Daylighting (architecture)

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 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.

Types

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

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).

Clerestory windows

Another important element in creating daylighting is the use of clerestory windows. These are high, vertically placed windows. They can be used to increase direct solar gain when oriented towards the equator. When facing toward the sun, clerestories and other windows may admit unacceptable glare. In the case of a passive solar house, clerestories may provide a direct light path to polar-side (north in the northern hemisphere; south in the southern hemisphere) rooms that otherwise would not be illuminated. Alternatively, clerestories can be used to admit diffuse daylight (from the north in the northern hemisphere) that evenly illuminates a space such as a classroom or office.

Often, clerestory windows also shine onto interior wall surfaces painted white or another light color. These walls are placed so as to reflect indirect light to interior areas where it is needed. This method has the advantage of reducing the directionality of light to make it softer and more diffuse, reducing shadows.

Sawtooth roof

Another roof-angled glass alternative is a sawtooth roof (found on older factories). Sawtooth roofs have vertical roof glass facing away from the equator side of the building to capture diffused light (not harsh direct equator-side solar gain). The angled portion of the glass-support structure is opaque and well insulated with a cool roof and radiant barrier. The sawtooth roof's lighting concept partially reduces the summer "solar furnace" skylight problem, but still allows warm interior air to rise and touch the exterior roof glass in the cold winter, with significant undesirable heat transfer.

Skylights

A skylight or rooflight is a light-permitting structure or window, usually of transparent or translucent glass, that forms part (or all) of the roof area of a building for daylighting or ventilation.

Laylights

As an element of architecture, a laylight is a glazed panel usually set flush with the ceiling for the purpose of admitting natural or artificial light. Laylights typically utilize stained glass or lenses in their glazing, but can also use alternative materials. For example, the Lyme Art Association Gallery utilizes translucent white muslin laylights below its skylights. A laylight differs from a glazed (or closed) skylight in that a skylight functions as a roof window or aperture, while a laylight is flush with the ceiling of an interior space. When paired with a roof lantern or skylight on a sloped roof, a laylight functions as an interior light diffuser. Before the advent of electric lighting, laylights allowed transmission of light between floors in larger buildings, and were not always paired with skylights.

Atrium

An atrium is a large open space located within a building. It is often used to light a central circulation or public area by daylight admitted through a glass roof or wall. Atria provide some daylight to adjacent working areas, but the amount is often small and does not penetrate very far. The main function of an atrium is to provide a visual experience and a degree of contact with the outside for people in the working areas. The daylighting of successive storeys of rooms adjoining an atrium is interdependent and requires a balanced approach. Light from the sky can easily penetrate the upper storeys but not the lower, which rely primarily on light reflected from internal surfaces of the atrium such as floor-reflected light. The upper stories need less window area than the lower ones, and if the atrium walls are light in color the upper walls will reflect light toward the lower stories.

Translucent walls

Walls made of glass brick are translucent-to-transparent. Traditionally they are hollow and grouted with a fine concrete grout, but some modern glass brick walls are solid cast glass grouted with a transparent glue. If the glue matches the refractive index of the glass, the wall can be fairly transparent.

Increasing the amount of concrete, bottle walls embed bottles that run right through the wall, transmitting light. Concrete walls with glass prisms running through them have also been made. With the advent of cheaper optical fibers and fiber-optic concrete walls, daylight (and shadow images) can then pass directly through a solid concrete wall, making it translucent; fiber optics will lead light around bends and over tens of meters. Typically only a few percent of the light is transmitted (the percent transmittance is about half the percent of the surface that is fibers, and usually only ~5% fibers are used).

Both glass and concrete conduct heat fairly well, when solid, so none of these walls insulate well. They are therefore often used outdoors, as a divider between two heated spaces (see images), or in very temperate climates.

Greenhouse walls (and roofs) are made to transmit as much light and as little heat as possible. They use a variety of materials, and may be transparent or translucent.

