Glossary

A-C     D-F     G-I     J-L     M-O     P-R     S-U     V-Z

A - C

Altitude (solar)

The vertical angular distance of a point in the sky (usually the sun) above the horizon. Altitude is measured positively from the horizon (0) to the zenith (the point in the sky straight overhead (90). (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Annealed glass

The most commonly used architectural glass. Because it is not heat-treated and therefore not subject to distortion typically produced during glass tempering, it has good surface flatness. On the downside, annealed glass breaks into sharp, dangerous shards. Heat-strengthened and fully-tempered glass are heat-treated glass products, heated and quenched in such a way to create residual surface compression in the glass. The surface compression gives the glass generally higher resistance to breakage than annealed glass. (Source: Nik Vigener, PE, Simpson Gumpertz & Heger Inc., Whole Building Design Guide)

Azimuth (solar)

  • The azimuth of the sun is the angle between the vertical plan containing sun and the plane of the horizon. (Source: Daylighting Performance and Design, Gregg Ander, FAIA 1995)
  • The horizontal angular distance between the vertical plane containing a poin in the sky (usually the sun) and true south. In other words, the angle of sun from true south as seen in plan view. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Baffle

A single opaque or translucent element used to shield a source from direct view at certain angels or to absorb unwanted light. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Ballast

  • Electrical device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current, waveform) for starting and operating. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)
  • Fluorescent and HID (high intensity discharge) lamps require ballast to apply starting voltage to the lamp and establish current flow (an "arc") between the lamp electrodes. Once the lamp is operating, the ballast also regulates the lamp current and power. Ballasts types fall into two broad categories: Magnetic: uses a core and coil assembly transformer to perform the minimum functions require to start and operate a lamp. Electronic: more complex device that substitutes electronic components fro the core and coil assemblies. They are generally smaller, lighter and quieter than magnetic counterparts and offer distinct advantages in energy efficiency and lamp operation. (Source: Osram Sylvania)

Ballast factor (BF)

  • Ratio of commercial electric ballast lamp lumens to a reference ballast lamp lumens, used to correct the lamp lumen output from rated to actual. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)
  • The measured ability of a particular ballast to produce light from the lamp(s) it powers; ballast factor is derived by dividing the lumen output of a particular ballast/lamp combination by the lumen output of the same lamps on a reference ballast.

Brightness

The subjective perception of illuminance. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)
(SEE ALSO: luminance)

Building envelope

Outer elements of a building, including walls, windows, doors, roofs, and floors, including those in contact with earth. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

Building-integrated photovoltaic (BIPV) systems

Integrated into the building envelope to convert sunlight into electrical energy. The solar cells that are part of the system are sometimes built into roofs and skylights rather than exterior walls to take advantage of the additional sunlight captured by sloped surfaces. Because the solar cells are generally not specifically designed as waterproofing elements, they typically require installation outboard of the building envelope. (Source: Whole Building Design Guide)

Chromatic glazing

Broad class of switchable glazings including active materials (electrochromic) and passive materials (photochromic and thermochromic).

Clear sky

Sky that is less than 30% cloud cover. (Source: Daylighting Design and Performance, Gregg Ander, FAIA, 1995)

Clerestory

Part of a building rising clear of the roof or other parts of the building, whose walls contain windows for lighting the interior.

Clerestory window

Window positioned about other windows or doors on the upper outside wall of a room.

Closed loop control system

Control system in which the effect of the control action on the controlled variable is sensed an used by the controller to provide a new output (feedback control). (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

Cloudy sky

Sky having more than 70% cloud cover. (Source: Daylighting Design and Performance, Gregg Ander, FAIA, 1995)

Coated glass

Covered with reflective or low-emissivity (low-E) coatings. In addition to providing aesthetic appeal, the coatings improve the thermal performance of the glass by reflecting visible light and infrared radiation. (Source: Whole Building Design Guide, National Institute of Building Sciences, by Nik Vigener, PE)

Color Rendering Index (CRI)

Measured on a scale of 0 to 100, CRI conveys the ability of a light source to accurately convey a sample of eight standard colors, relative to a standard source wit excellent color. In general, the higher the number, the better the source. Color rendering indices may only be directly compared for lamps of similar color temperature. (Source: ESource Technology Atlas Series Lighting Volume 1 2005)

