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Daylighting is the practice of placing windows or other openings and reflective surfaces so that during the day natural light provides 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 either from the reduced use of artificial (electric) lighting or from passive solar heating or cooling. Artificial lighting energy use can be reduced by simply installing fewer electric lights because daylight is present, or by dimming/switching electric lights automatically in response to the presence of daylight, a process known as daylight harvesting.
Daylighting is a technical term given to a common centuries-old, geography and culture independent design basic when "rediscovered" by 20th century architects. The amount of daylight received in an internal space can be analyzed by undertaking a Daylight factor calculation. Today, the use of computers and proprietary industry software such as Radiance can allow an Architect or Engineer to quickly undertake complex calculations to review the benefit of a particular design.
There is no direct sunlight on the polar-side wall of a building from the autumnal equinox to the spring equinox. Traditionally, houses were designed with minimal windows on the polar side but more and larger windows on the equatorial-side. Equatorial-side windows receive at least some direct sunlight on any sunny day of the year (except in tropical latitudes in summertime) so they are effective at daylighting areas of the house adjacent to the windows. Even so, during mid-winter, light incidence is highly directional and casts deep 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:
- Placing the window close to a light colored wall.
- Slanting the sides of window openings so the inner opening is larger than the outer opening.
- 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.
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.
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Skylights are light transmitting fenestration (products filling openings in a building envelope which also includes windows, doors, etc.) forming all, or a portion of, the roof of a building space. Skylights (roof windows, unit skylights, tubular daylighting devices (TDDs), sloped glazing) are used to convey abundant daylight or toplighting, provide a connection to the outdoor environment to occupants, and often to help fresh outside air enter the space below.
Skylight Basics A basic fixed unit skylight consists of a structural perimeter frame supporting one panel of glazing infill (the light-transmitting portion, which is made primarily of glass or plastic). An operable (venting) unit skylight uses a glazed sash attached to and supported by the frame. When within reach of the occupants, this type is also called a roof window. Unit skylights are typically shipped fully assembled to the jobsite.
A TDD has a roof-mounted fixed unit skylight element connected by a light conveying conduit to a light diffusing element, shipped in an unassembled, complete kit to accommodate infinite variations in site configurations.
Sloped glazing differs from other “skylights” in that one assembly contains multiple infill panels in a framing system, usually designed for a specific project and installed in sections on site.
Benefits of daylighting with skylights are numerous. Skylights are widely used in daylighting design in residential and commercial buildings, mainly because they are the most effective source of daylight on a unit area basis. The concept is simple; more daylighting means less artificial light and fewer square feet of necessary glazing, thus saving significant energy and resulting in lower financial and environmental costs.
Skylights are good for people. An independent study analyzed test score results for over 21,000 students from three districts located in Orange County, California; Seattle, Washington; and Fort Collins, Colorado, and concluded that students have significantly higher test scores in classrooms that optimize daylighting design than classrooms that do not. It is becoming clear through several other recent studies that daylight positively affects physiological and psychological well-being and has been shown to increase sales and productivity when properly designed.
Exploring the energy equation in more depth. Savings from daylighting can cut lighting energy use in some buildings by up to 80%, according to the U.S. Department of Energy's (DOE) Federal Energy Management Program. In terms of cost savings, the DOE reported that many commercial buildings can reduce total energy costs by up to one-third through the optimal use of daylighting. Efficiencies achievable in residential construction are not yet quantified, but should be sizable as well. The majority of commercial warehouses and box stores built in recent years have used skylights extensively in daylighting design.
Toplighting (skylights) works well with sidelighting (windows) to maximize daylighting in that 1) toplighting is able to bring light into centralized areas of a building, 2) daylight is available throughout the day from both ambient lighting from the sky and direct exposure to the sun, 3) modern transparent and/or translucent glazing can be utilized to avoid glare, aid in capturing sunlight at low angles and diffuse light to wider areas of floor space. Even on a cloudy day, toplighting is three to ten times more efficient than sidelighting. (source – AAMA Daylighting Fact Sheet).