Remote distribution

It is possible to provide some daylight into spaces that have low possibility of windows or skylights through remote distribution devices such as mirrors, prisms, or light tubes. This is called anidolic lighting, from anidolic (non-image-forming) optics. The non-linear response of the human eye to light means that spreading light to a broader area of a room makes the room appear brighter, and makes more of it usefully lit.

Remote daylight distribution systems have losses, and the further they have to transmit the daylight and the more convoluted the path, the greater the inefficiency. The efficiency of many remote distribution systems can also vary dramatically from clear to overcast skies. Nonetheless, where there is no other possibility of providing daylight to a space, remote distribution systems can be appreciated.

Light reflectors and shelves

Once used extensively in office buildings, the manually adjustable light reflector is seldom in use today having been supplanted by a combination of other methods in concert with artificial illumination. The reflector had found favor where the choices of artificial light provided poor illumination compared to modern electric lighting.

Light shelves are an effective way to enhance the lighting from windows on the equator-facing side of a structure, this effect being obtained by placing a white or reflective metal light shelf outside the window. Usually the window will be protected from direct summer season sun by a projecting eave. The light shelf projects beyond the shadow created by the eave and reflects sunlight upward to illuminate the ceiling. This reflected light can contain little heat content and the reflective illumination from the ceiling will typically reduce deep shadows, reducing the need for general illumination.

In the cold winter, a natural light shelf is created when there is snow on the ground which makes it reflective. Low winter sun (see Sun path) reflects off the snow and increases solar gain through equator-facing glass by one- to two-thirds which brightly lights the ceiling of these rooms. Glare control (drapes) may be required.

Prisms

The oldest use of prisms for daylighting may well be deck prisms, let into the decks of ships to transmit light below. Later, pavement lights or vault lights were used to light basement areas under sidewalks.

Prisms that used total internal reflection to throw light sideways, lighting the deeper portions of a room, later became popular. Early thick, slow-cooling cast glass prism tiles were often known as "luxfer tiles" after a major manufacturer. They were and are used in the upper portions of windows, and some believe that they contributed to the trend from dark, subdivided Victorian interiors to open-plan, light-coloured ones.

Daylight redirecting window film (DRF) is a thin plastic version of the old glass prism tiles. It can be used as a substitute for opaque blinds.

Light tubes

Another type of device used is the light tube, also called a tubular daylighting device (TDD), which is placed into a roof and admits light to a focused area of the interior. These somewhat resemble recessed ceiling light fixtures. They do not allow as much heat transfer as skylights because they have less surface area.

TDDs use modern technology to transmit visible light through opaque walls and roofs. The tube itself is a passive component consisting of either a simple reflective interior coating or a light conducting fiber optic bundle. It is frequently capped with a transparent, roof-mounted dome "light collector" and terminated with a diffuser assembly that admits the daylight into interior spaces and distributes the available light energy evenly (or else efficiently if the use of the lit space is reasonably fixed, and the user desired one or more "bright-spots").

The tubular daylighting device was invented by Solatube International in 1986 and brought to market first in Australia in 1991.

Active daylighting

Active daylighting is a system of collecting sunlight using a mechanical device to increase the efficiency of light collection for a given lighting purpose. Active daylighting systems are different from passive daylighting systems in that passive systems are stationary and do not actively follow or track the sun. There are two types of active daylighting control systems: closed loop solar tracking, and open loop solar tracking systems.

• Closed loop systems track the sun by relying on a set of lens or sensors with a limited field of view, directed at the sun, and are fully illuminated by sunlight at all times. As the sun moves, it begins to shade one or more sensors, which the system detect and activates motors or actuators to move the device back into a position where all sensors are once again equally illuminated.
• Open loop systems track the sun without physically following the sun via sensors (although sensors may be used for calibration). These systems typically employ electronic logic which controls device motors or actuators to follow the sun based on a mathematical formula. This formula is typically a pre-programmed sun path chart, detailing where the sun will be at a given latitude and at a given date and time for each day.