Color temperature

Metric used to describe the "whiteness" of a light source. It is expressed in kelvins (K) (units which typically have no relation to the actual temperature of the light source ). It corresponds to the temperature of a blackbody radiator emitting light of a comparable quality. (Source: ESource Technology Atlas Series Lighting Volume 1 2005)

Commissioning

Process for achieving, verifying, and documenting the performance of buildings to meet the operational needs of the building within the capabilities of the design and to meet the design documentation and the owner's functional criteria, including preparation of operator personnel. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

Commissioning authority (or agent)

Qualified person, company, or agency that will plan and carry out the overall commissioning process. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

Cool daylit glazing

Spectrally neutral glazing that employs tinting and/or surface coatings to achieve a visiliby transmittance that exceeds the solar heat gain coefficient (total solar transmittance). See Light to Solar Gain Ratio. (Source: Center for Sustainable Building Research)

Cooling load

Amount of cooling per unit of time required by the conditioned space or product; heat a cooling system must remove from a controlled system over a period of time. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

Cut-off angle

The critical viewing angle beyond which a source can no longer be seen because of an obstruction, such as a baffle or overhang. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

D - F

Daylight Pipes

See Tubular Daylighting Devices.

Daylight factor

The ratio of daylighting illumination on a horizontal point indoors to the horizontal illumination outdoors, expressed as a percentage. Direct sunlight is excluded [only evaluated under cloudy sky conditions]. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Daylight zone

Term indicating floor area that has sufficient daylight available to reduce or eliminate the need for electric lighting during daytime hours.

Diffuse light

Lighting on a workplane or object that is not predominantly incident from any particular direction. (Source: Center for Sustainable Building Research)

Distribution

Introduce as much controlled daylight as deep as possible into a building interior. The human eye can adjust to high levels of luminance as long as it is evenly distributed. In general, light which reaches a task indirectly (such as having bounced from a white wall) will provide better lighting quality than light which arrives directly from a natural or artificial source. (Source: Whole Building Design Guide, National Institute of Building Sciences, by Greg Ander, FAIA)

Efficacy

Term for the amount of light produced per watt of electricity (comparable to efficiency). The rate at which a light bulb is able to convert elecrtrical power (watts) into light (lumens), expressed in term of lumens per watt (LPW). Example: a 100 watt lamp producing 1,750 lumens gives 17.5 lumens per watt. (Source: Oregon Department of Energy)

Electric power factor (PF)

Ratio of real power (kW) to apparent power (kVA) at any given point and time in an electrical circuit. Generally expressed as a percentage. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)
A measure of the effectiveness with which an electrical device converts volt-amperes to watts; devices with power factors greater than .90 are "high power factor" devices (Source: Osram Sylvania)

Electrochomic glass

Glass with electrochromic coatings utilizes a small electrical voltage, adjusted with dimmable ballasts, to adjust the shading coefficient and visible light transmission. Like photochromic coatings, they are intended to attain lighting energy savings. (Source: Nik Vigener, PE, Simpson Gumpertz & Heger Inc., Whole Building Design Guide)

Electrochromics

Glazing with optical properties that can be varied continuously from clear to dark with a low-voltage signal. Ions are reversibly injected or removed from an electrochromic material, causing the optical density to change. (Source: www.efficientwindows.org)

Electromagnetic spectrum

Vision depends on light: the band of electromagnetic radiation with wavelengths ranging from about 380 to 770 nanometers (one nanometer = 10-9 meters). Humans sense different wavelengths as color. Most light sources also emit energy in the nonvisible ultraviolet and infrared ranges as well. (Source: ESource Technology Atlas Series Lighting Volume 1 2005)
Radiant energy over a broad range of wavelengths. (Source: www.efficientwindows.org)

Externally or envelope dominated

Buildings whose heating and cooling loads are determined primarily by external factors. Typically smaller buildings with high surface to volume ratios.

Fenestration

Any opening in a building's envelope including windows, doors and skylights. (Source: National Fenestration Rating Council)

Fluorescent lamp

A low pressure mercury electric discharge lamp in which a fluorescent coating (phosphor) transforms ultraviolet energy into visible light.

