Daylighting

Daylighting is the practice of placing windows, skylights, other openings, and reflective surfaces so that sunlight (direct or indirect) 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/switching off electric lights in response to the presence of daylight – a process known as daylight harvesting. Daylighting is the practice of placing windows, skylights, other openings, and reflective surfaces so that sunlight (direct or indirect) 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/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. Light that is scattered in the atmosphere is diffused daylight. Ground reflected light also contributes to the daylight. Each climate has different composition of these daylights and different cloud coverage, so daylighting strategies vary with site locations and climates. There is no direct sunlight on the polar-side wall (north-facing wall in the Northern Hemisphere and south-facing wall in the Southern Hemisphere) of a building from the autumnal equinox to the spring equinox at latitudes north of the Tropic of Cancer and south of the Tropic of Capricorn. 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 summertime), so they are effective at daylighting areas of the house adjacent to the windows. In 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. In fairly low latitudes in summertime, windows that face east and west and sometimes those that face toward the pole receive more sunlight than windows facing toward the equator. 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. 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.