Daylighting

Summary: Daylight refers to the visible spectrum of radiation received from the sun, and daylighting is used to indicate the capture and use of this visible spectrum for illuminating spaces inside buildings and underground.

The capture of visible light for use in interior spaces, in its simplest form, is achieved by allowing it to penetrate into a building through side windows, roof windows, and other areas of glazing, such as glass doors and curtain walling. Energy savings are accrued from daylighting, as a result of the reduced use of electrical lighting in the daytime as well as reduced demand for space heating in cold climates resulting from passive solar heating.

Daylight or natural light is important for maintaining physiological processes in the human body. The hormone melatonin, which induces sleep, is stimulated by darkness and secreted by the pineal gland in the brain, and it is at its highest levels in the dark hours of the night. The normal day-night cycle sees melatonin levels rise toward the evening and night and decline in the early morning hours, which enables humans to wake up from their night’s sleep. In polar regions, the length of day varies according to the time of the year; the day increases up to 24 hours in the summer months and decreases up to zero hours in the winter months. It is now known that seasonal affective disorder (SAD), also called winter depression, occurs among the populations of some countries in the polar regions, which have long nights for five to six months of the year. Those affected by this disorder show symptoms of depression, oversleeping, and related social problems. One of the recommended treatments for this condition is light therapy, whereby the patient is exposed to a bright white, full spectrum light for prescribed durations to simulate daylight in order to suppress the secretion of melatonin in alignment with a more normal pattern and level.

Thus, the extent of daylight received on the Earth varies according to latitude: Locations close to the equator have roughly 12 hours of day and 12 hours of night all year round, while in the northern and southern regions from the tropics of Cancer and Capricorn, the total daylight received is much lower than that received in equatorial regions. Moreover, daylight varies according to local weather conditions and cloud cover. Therefore, designing buildings to maximize opportunities for illuminating living and working spaces by daylighting is a critical requirement in places where daylight is scarce for the reasons of geographical location, climatic patterns, or both.

The strategies used to maximize daylighting in buildings are varied and serve different requirements. Perimeter daylighting zones, to capture daylight through optimally glazed facades, are used in large buildings where the majority of spaces needing daylight are based along the perimeter. The core, which is cut off from the outside environment, generally contains spaces that do not require direct daylight, such as conference and audiovisual rooms. Even there, however, advanced methods of capturing daylight can be used; these take advantage of reflective surfaces to deliver daylight by multiple reflections into areas that otherwise are completely cut off from the outdoor environment.

One such device is called a light tube or solar tube, which has a transparent receptor on the roof to admit light into highly reflective flexible tubing. The diffuser at the end of the tube is located in the space to be illuminated. These tubes can also have directional collectors, reflectors, or even Fresnel lens devices that assist in collecting additional directional light down the tube. In regions where window shading is essential to keep out the glare from the bright tropical or equatorial sun, light shelves are used to bring diffused natural light into indoor spaces and eliminate overheating and glare. These light shelves are placed in positions such that the sunlight incident on their surfaces is reflected inside the building spaces onto white or light-colored walls or ceilings. The light is then reflected back into the room from the white surfaces as a uniformly diffused natural light without heat.

In equatorial regions, factory buildings were traditionally designed with north-facing roof windows, since the light received from this direction is without glare and suitable for direct illumination without the need for window shading or glare control. Buildings in colder regions typically have large glazed areas along the sun’s path to maximize the daylight received, whereas buildings in hot regions in the south, southeast, or southwest have heavily shaded windows or deep verandas to take advantage of air circulation and diffused daylight but protect living and working spaces from direct sunlight. Thus, strategies for daylighting differ according to the geographical locations of buildings as well as the functions that they are intended to serve.

Bibliography

Alaqtum, Tasbeeh, Altaf Engineer, and Colby Moeller. "Daylighting Glare and Design for Visual Comfort." Technology/Architecture + Design, vol. 8, no. 1, 24 May 2024, pp. 126-142, doi.org/10.1080/24751448.2024.2322924. Accessed 30 July 2024.

Bainbridge, David, and Ken Haggard. Passive Solar Architecture: Heating, Cooling, Ventilation, Daylighting, and More Using Natural Flows. White River Junction, VT: Chelsea Green, 2011.

“Building Envelope Design Guide and Whole Building Design Guide.” National Institute of Building Sciences, 14 Dec. 2021, www.wbdg.org/design/envelope.php. Accessed 30 July 2024.

Schiller, Mark. Simplified Design of Building Lighting. Hoboken, NJ: John Wiley and Sons, 1997.