Footcandle

A unit of illumance, equal to 1 lumen per square foot or 10.76 lux. (Source: ESource Technology Atlas Series Lighting Volume 1 2005)

Fully-tempered glass

Provides at least four times the strength of annealed glass, which gives it superior resistance to glass breakage. Similar to heat-strengthened glass, the heat-treatment generally results in some distortion. If it breaks, fully-tempered glass breaks into many small fragments, which makes it suitable as safety glazing under certain conditions. (Source: Nik Vigener, PE, Simpson Gumpertz & Heger Inc., Whole Building Design Guide)

G - I

Glare

  • The aim of an efficient daylighting design is not only to provide illuminance levels sufficient for good visual performance, but also to maintain a comfortable and pleasing atmosphere. Glare, or excessive brightness contrast within the field of view, is an aspect of lighting that can cause discomfort to occupants. The human eye can function quite well over a wide range of luminous environments, but does not function well if extreme levels of brightness are present in the same field of view. (Source: Whole Building Design Guide, National Institute of Building Sciences, by Greg Ander, FAIA)
  • The sensation produced by brightness within the visual field that is greater than the brightness to which the eye is adapted and thus caused annoyance, discomfort, or loss in visual performance and visibility. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Various types of glare include:

  • Reflected glare or veiling reflection
  • Discomfort glare: glare which is distracting or uncomfortable and interferes with the perception of visual information but does not significantly reduce ability to see.
  • Disability glare: glare which reduces the ability to perceive visual information needed for an activity.
  • Direct glare: glare resulting from high luminances in the visual environment that are directly visible from a viewers position.
  • Blinding glare: glare so intense that for a period of time after the glare has been removed, no visual perception is possible.

Glass

An inorganic transparent material composed of silica (sand), soda (sodium carbonate), and lime (calcium carbonate) with small quantities of alumina, boric, or magnesia oxides. (Source: www.efficientwindows.org)

Glazing

The glass or plastic panes in a window, door, or skylight. (Source: www.efficientwindows.org)

Heat-strengthened glass

Glass that is at least twice the strength and resistance to breakage from wind loads or thermal stresses as annealed glass. The necessary heat treatment generally results in some distortion compared to annealed glass. Like annealed glass, heat-strengthened glass can break into large shards. Whole Building Design Guide)

Heating degree day

Term used to relate the typical climate conditions to the amount of energy needed to heat a building. The base temperature is usually 65 degrees Fahrenheit. A heating day is counted for each degree below 65 degrees that the average outside temperature reaches in the winter.

Illuminance

  • Amount of light incident on a surface. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)
  • Light arriving at a surface, expressed in lumens per unit area; 1 lumen per square foot equals 1 footcandle; 1 lumen per square meter equals 1 lux. (Source: Osram Sylvania)

Indirect lighting

Lighting achieved by reflection, usually from wall and ceiling surfaces. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Infrared (IR)

IR is part of the solar spectrum, or sunlight, that is invisible to the human eye. It has a wavelength range of ~790-3000 nanometers and has a penetrating heat effect. Short-wave IR converts to heat when it is absorbed by an object. (Source: www.viracon.com)

Insulating glass units (ig units)

Consists of two or more lites of glass with a continuous spacer that encloses a sealed air space. The spacer typically contains a desiccant that dehydrates the sealed air space. The air space reduces heat gain and loss, as well as sound transmission, which gives the ig unit superior thermal performance and acoustical characteristics compared to single glazing. Most commercial windows, curtain walls, and skylights contain ig units. Most perimeter seals consist of a combination of non-curing (typically butyl) primary seal and cured (frequently silicone) secondary seal. The service life of an ig unit is typically determined by the quality of the hermetic sealants installed between the glass and the spacers, and the quality of the desiccant. (Source: Nik Vigener, PE, Simpson Gumpertz & Heger Inc., Whole Building Design Guide)

Integrated system

More than one building system, such as lights and sir distribution, combined into a common design. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

Internally load dominated

Heating and cooling loads determined by internal factors including lighting, equipment and occupants. Typically larger buildings with low surface to volume ratios.