Many recent advances in both glass and plastic infill systems have greatly benefited end users of all skylight types. Some are mainly intended to increase thermal performance, some are focused on preserving and utilizing daylight potential and some are designed to enhance strength, durability, fire resistance and other performance measures.
U-factor* expresses the heat loss performance of any building assembly. Solar heat gain coefficient (SHGC) measures the assembly’s transfer of heat from outside to inside that is caused by sunlight. These properties are labeled in the U.S. as a decimal between zero and one, with lower numbers indicating lower heat transfer rates. Depending on the geographic region, optimal U-factor and SHGC performance will vary. In the sunny southern climate zones, a lower SHGC is more important than lower U-factor. In the cooler northern climate zones, lower U-factor is more important, and higher SHGC can be justified.
- It is important to note that U-factor for National Fenestration Rating Council (NFRC)-certified skylights are based on a 20°-from-horizontal orientation and assume a minimum four-inch projection from the mounting surface, whereas windows and doors are tested vertically and are assumed to be inset mounted. Skylight thermal performance is, therefore, often perceived to be inferior to windows, but this is not the whole energy story as indicated above. Window U-factor and SHGC specification criteria are, therefore, NEVER applicable to skylight products.
- It is also worth noting that sloped glazing suppliers often provide a different set of performance values as a means of comparison for selection. Center of Glazing (COG) U-factors will always be significantly better than full product NFRC-rated values. COG values are measured from a single (optimal) center point whereas NFRC-rated values measure the entire assembly performance.
Careful selection of skylights is an important step in good daylight design. A balance must be struck between low U-factor and optimal SHGC values while preserving enough daylight supply to allow for artificial lights to be used only when absolutely necessary. Use of automatic electric lighting controls should be used to maximize energy savings. Unfortunately, NFRC has not yet decided how to rate many popular skylight products for their daylighting potential, so specifiers have to rely on manufacturer-derived claims.
Modern skylights using glass infill, like windows, typically use sealed insulating glass units (IGU) made with two panes of glass. These types of products are NFRC-ratable for visible transmittance. Assemblies with three panes can sometimes be cost-justified in the coldest climate zones, but they lose some light by adding the third layer of glass. Glass units typically include at least one low emissivity (Low-E) coating applied to one or more glass surfaces to reduce the U-factor and especially SHGC by suppressing radiant heat flow. Many varieties of Low-E coatings also reduce daylight potential to different degrees. High purity inert gas is frequently used in the space(s) between panes, and advances in thermally efficient glass spacing and supporting elements can further improve thermal performance of glass-glazed skylight assemblies.
Plastic glazing infill is commonly used in many skylights and TDDs. These assemblies typically contain thermally formed domes, but molded shapes are not uncommon. Domed skylights are typically used on low slope roofs. The dome shape allows for shedding of water and burning embers. Acrylic is the most common plastic glazing used for dome skylights today; however, polycarbonate and copolyester materials are also often used as glazing, where additional properties such as impact resistance may be required to meet specific demands. See AAMA Skylight Council’s Plastic Glazing 101 article for more information on this subject. Plastics used in skylights are UV stabilized and may feature other advances to improve thermal properties. Lack of an accepted procedure for measuring light transmittance is one disadvantage when specifying this type of skylight glazing infill.
Light reflectors 
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 
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.
Light tubes 
Another type of device used is the light tube, also called a tubular daylighting device, 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.
Tubular Daylighting Devices (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 1993 and is used to provide daylighting to residential and commercial buildings, contributing to sustainability from a lighting standpoint and reducing the carbon footprint.
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.
The use of heliostats, mirrors which are moved automatically to reflect sunlight in a constant direction as the sun moves across the sky, is gaining popularity as an energy-efficient method of lighting. A heliostat can be used to shine sunlight directly through a window or skylight, or into any arrangement of optical elements, for example light tubes, that distribute the light where it is needed.
