Daylighting (architecture)

Daylighting is the practice of placing windows, skylights, other openings, and reflective surfaces so that direct or indirect sunlight can provide effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use. Energy savings can be achieved from the reduced use of artificial (electric) lighting or from passive solar heating. Artificial lighting energy use can be reduced by simply installing fewer electric lights where daylight is present or by automatically dimming or switching off electric lights in response to the presence of daylight – a process known as daylight harvesting.

The amount of daylight received in an internal space can be analyzed by measuring illuminance on a grid or undertaking a daylight factor calculation. Computer programs such as Radiance allow an architect or engineer to quickly calculate benefits of a particular design. The human eye's response to light is non-linear, so a more even distribution of the same amount of light makes a room appear brighter.

The source of all daylight is the Sun. The proportion of direct to diffuse light impacts the amount and quality of daylight. "Direct sunlight" reaches a site without being scattered within Earth's atmosphere. Sunlight that is scattered in the atmosphere is "diffused daylight". Sunlight reflected off walls and the ground also contributes to daylighting. Each climate has different composition of these daylights and different cloud coverage, so daylighting strategies vary with site locations and climates. At latitudes north of the Tropic of Cancer and south of the Tropic of Capricorn, there is no direct sunlight on the polar-side wall of a building between the autumnal equinox and the vernal equinox (that is, from the September equinox to the March equinox in the Northern Hemisphere, and from the March equinox to the September equinox in the Southern Hemisphere.) In the Northern Hemisphere, the north-facing wall is the "polar-side" and in the Southern Hemisphere, it is the south-facing wall.

Traditionally, houses were designed with minimal windows on the polar side, but more and larger windows on the equatorial side (south-facing wall in the Northern Hemisphere and north-facing wall in the Southern Hemisphere). Equatorial-side windows receive at least some direct sunlight on any sunny day of the year (except in the tropics in summer), so they are effective at daylighting areas of the house adjacent to the windows. At higher latitudes during midwinter, light incidence is highly directional and casts long shadows. This may be partially ameliorated through light diffusion, light pipes or tubes, and through somewhat reflective internal surfaces. At fairly low latitudes in summertime, windows that face east and west and sometimes those that face toward the nearer pole receive more sunlight than windows facing toward the equator.

Passive daylighting is a system of both collecting sunlight using static, non-moving, and non-tracking systems (such as windows, sliding glass doors, most skylights, light tubes) and reflecting the collected daylight deeper inside with elements such as light shelves. Passive daylighting systems are different from active daylighting systems in that active systems track and/or follow the sun, and rely on mechanical mechanisms to do so.

Windows are the most common way to admit daylight into a space. Their vertical orientation means that they selectively admit sunlight and diffuse daylight at different times of the day and year. Therefore, windows on multiple orientations must usually be combined to produce the right mix of light for the building, depending on the climate and latitude. There are three ways to improve the amount of light available from a window: (a) placing the window close to a light colored wall, (b) slanting the sides of window openings so the inner opening is larger than the outer opening, or (c) using a large light colored window-sill to project light into the room. Besides permitting daylighting into the building, windows serve another function in daylighting practice, providing views out. To enhance the quality of the view seen from a window, three primary variables need to be ensure: view content (what can be seen in the view), view access (how much of the window view can be seen), and view clarity (how clearly the view can be seen). View clarity is often influenced by the amount of shading provided by blinds or devices used to protect occupants from harsh daylight (e.g. glare) or for reasons of visual privacy. Environmental criteria serve as important criteria to gauge the quality of window view content. These criteria can be distilled into five important factors, namely: Location, time, weather, people, and nature. Notably, views that are able to provide building inhabitants with content of nature far outweigh the other four Environmental Information Criteria.

Different types and grades of glass and different window treatments can also affect the amount of light transmission through the windows. The type of glazing is an important issue, expressed by its VT coefficient (Visual Transmittance), also known as visual light transmittance (VLT). As the name suggests, this coefficient measures how much visible light is admitted by the window. A low VT (below 0.4) can reduce by half or more the light coming into a room. But be also aware of high VT glass: high VT numbers (say, above 0.60) can be a cause of glare. On the other hand, you should also take into account the undesirable effects of large windows.

Windows grade into translucent walls (below).