06.03 Visual comfort
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Introduction
Visual comfort criteria identify the factors that allow an individual to carry out tasks comfortably in terms of their photo-sensory perception of their environment. They include: the intensity and direction of light sources, reflections from surfaces, contrast between surfaces, the nature of any task being undertaken and the photo-sensory response of the eye. International recommendations exist that specify the minimum level of illuminance required to provide task visibility for various tasks and locations. Additional criteria are given that promote visual comfort. These may include advice on the appropriate background for a task, the maximum luminance of a source close to the line of sight, illuminance ratios that direct visual attention and limits on the punctuation of light levels that might cause distraction or advere physiological reactions. As with thermal comfort temperatures, these light levels were derived experimentally and reflect an average person engaged in a specific activity within a defined environment. Click here to see the visual comfort diagram.
A specified light level could be provided by either natural light or artificial light, but in order to satisfy energy concerns it is often necessary to maximise the degree of daylight contribution and minimise the artificial lighting contribution. A room designed to utilise natural lighting will only save energy if the electrical lighting is adequately controlled. However the introduction of natural light into a space can have certain drawbacks, namely glare and unwanted solar heat gain. Blinds may be used to alleviate these problems but they can have an important impact of the visual environment within a room. Indeed, it is a misconception that tightly controlled uniform lighting levels are needed to achieve occupant visual comfort. The contrast in light levels between the front and back of the room may be as important as the overall illuminance. Preferences for visual conditions vary from person to person depending on gender, age and past experience. Individuals can tolerate a wide range of light levels, and reading performance of large high contrast print does not show any significant variation between 100 and 5000 lux.
Different people react to windows, views, natural light and direct sunlight in different ways. Research has shown that these aspects of natural lighting can have real psychological benefits to occupants, although whether or not they have a positive affect on productivity depends on the task being undertaken. The variable nature of natural light is one of the fundamental aspects of human life; it provides us with a perception of the time and stimulates conditions that aid psychological well-being in closed environments. Cool daylight which is filtered through trees or shading devices can provide visual relief to mentally counteract the thermal rigors of hot days and a carefully placed sparkle of sunlight can help relieve the impression of cold in the winter. Empirical visual comfort assessments do not take many of these factors and other psychological factors into account, Section 06.01. Therefore when designing an automatic control system for a blind it is often wise to provide an occupant override, Section 06.01.
Glare
Clearly the utilisation of daylight reduces the need for artificial light and thus should form an important part of a strategy for reducing building energy use with anadaptive facade. 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 cannot 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.