Daylighting (architecture)

Daylighting is a design strategy that uses windows, skylights, openings, and reflective surfaces to channel direct or indirect sunlight into a building for interior lighting. Architects focus on it when the goal is to improve visual comfort or cut energy use. Energy savings come from relying less on electric lights or from passive solar heating. Using fewer electric fixtures in daylit areas, or automatically dimming or switching off lights when sunlight is available—called daylight harvesting—reduces artificial lighting consumption.

To gauge how much daylight enters a space, designers measure illuminance on a grid or calculate a daylight factor. Software like Radiance lets architects and engineers quickly assess a design’s performance. Because the human eye responds to light in a non-linear way, spreading the same amount of light more evenly makes a room feel brighter.

The balance between direct and diffuse light affects both the quantity and quality of daylight. Direct sunlight reaches a site without being scattered by the atmosphere, while scattered sunlight is diffuse daylight. Light bouncing off walls and the ground also contributes. Since climates vary in cloud cover and the mix of direct and diffuse light, daylighting strategies depend on location. At latitudes north of the Tropic of Cancer or south of the Tropic of Capricorn, the polar-side wall (north-facing in the Northern Hemisphere, south-facing in the Southern Hemisphere) receives no direct sunlight between the autumnal and vernal equinoxes. Traditionally, houses had few windows on the polar side and larger ones on the equatorial side (south-facing in the Northern Hemisphere, north-facing in the Southern Hemisphere). Equatorial-side windows get at least some direct sunlight on any sunny day, except in the tropics during summer, making them effective for lighting nearby rooms. In higher latitudes during midwinter, light is highly directional and casts long shadows, which can be partly offset by light diffusion, light pipes or tubes, and reflective interior surfaces. At low latitudes in summer, east- and west-facing windows, and sometimes those facing the nearer pole, receive more sunlight than equator-facing ones.

**Types**

Passive daylighting uses static, non-moving systems—like windows, sliding glass doors, skylights, and light tubes—to collect sunlight, and elements such as light shelves to reflect it deeper indoors. This differs from active daylighting, which relies on mechanical systems to track and follow the sun.

**Windows**

Windows are the most common way to bring daylight inside. Their vertical orientation means they admit sunlight and diffuse light selectively depending on the time of day and year, so windows on multiple sides are often needed to get the right light mix for a given climate and latitude. Three methods can boost the light from a window: placing it near a light-colored wall, slanting the window opening so the inner side is larger than the outer side, or using a large, light-colored sill to project light into the room. Windows also provide views, and to improve view quality, designers consider three variables: view content (what is seen), view access (how much of the window view is visible), and view clarity (how clearly it is seen). Clarity is often affected by blinds or devices used to control glare or privacy. Environmental criteria—location, time, weather, people, and nature—help gauge view content, with nature views being the most valuable.

Different glass types and window treatments affect light transmission. The key measure is the visual transmittance (VT) coefficient, or visible light transmittance (VLT). A VT below 0.4 can cut incoming light by half or more, while a VT above 0.6 can cause glare. Large windows also have drawbacks. Windows grade into translucent walls.

**Clerestory windows**

Clerestory windows are high, vertically placed windows. When oriented toward the equator, they can increase direct solar gain. However, if facing the sun, clerestories and other windows may create unacceptable glare.