Smart glass 
Smart glass is the name given to a class of materials and devices that can be switched between a transparent state and a state which is opaque, translucent, reflective, or retro-reflective. The switching is done by applying a voltage to the material, or by performing some simple mechanical operation. Windows, skylights, etc., that are made of smart glass can be used to adjust indoor lighting, compensating for changes of the brightness of the light outdoors and of the required brightness indoors.
Fiber-optic concrete wall 
Hybrid solar lighting 
Oak Ridge National Laboratory (ORNL) has developed a new alternative to skylights called Hybrid Solar Lighting. This design uses a roof-mounted light collector, large-diameter optical fiber, and modified efficient fluorescent lighting fixtures that have transparent rods connected to the optical fiber cables. Essentially no electricity is needed for daytime natural interior lighting.
Field tests conducted in 2006 and 2007 of the new HSL technology were promising, but the low-volume equipment production is still expensive. HSL should become more cost effective in the near future. A version that can withstand windstorms could begin to replace conventional commercial fluorescent lighting systems with improved implementations in 2008 and beyond. The U.S. 2007 Energy Bill provides funding for HSL R&D, and multiple large commercial buildings are ready to fund further HSL application development and deployment.
At night, ORNL HSL uses variable-intensity fluorescent lighting electronic control ballasts. As the sunlight gradually decreases at sunset, the fluorescent fixture is gradually turned up to give a near-constant level of interior lighting from daylight until after it becomes dark outside.
HSL may soon become an option for commercial interior lighting. It can transmit about half of the direct sunlight it receives.
In a well-designed isolated solar gain building with a solarium, sunroom, greenhouse, etc., there is usually significant glass on the equator side. A large area of glass can also be added between the sun room and your interior living quarters. Low-cost high-volume-produced patio door safety glass is an inexpensive way to accomplish this goal.
The doors used to enter a room, should be opposite the sun room interior glass, so that a user can see outside immediately when entering most rooms. Halls should be minimized with open spaces used instead. If a hall is necessary for privacy or room isolation, inexpensive patio door safety glass can be placed on both sides of the hall. Drapes over the interior glass can be used to control lighting. Drapes can optionally be automated with sensor-based electric motor controls that are aware of room occupancy, daylight, interior temperature, and time of day. Passive solar buildings with no central air conditioning system need control mechanisms for hourly, daily, and seasonal, temperature-and-daylight variations. If the temperature is correct, and a room is unoccupied, the drapes can automatically close to reduce heat transfer in either direction.
To help distribute sun room daylight to the sides of rooms that are farthest from the equator, inexpensive ceiling-to-floor mirrors can be used.
Building codes require a second means of egress, in case of fire. Most designers use a door on one side of bedrooms, and an outside window, but west-side windows provide very-poor summer thermal performance. Instead of a west-facing window, designers use an R-13 foam-filled solid energy-efficient exterior door. It may have a glass storm door outside with the inner door allowing light to pass through when opened. East/west glass doors and windows should be fully shaded top-to-bottom or a spectrally selective coating can be used to reduce solar gain.
See also 
- Active daylighting
- Architectural glass
- Passive daylighting
- Passive solar building design
- Daylight harvesting
- Daylight factor
- Deck prism
- Sun path
- Transom (architectural)
- Sun/Earth Buffering and Superinsulation page 68 ISBN 0-9604422-4-3
- Oliver Graydon (March 11, 2004). "Concrete casts new light in dull rooms". optics.org. Retrieved 2010-08-27.
- Muhs, Jeff. "Design and Analysis of Hybrid Solar Lighting and Full-Spectrum Solar Energy Systems". Oak Ridge National Laboratory. Retrieved 2007-12-23.
- U.S. Department of energy page on passive daylighting
- Daylighting, Chapter 2 of the SynthLight Handbook, Low Energy Architecture Research Unit, London Metropolitan University, April 2004
- Sun Light Redirecting Devices - examples of geometrical set-up of light shelves etc.
- Solar control façades and Daylighting façades, University of California, Berkeley
- MIT, Building Technology Program, Daylighting Lab
- Photos of a small-scale heliostat system in action