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Article Excerpt
Abstract: This study outlines a simplified mathematical approach to determine window dimensions for a given room based on two lighting quality indicators, namely, discomfort glare and vertical illuminance. The former represents visual comfort and is expressed in terms of daylight glare index (DGI). The latter reflects on the aspect of receiving sufficient light exposure to maintain optimal health and psychological wellbeing. It is represented by vertical illuminance at the eye level. The result offers a formula that puts forward a simple relationship between the window width, the window height and a specific (desired) depth of a given point in the space. A generic design scenario is used to illustrate the current method. The examined daylighting design criteria are of DGI=20 and [E.sub.v] = 2500 lux. In conclusion, this paper proposes the glare criterion as additional to existing national window sizing procedures based on critical illuminance levels (or minimum daylight factor on the working plane corresponding to the critical visual tasks) and critical exterior conditions (that occur with absence of sunlight and a CIE Overcast Sky standard).
Keywords: CIE standards, Daylighting calculations, Discomfort glare, Flux transfer method, Vertical illuminance, Visual comfort, Window sizing
Introduction
Maximum utilization of daylight is often contrary to optimum visual comfort. Large windows admit large amounts of daylight that can impede vision, cause excessive glare and negatively affect performance. In the case of side lighting, visual discomfort is usually represented by the degree of discomfort glare cased by large luminance contrast between the light source and its surrounding. While much attention has been given to evaluating discomfort glare from windows through the daylight glare index (DGI) and to the methods of quantifying interior light levels, little attention has been given to analyzing these two conditions simultaneously. A successful daylighting strategy is one that optimizes the window size while balancing the criteria of visual comfort and illuminance penetration. It is necessary to find the right balance between the two criteria in order to meet user's preferences and best working and wellbeing conditions.
The major factor affecting discomfort glare sensation is high source luminance (Chauvel, Collins, Dogniaux, & Longmore, 1982; Chauvel & Perraudeau, 1995; Nazzal & Oki, 2007). Osterhaus (2001) has found that direct vertical illuminance at the eye and the overall brightness in the visual field correlate well with glare sensation. Aizlewood (2001) measured vertical illuminance levels falling on a point inside a room to assist in determining the source and background luminances in the room. Aries, Begemann, Tenner, and Zonneveldt (2003); Cuttle (2003); Osterhaus (1998) and Osterhaus and Bailey (1992) and followed similar approach. Thus, vertical illuminance at the eye level is related to the average luminance of the visual field, including background luminance.
Besides providing illuminance levels for task performance, recent daylighting research has stressed the need for providing natural lighting in buildings for health and psychological wellbeing (Boubekri, 2004). In order to optimize our circadian rhythm, research indicates standard illuminance levels that must be received at the eye level for certain duration. Vertical illuminance at the eye level is a key factor.
Current recommendations for visual task performance in an office prescribe horizontal illuminance levels from 200 to 700 lux (IES, 1993). Recommendations for bright light therapy to overcome seasonal depression are about 1000 to 1500 lux (NSVV, 2003), 2500 lux for two hours per day or 10000 lux for three times less that duration (Graw, Recker, Sand, Krauchi, & Wirz-Justice, 1999; Wirz-Justice, 1998). These levels must reach the eye of a room occupant at the vertical plane in order to be relevant for the non-visual stimulation where vertical illuminance is important.
This paper outlines a simplified mathematical approach to determine window dimensions based on a predetermined level of DGI and a desired level of vertical illuminance inside a room.
Background
Design practices aim at maximizing daylight levels inside buildings, as daylight is known to improve health and psychological wellbeing of occupants in addition to saving energy when properly used. Visual comfort is also an integral part of the daylighting design question. Typically, visual comfort can be achieved by providing adequate but only moderate levels of illumination in order to avoid excessive glare and harsh luminance contrasts from the daylight sources. A successful window design must provide a balance between visual comfort and illuminance exposure. The ideal design may also imply that designers can meet desired illuminance levels for occupants' wellbeing and specify a visual comfort level for which the marker is in this case DGI.
Quantifying Daylight Illuminance: The Flux Transfer Method
Daylight levels can be calculated through numerous methods. The flux transfer method is one that has been used most. With daylight being an electromagnetic form of energy, this method is used to quantify the transfer of visual energy from a surface to another surface or to a point in the space. The quantity of light received at a point depends on the incident amount of daylight at the exterior of the window, the size of the window and its glazing transmission factor, the three dimensional view factor between the surface and the point and the nature of the intensity distribution of the source (REA, 2000) (see Figure 1). The flux transfer method is applied in diffuse conditions only whereby any radiating surface is considered a perfectly diffuse one (Lambertian). The source can be either primary source, as is the case of a...
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