylight within buildings is provided for people, therefore daylighting design should respond to their visual and perceptual needs. As these needs are so variable and difficult to measure we must allow the occupant the luxury of being able to make adjustments.