J - L

Kilowatt (kW)

Unit of electrical power; rate at which energy is used. (Source: Energy Center of Wisconsin)

Kilowatt hour (kWh)

Unit of energy; equal to 1,000 Watts per hour. (Source: Energy Center of Wisconsin)

Laminated glass

Consists of two or more lites of glass adhered together with a plastic interlayer. Because it can prevent the fall-out of dangerous glass shards following fracture, it is often used as safety glazing and as overhead glazing in skylights. The plastic interlayer also provides protection from ultraviolet rays and attenuates vibration, which gives laminated glass good acoustical characteristics. Because laminated glass has good energy absorption characteristics, it is also a critical component of protective glazing, such as blast and bullet-resistant glazing assemblies. (Source: Nik Vigener, PE, Simpson Gumpertz & Heger Inc., Whole Building Design Guide)

Light

Visually perceived radiant energy. Visible light is just a small segment of the electromagnetic spectrum - a broad range of radiant energy which includes x-rays, ultraviolet and infrared energy, microwaves and radio waves. Any form of electromagnetic energy, including light, radiates outward from its source in straight lines at the "speed of light" (300,000kilometers per second) and dissipates as its gets further from its source. (Source: Osram Sylvania)

Light scoops

Clerestory roof monitors oriented away from the sun, utilized when and where indirect light s desired and solar heat gains are undesirable.

Light pipe

See tubular daylighting devices.

Light shelf

Horizontal element positioned above the eye level to reflect daylight onto the ceiling. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Light-to-solar-gain ratio (LSG)

  • A measure of the ability of a glazing to provide light without excessive solar heat gain. It is the ratio between the visible transmittance of a glazing and its solar heat gain coefficient. Abbreviated LSG. (Source: www.efficientwindows.org)
  • The ratio is equal to the Visible Light Transmittance divided by the Solar Heat Gain Coefficient. The Department of Energy's Federal Technology Alert publication of the Federal Energy Management Program (FEMP) views and LSG of 1.25 or greater to be Green Glazing/Spectrally Selective Glazing (Source: www.viracon.com)

Ratio between visible transmittance (VT) and solar heat gain coefficient (SHGC).
LSG = VT/SHGC

A value below one means the glazing transmits more light than heat. Spectrally selective glazing is characterized by having a high LSG value (=1.25).

Lighting power density (LPD)

Measure of the amount of electric lighting installed in a building. Expressed as the number of watts of lighting power required for the luminaires and lamps installed in a building, divided by the gross number of square feet in the building (watts per square foot). (Source: Daylighting Design and Performance, Gregg Ander FAIA, 1995)

Low E coatings

Invisible coating applied to glass to improve its thermal characteristics. E stands for emissivity, with is the ability of a material to emit radiant energy. (Source: The Dumb Architect's Guide toGglazing Selection)

Low-emittance (Low-E) coating

Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. A typical type of low-E coating is transparent to the solar spectrum (visible light and short-wave infrared radiation) and reflective of long-wave infrared radiation.

Luminance

  • Amount of light coming from a surface; in other works, how bright it is. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)
  • Light reflected in a particular direction; the photometric quantity most closely associated with brightness perception, measured in units of luminous intensity (candelas) per unit area (square feet or meters). (Source: Osram Sylvania)

Luminance contrast

The relationship between the luminances of an object and its immediate background. (Source: ESource Technology Atlas Series Lighting Volume 1 2005)

Luminance ratios

Ratio between different brightnesses in the visual field. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Lux (lx)

A unit of illuminance equal to 1 lumen per square meter.

M - O

Open loop control system

Signal path with includes a forward path, a feedback path, and a summing point, and forms a closed circuit. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

Overcast sky

Sky completely covered by clouds, no sun visible. (Source: Daylighting Design and Performance, Gregg Ander, FAIA, 1995)

P - R

Particle dispersed glazing

Glazing in which the orientation of small particles between two sheets of glass is controlled electrically, thus changing its optical properties. (Source: www.efficientwindows.org)

Peak demand

Maximum hourly total building electricity use in the year, includes space conditioning, electric lighting and equipment. (Source: Energy Center of Wisconsin)

Peak load

Maximum hourly total building heating or cooling load in the year. (Source: Energy Center of Wisconsin)

Photochromic coatings

Incorporate organic photochromic dyes to produce self-shading glass. Originally developed for sunglasses, these coatings are self-adjusting to ambient light and reduce visible light transmission through the glass. In architectural glass they are typically used to provide shading. (Source: Nik Vigener, PE, Simpson Gumpertz & Heger Inc., Whole Building Design Guide)

