Skylights are light transmitting fenestration (elements filling the building openings), forming a part of, the roof of a building space for daylighting purposes.
Open skylights were used in ancient Roman architecture , such as the oculus of the Pantheon . Glazed ‘closed’ skylights have been in use since the Industrial Revolution made advances in glass production manufacturing. Mass production units since the mid-20th century have brought many skylights to many uses and contexts. Energy conservation has brought new motivation, innovation design, transmission options, and efficiency rating systems for skylights
Skylighting types include roof windows , unit skylights, tubular daylighting devices (TDDs), sloped glazing, and custom skylights. Uses include:
- daylighting elements to allow direct and / or indirect sunlight, via toplighting.
- providing a connection to the outdoor environment to interior occupants.
- sustainable building – passive solar heating, and with operable units; ventilation for passive cooling and fresh air exchange.
- Open skylight
An unglazed hole in a roof.
- Fixed unit skylight
A fixed skylight consists of a structural perimeter frame supporting glazing infill (the light-transmitting portion, which is made primarily of glass or plastic). A fixed skylight is non-operable, meaning there is no ventilation. 
- Operable skylight
An operable (venting) unit skylight uses a hinged sash attached to and supported by the frame. When within reach of the occupants, this type is also called a roof window.
- Retractable skylight
A wide (20 ‘x 20’) steel and glass retractable skylight, seen from the roof. Note the steel tracks that the skylight rolls on, to retract.
A retractable skylight rolls – we have a set of tracks – off the frame, so we are not open to the outdoors, ie, not impeded by a hinged skylight. The terms retractable skylight and retractable roof are often used interchangeably, though skylight implies a degree of transparency.
- Tubular daylight device
Active daylighting uses a tubular daylighting device- TDD. It is a roof-mounted fixed unit skylight element, condensing sunlight, distributed by a light conveying optic led to a light diffusing element. Being small in diameter, they can be used for daylighting smaller spaces such as hallways , and bouncing light in darker corners of spaces. TDDs harvest daylight through a roof-mounted dome with diameters ranging from about 10 inches for residential applications to 22 inches for commercial buildings. Made from acrylic or polycarbonate formulated to block ultraviolet rays, the dome catches and redirects light rays into an aluminum tubing system that will resemble ductwork.  Image: Skylight on the roof terrace of Liverpool Central Library (2) .JPG | TDD skylight on the roof terrace of Liverpool Central Library
- Sloped glazing
Sloped glazing differs from other “skylights” in one assembly contains multiple infill panels in a framing system, usually designed for a specific project and installed in sections on site.
- Pavement lights
Pavement lights are walk-on skylights. They are set into sidewalks, open areas, and well-lit interior floors.
- Prism lights
Prism lights are sometimes used as skylights; they redirect the light passing through.
Skylights are widely used in designing daylighting for residential, public, and commercial buildings. Increased daylighting can result in lower energy costs, lower energy costs, and lower environmental costs. Daylighting can be used in some buildings by up to 80%. 
Toplighting (skylights) works with sidelighting (windows) to maximize daylighting:
- toplighting is able to bring light to a centralized areas of a building
- daylight is available throughout the day from both ambient lighting and direct exposure to the sun.
- modern transparent and / or translucent glazing can be used to avoid glare, helps in capturing sunlight at low angles and diffuses light to wider areas of floor space.
Even on overcast days, toplighting from skylights is more efficient than sidelighting. 
Many recent [ when? ] advances in both glass and plastic infill systems have greatly benefited all skylight types. Some of them are focused on preserving and utilizing daylight potential, and some are designed to enhance strength, durability, fire resistance and other performance measures.
Contemporary skylights using glass infill (windows) with two glass panes of glass. These types of products are NFRC-ratable for visible transmittance. Can be used in the coldest climate zones, but they lose some light by adding the third layer of glass.
Low-E (low-E) coating is one of the most important applications of UV radiation. 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 shaped 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.
Plastics used in skylights are UV stabilized and may feature other advances to improve thermal properties. Lack of accepted standards for measuring light transmittance is a disadvantage for comparing and choosing skylights with plastic glazing.
Acrylic is the most common plastic glazing used for dome skylights. However, polycarbonate and copolyester materials are also used as glazing, where additional properties such as impact resistance may be required. 
NFRC – rating for visible transmittance
U-factor – expresses the heat loss performance of any building assembly.
SHGC-Solar Heat Gain Coefficient – measures the assembly of heat from outside the sun.
These properties are labeled in the US as a decimal between zero and one, with lower numbers indicated 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.
In selection of skylights, a balance is sought between low U-factor and optimal SHGC values, while preserving enough daylight supply to minimize artificial light use. Automatic light sensing controls for electric lighting
Classrooms that optimize classlighting, that classrooms that do not.  Other studies show that daylight positively affects physiological and psychological well-being, which can increase productivity in many contexts, such as sales in retail spaces.
In terms of cost savings, US DOE reported that many commercial buildings can reduce total energy costs by up to one-third through the optimal use of daylighting. The majority of commercial warehouses and ‘big box stores’ have been used extensively for energy / costs savings.
- Roof lantern , Cupola
- Light Pavement (a walk-on skylight)
- Passive daylighting
- Energy-saving lighting
- Daylight factor
- Passive solar building design
- Light tube
- Daylighting / Passive solar lighting
- Jump up^ “Terms defined: Types of skylight” . Sunsquare . Retrieved 20 Sep 2017 .
- Jump up^ Lau, Wanda (Feb 6, 2012). “Lighten Up Tubular daylighting devices turn natural light into a deployable technology” . EcoBuilding Pulse . Retrieved April 3, 2014 .
- Jump up^ US Department of Energy (DOE): Federal Energy Management Program; toplighting_final_report
- Jump up^ AAMA: Daylighting Fact Sheet
- Jump up^ AAMA Skylight Council
- Jump up^ coe.uga.edu/sdpl/research/daylightingstudy: study of 21,000 students.