Daylighting design factors for visual comfort

Learning objectives

• Learn about daylighting benefits and daylighting glare.
• Know what tools and metrics can be used to evaluate visual comfort.
• Understand the daylighting design process is responsive to human visual comfort.

Before the advent of electrical lighting, daylight was the prominent source of illumination in buildings. The architectural layout of significant preindustrial buildings ensured that most enclosed spaces had access to daylight to reduce the use of candles, oil lamps, or gaslight. In the preindustrial era, the daylighting design was based on the past experience of the designer/builder or general guidelines. The introduction of electrical lighting transformed the architectural plan—without the necessary access to daylight. Consequently, some spaces lost their access to the outside view, leading to occupants’ dissatisfaction due to losing their visual connection to the outside environment and natural landscape. Using daylight has gained more attention recently because of its significant benefits, including being the most sustainable source of energy.

Daylighting benefits

Daylight has quantitative benefits that are relatively easy to measure. For instance, it could be used as a source of illumination to dim (or even turn off) the electrical lighting system. This can be achieved by designing the electrical lighting layout and controls to align with the areas that receive plenty of natural light during occupied hours. The electrical control system is essential to realizing the benefits of daylighting. By dimming electrical lighting, it helps harvest the available daylight while ensuring occupants always receive the required level of illumination. In cold climates, daylight can also be used to assist in heating the building by admitting the sun’s energy into the lived space, providing illumination and heat—the method that is implemented in passive house design for energy efficiency.

The energy codes and regulations tend to focus on the energy-saving aspects of daylighting, and often the visual comfort and occupant visual satisfaction is not part of the criteria. If measures for visual comfort are not considered, occupants will override the daylight system. For example, an occupant could pull the shade down; as a result, the intended energy-savings goal will not be met.

Daylight also has many subjective benefits that increase the quality of the interior environment. The health benefits and the human desire for daylight are intuitively based on human evolution over the ages, and there is plenty of research demonstrating the effect of daylight on human health including its effect on our circadian rhythm—the regulation of our sleep/wake patterns and other important biological processes. Additionally, daylight has incredible color-rendering properties that can make the architectural finishes, interior furniture, or artwork look more vivid. Providing daylight with windows also offers views to the exterior, which increases occupants’ productivity and satisfaction with the environment. Daylight also can be used to enhance the overall architectural experience due to its high color-rendering properties as a primary source of illumination to create a dynamic atmosphere (Figure 5).

Glare and daylighting design 

While there are many benefits to designing with daylighting in mind, it also presents complex challenges. Daylight is an inherently dynamic source—its location and intensity changes with time of day, seasons, and weather. Compared with designing for electrical lighting, which is static and makes it relatively easier to establish a lighting layout, designing for daylight requires a strategy that works across a wide variety of daylight conditions and site-specific factors (Figure 2).

The source of daylight is also immensely bright, and direct viewing with the naked eye for more than a few seconds could cause eye damage. In an outdoor environment, people experience sunlight indirectly through the reflection of light off nearby surfaces. This light is a fraction of incident light on the surface and depends on each object’s relative reflectance value. Even without sunglasses, people enjoy outdoor activity without serious visual discomfort because our visual system uses adaptive mechanisms: We avoid directly viewing the sun, and reflexes in our irises and eyelids control the amount of light admitted onto the retina.

However, in the indoor environment, sunlight can cause serious visual discomfort by seeing either the sun directly in the visual field or the excessive light reflected off architectural finishes. Particularly in workspaces and classrooms, there is often less flexibility to reorient our point of view to avoid the sun. So why don’t we experience glare walking in the park in bright daylight, but as soon as we walk into a building, sunlight can become a very glary light source? Human vision can adapt to a wide range of luminous environments, from very high ambient light levels (i.e., outdoors on a clear day) to very low light levels (moonlight). Visual discomfort occurs when the visual system must frequently switch its focus between relatively dimmer interior areas in our visual field to high-brightness areas (bright sun or exterior views).