Power factor

See electric power factor

R-value

  • Thermal resistance; ability to resist heat flow.
  • Under steady conditions, the mean temperature difference between two defined surfaces of material or construction that induces unit heat flow through unit area; quantity determined by the temperature difference, at steady state, between two defined surfaces of a material. NOTE: Thermal resistance and thermal conductance are reciprocals. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)
  • A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value. (Source: www.efficientwindows.org)

Radiation

The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches the earth by radiation, and a person's body can lose heat to a cold window or skylight surface in a similar way. (Source: www.efficientwindows.org)

Reflectance

The ratio of light reflected by a body to the incident light; the total reflection factor of a layer of material of such thickness that there is no change of reflection factor with increase in the thickness. (Source: Daylighting Performance and Design, Gregg Ander, FAIA, 1995)

Reflected glare

See Veiling reflection

Reflective glass

Window glass coated to reflect radiation striking the surface of the glass. (Source: www.efficientwindows.org)

Refraction

Deflection of a light ray from a straight path when it passes at an oblique angle from one medium (such as air) to another (such as glass). (Source: Center for Sustainable Building Research)

S - U

Shading device

Use of a fixed or movable devices that can block, diffuse or redirect incoming light to control direct beam penetration and unwanted heat gains and glare.

Sidelighting

Lighting from windows and translucent walls. Usually defined at vertical glazing below 10 feet.

Shading coefficient (SC)

  • Ratio of absorbed and transmitted solar heat relative to fenestration fitted with shading devices to that occurring with unshaded single strength glass. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)
  • A measure that describes a window's ability to block heat relative to clear glass.
  • A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8-inch clear, double- strength, single glass. It is being phased out in favor of the solar heat gain coefficient, and is approximately equal to the SHGC multiplied by 1.15. It is expressed as a number without units between 0 and 1. The lower a window's solar heat gain coefficient or shading coefficient, the less solar heat it transmits, and the greater is its shading ability. (Source: www.efficientwindows.org)
  • Shading coefficient is the ratio of solar heat gain through a specific type of glass that is relative to the solar heat gain through a 1/8" (3mm) ply of clear glass under identical conditions. As the shading coefficient number decreases, heat gain is reduced, which means a better performing product. (Source: www.viracon.com)

Skylights

Sloped or horizontal application of a fenestration product which allows for daylighting. Skylights may be either fixed (non-operable), or venting (operable). (Source: Center for Sustainable Building Research)

Solar heat gain coefficient (SHGC)

  • A measure that describes how much heat makes it through the glass compared to the amount that strikes it.
  • Measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window, both admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits. (Source: National Fenestration Rating Council)
  • The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly. (Source: www.efficientwindows.org)

Solar heat gain

Quantity of heat absorbed by an enclosed space or system. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

Spectrally selective glazing

A glazing's ability to transmit visible light while reducing solar heat gain denoted as a value between 0 and 1. Glazing using low-E coatings or tinted that selectively blocks certain wavelengths of light more readily that others. In general, designed to block heat while allowing visible light to pass.

Specular surface

A shiny, highly polished surface which reflects light at an angle equal to that of the incident light.

Spectrally selective coating

A coated or tinted glazing with optical properties that are transparent to some wavelengths of energy and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared radiation. (Source: www.efficientwindows.org)

Spectral selectivity

Windows ability to transmit visible light while reducing solar heat gain. Expressed by a number between 0 and 1. (Source: Architectural Lighting)

Sunpath diagram

Sun path diagrams or sun charts are two-dimensional projections of the sky dome onto a surface, including solar altitude and azimuth for a specific latitude. Two different types exist with advantages and disadvantages. Both show same information but in different forms:

  • Cylindrical projection - Shows the suns apparent path for an observer who is looking due south. Useful for conducting shading calculations, especially for high geographical altitudes.
  • Pola diagram - Sky dome is projected onto a horizontal plane with the observer being in the center. Ideal for visualizing the compass direction of the sun at any point in time, especially at tropical altitudes and during summer months. Different ways of mapping the altitude angles maybe used (stereographic projection, spherical projection, equidistant projection) but stereographic is most commonly used. (Source: CLEAR: Comfortable Low Energy Architecture)

Sunlight

Composed of ultraviolet light (radiation), visible light and infrared radiation.

