Issues in Window Selection: Human Factors

Daylight | Glare | View | Thermal Comfort

Daylight

A recent study suggests "that the frequently reported desire for windows in the workplace is more than a matter of simple preference. Underpinning this preference is a fundamental issue of emotional and psychological well-being" (Leather et al. 1998). Daylight is an important part of this desire for windows. The changing direction, intensity, and even color of daylight are stimulating and connect people to the time of day and the natural world. Recent studies have suggested numerous benefits including links between daylight and increased sales in retail facilities and better performance in schools (Boyce et. al., 2003, Edwards and Torcellini 2002, Heschong 1999).

Windows, of course, permit daylight to enter a space. The design of a window and choice of glazing can dramatically affect the quantity and quality of daylight in a space and how it is experienced. To illustrate this point, a series of DOE-2.1E computer simulations were conducted on perimeter zones in office buildings. This indicates the average annual workplane illuminance in a south-facing perimeter office in Chicago with moderately sized windows (WWR=0.30) at a point ten feet from the window. Average daylight illuminance is relatively high in this example because at the more northerly latitude in Chicago, the lower sun angle penetrates a south-facing facade more deeply than in more southerly latitudes, and there is no interior shading assumed in these calculations. Most visual tasks require only 30-70 footcandles. Figure 2-31 shows there is also great variability in daylight illuminance with different window types. For example, the reflective glazing (Window D) has a very low VT and provides much less daylight than any other option.

Simply using average annual daylight illuminance as an indicator of the presence of daylight can be misleading. Another measure of the effectiveness of daylight is to determine the percentage of time the illumination is above 50 footcandles (an adequate level of general office lighting) at a given location within the space. Figure 2-33 indicates that daylight levels exceeding 50 footcandles occur at least 70 percent of the time ten feet from the window for all cases except Window D. This occurs in part because it is assumed there is no control of direct sunlight with shades or blinds.

There is an additional reason that simply using average daylight illuminance as an indicator does not reflect the true impact on humans. The perceived brightness of the interior due to daylight is not directly proportional to the amount of daylight. For example, a window that results in an average illuminance of 60 footcandles may be perceived as considerably brighter than one that produces 20 footcandles. However, a window resulting in an average illuminance of 300 footcandles is not necessarily perceived as three times brighter than one with 100 footcandles. In order to make a meaningful comparison between windows on a 0-10 scale that reflects the visual impact of brightness on human beings, a brightness index was developed (see Appendix A). Figure 2-34 illustrates the brightness index for the same set of nine windows shown in the previous figure. The relative ranking of the windows remains the same but the differences are not so extreme. The percentage of time over 50 footcandles (Figure 2-33) has a very similar pattern to the brightness index (Figure 2-34).

While the measures shown in Figures 2-31 and 2-34 are good general indicators, they are taken at one point in the center of the space 10 feet from the window. This approach does not illustrate the variation in light levels that occurs across a room. Figure 2-35 shows the spatial distribution of daylight at noon on June 21 in a south-facing perimeter zone with six window types. The differences between cases is due to the variations in the visible transmittance of the windows. The contours illustrate the intense daylight near the windows that diminishes rapidly moving away from a window. This lack of uniform light distribution is particularly notable at noon with an unshaded, south-facing window.

Daylight illuminance not only varies depending on location within a space; it also varies with the time of day and season of the year. The variation in the location of the sun depends on specific location. Figure 2-37 illustrates the considerable differences in daylight intensity and distribution that occur at different times of day throughout the year with an unshaded south-facing window in Chicago.

Daylight design is far more sophisticated than simply providing a window with a high enough visible transmittance. More daylight does not necessarily equate to better lighting conditions. It is a matter of balancing daylight admission with glare control, as well as providing uniform light distribution. Remember that the usability of daylight is dependent on the task. For some tasks requiring a high level of visual acuity not subject to glare, the brighter the interior the better. For computer tasks, particularly with older CRT screens, reducing and controlling illuminance levels is better.