Daylighting metrics and tools to assess visual comfort 

To evaluate the visual comfort of an interior space, designers need reliable metrics and clear benchmark criteria to test how the intended design performs. A suitable metric for glare analysis should factor in quantitative aspects of the luminous environment as well as qualitative evaluations of occupant comfort or discomfort. One of the metrics that is widely used by the Illuminating Engineering Society (IES) to evaluate daylight glare (specifically in offices) is daylight glare probability (DGP), which evaluates a view in terms of the probability of being perceived as glary by an average observer. Simulation tools that can compute DGP for every hour of the year provide valuable insight into the performance of a space. Once the sources of glare are identified for certain daylight hours using a DGP plot, a daylighting design strategy can be developed to mitigate the glare.

A good daylighting design that is responsive to occupants’ visual comfort should consider lighting conditions throughout the year to account for changes in the sun’s angle and intensity, weather, etc. Design and analysis tools and metrics must include annual capabilities to ensure the space performs well for most daylight hours (if not all). One of the advanced tools available can be used with customized scripting to make it very powerful in terms of predicting the space’s luminous characteristics with high accuracy and identifying sources of glare. These tools can also use the weather data and simulate complex architectural materials and systems. There are other computer tools that are available that can be very useful in the design process, including many toolkits, with streamlined user interfaces or connections to 3-D modeling software or parametric modeling technics. Other tools also have the capability of simulating daylight and computing glare metrics.

Figure 8: Annual daylight glare probability computed for a view (this particular view is exposed to disturbing/intolerable glare for a significant number of daylight hours)[/caption]

Daylighting design factors and strategies to achieve visual comfort 

Daylighting design is a microcosm of architectural design and can use a procedural and heuristic architectural design approach to solve a problem. In this approach, we can generate design solutions by changing the important factors that are implemented and evaluate each potential design strategy against criteria and metrics using appropriate analysis tools. The factors impacting the luminous properties of the interior environment could be divided into exterior, façade, and interior factors (Figure 2). Exterior factors include building orientation relative to the sun’s path, surrounding buildings, and landscape geometries that alter the daylight by blocking, absorbing, and redirecting light. Sometimes the surrounding landscape can be part of the design solution, and foliage could be arranged carefully relative to the daylight apertures to provide shading. For instance, in cold climates, deciduous trees can be planted in a way that provides shading to the daylight aperture during the summer (high-angle sun) and allows sunlight through for illumination and heat in the winter time as they shed their leaves. Trees are also very effective in reducing excessive outdoor daylight level from tens of thousands of lux (for reference, office electrical lighting is 300 to 500 lux) to a more useful light level (illuminance value lower than 3,000 lux) because of their unique 3-D geometries and luminous properties, such as light absorbance, scattering, and transmitting.

Façade factors influence the quantity and quality of daylight to an even greater extent, and these variables are within the primary scope of daylighting design. Façade factors can be broken down into three items:

• External shading devices, such as overhangs or vertical/horizontal louvers, are often static and permanent, with a strong influence on the architectural appearance.
• Daylight aperture glazing systems consist of layers of transparent substrate (usually architectural glass), coatings (for example, Low-E, frit, etc.), interlayers such as PVB, filling (air, argon, aerogel, etc.), and films for light- or UV-transmittance reduction (for artwork preservation). Visible light transmittance (VLT) is one of the variables used to evaluate the glazing system’s optical performance—in other words, it is the percentage of daylight that is admitted into the interior.
• Internal shading devices, such as Venetian blinds, fabric shades, and light shelves, are the last layer of façade daylight control to further influence the daylight entering the space.

Architectural interior design including the layout, ceiling height and geometry, finishes, furniture or partitions, and electrical lighting all play an important role in occupants’ visual comfort. For instance, changing the reflectance of floor carpet could reduce excessive reflected light, or changing the interior layout could avoid the direct view of the sun for workstations in an open-office plan and reduce glare probability.

To maximize the benefits of daylighting, a more holistic daylighting design process is needed that addresses the occupants’ visual comfort. The available tools and metrics can be used to evaluate the intended design setting. A wide gamut of architectural daylighting design factors contributes to visual comfort of occupants, and they need to be adjusted sometimes through several iterations to find the optimum solution, which could be automated by using available parametric design tools to optimize the design solutions. The IES and academia are putting in a lot of effort to further enhance visual-comfort metrics and tools. Moving into the future, we should remember daylight’s historic legacy while embracing development and using technology to elevate the architectural experience.

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