Superwindow

A window with a very low U-factor, typically less than 0.15, achieved through the use of multiple glazings, low-E coatings, and gas fills. (Source: www.efficientwindows.org)

Task-ambient lighting

A combination of ambient and task lighting yielding a general level of ambient lighting that is lower than, and complementary to, task lighting. (Source: ESource Technology Atlas Series Lighting Volume 1 2005)

Thermochromic glass

Glass that has been chemically designed to change opacity and translucency in response to the surface temperature of the glass.

Tinted glass

Contains minerals that color the glass uniformly through its thickness and promote absorption of visible light and infrared radiation. (Source: Nik Vigener, PE, Simpson Gumpertz & Heger Inc., Whole Building Design Guide)

Toplighting

Lighting from skylight, roof monitors or clerestories. Generally vertical and horizontal daylighting apertures above 10 feet.

Transmittance

The percentage of radiation that can pass through glazing. Transmittance can be defined for different types of light or energy, e.g., visible light transmittance, UV transmittance, or total solar energy transmittance. (Source: www.efficientwindows.org)

Tubular daylighting devices

Generic term for a system employing bulk optics (lenses, mirrors, reflective ducts or other optical waveguide technology) designed to transport daylight to parts of a building remote from the envelope. (Source: Center for Sustainable Building Research)

U-value

See U-factor. Per ASHRAE, this is improper terminology. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)

U-factor

  • Thermal transmittance. Heat transmission in unit time through unit area of a material and the boundary air films, induced by unit temperature difference between the environment on each side. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)
  • Measure of the rate of heat loss and gain through a material by conduction - expressed in Btu/hr per square foot.
  • U-factor measures how well a product prevents heat from escaping. The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. U-Factor ratings generally fall between 0.20 and 1.20. The insulating value is indicated by the R-value which is the inverse of the U-value. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value. (Source: National Fenestration Rating Council)
  • U-factor (U-value). A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-°F (W/sq m-°C). Values are normally given for NFRC/ASHRAE winter conditions of 0° F (18° C) outdoor temperature, 70° F (21° C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value. (Source: www.efficientwindows.org)

Ultraviolet light (UV)

The name of the invisible portion of the light spectrum with wavelengths shorter than 390 nanometers. The damaging effects on long-term UV exposure results in fabric fading and plastic deterioration. (Source: www.viracon.com)

V - Z

Veiling reflection

Glare from reflected sources. Reflections that partially or totally obscure the details of objects in the visual field by reducing contrast.
Veiling reflections of high brightness light sources off specular (shiny) surfaces obscure details by reducing contract. They should be avoided, particularly where critical visual tasks occur. (Source: Whole Building Design Guide, National Institute of Building Sciences, by Greg Ander, FAIA)

Visible light

The portion of the electromagnetic spectrum that produces light that can be seen. Wavelengths range from 380 to 720 nanometers. Source: www.efficientwindows.org)

Visible light transmittance (VLT, VT, VT)

  • Fraction of the visible electromagnetic spectrum that is transmitted through the glazing in the visible spectrum (380 to 720 nanometers). The higher the number the higher the visible light incident on the window that is transmitted. Florida Solar Energy Center)
  • Measure of the amount of light that makes it through glass compared to the amount striking the surface.
  • Expressed as a percentage. VLT of 0.50 indicates that only 50% of the light striking the glass in being transmitted through.
  • Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted. (Source: National Fenestration Rating Council)
  • The percentage or fraction of the visible spectrum (380 to 720 nanometers) weighted by the sensitivity of the eye, that is transmitted through the glazing. (Source: www.efficientwindows.org)
  • The percentage of visible light (380 - 780 nanometers) that is transmitted through the glass. (Visible light is the only portion of the solar spectrum visible to the human eye.) (Source: www.viracon.com)

Watt (W)

A unit of electrical power equal to 1 joule per second.

Window to wall ratio (WWR)

Ratio of the total area of a building façade which is occupied by windows (glass area and frame). (Source: Center for Sustainable Building Research)

Workplane

The plane at which work is performed, usually taken as horizontal and at desk height (30 inches) from the floor. (Source: Tips for Daylighting with Windows, LBNL - 39945, 1997)

Zenith

Point on the skydome directly overhead, a 90 degree altitude.

Zoning

Division of a building into separately controlled spaces. (Source: ASHRAE Terminology of Heating, Ventilation, Air Conditioning & Refrigeration, ASHRAE, Inc. Atlanta, GA, 1991)