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Absorption Chiller – A water chilling process in which cooling is accomplished by the evaporation of a fluid (usually water), which is then absorbed by a different solution (usually lithium bromide), then evaporated under heat and pressure. The fluid is then condensed with the heat of condensation rejected through a cooling tower.

Air Changes Per Hour (ACH) – The movement of a volume of air in a given period of time; if a house has one air change per hour, it means that the air in the house will be replaced in a one-hour period.

Acid Rain – Precipitation that is contaminated with acid due to sulfur dioxide and oxides of nitrogen in the air.

Bauxite – the raw material mined from the earth we use to make aluminum.

Biodegradable – capable of being broken down by living organisms, principally bacteria and fungi.

Biomass – biomass energy is derived from plants. Alcohol fuels are produced from wood, sugarcane and corn. Firewood, crop residue and cattle dung can also be burned as biomass fuel. As long as the amount of plants regrown equals the amount of fuel burned there will be no additional carbon dioxide produced to contribute toward global warming.

Btu – a British Thermal Unit. A measure of energy in the English system measurement, roughly the amount of heat required to raise one pound of water one degree Fahrenheit. This unit of measuring heat will soon no longer be used and will be replaced in usage by “joule.”

Calorie – currently the most common unit for measuring heat and soon to be replaced by joules (J). The calorie is the amount of energy required to raise the temperature of one cubic centimeter of water one degree Celsius (formerly called centigrade).

Carbon Dioxide (CO2) – a colorless, odorless, nonflammable gas formed during decomposition, combustion and respiration. CO2 is used in food refrigeration (dry ice), carbonated beverages, fire extinguishers and aerosol cans. Whenever something burns — such as gasoline, wood or a candle — CO2 is produced from the available oxygen combined with the carbon in the fuel.

Carbon Monoxide – (CO) a colorless, odorless gas formed when carbon is oxidized in a limited supply of air. It is a poisonous constituent of car exhaust fumes, forming a stable compound with hemoglobin in the blood, thus preventing the hemoglobin from transporting oxygen to the body tissues.

Chlorofluorocarbon – (CFC) synthetic chemical that is odorless, nontoxic, nonflammable but reacts with chemicals high in the atmosphere resulting in the depletion of the earth’s protective ozone layer. CFCs have been used as propellants in aerosol cans, as refrigerants in refrigerators and air conditioners, and in the manufacture of foam packaging. They are partly responsible for the destruction of the ozone layer.

Coal – coal is a form of stored solar energy. It is created from the remains of plants that have been concentrated by heat and pressure for millions of years. Coal is found in various forms or “grades,” which depend on the ratio of carbon mass to energy content. Represented in descending order of hardness and energy content per pound, these grades are anthracite, bituminous, sub-bituminous and lignite.

Cogeneration – the use of waste heat from an electrical generating plant for other purposes, such as heating. Also, the use of waste heat from a high-temperature industrial process to generate electricity.

Cooling Load – A measure of the energy that must be expended by a building’s systems to cool the indoor to a desired temperature.

Crude Oil – crude oil is petroleum direct from the ground, prior to refinement or processing.

Daylighting – The means by which daylight is brought into a building to either supplement or replace electrical lighting in order to allow the occupants to perform their tasks.

Distribution – the facilities of the electric system that deliver electricity from substations to customers. The distribution system “steps down” power from high-voltage transmission lines to a level that can be used in homes and businesses.

Ecology – study of the relationship among organisms and the environments in which they live, including all living and nonliving components.

Efficiency – output of a machine (work done by the machine) divided by the input (work put into the machine), usually expressed as a percentage. Because of losses caused by friction, efficiency is always less than 100%, although it can approach this for electrical machines with no moving parts (such as a transformer).

Electric Current – the flow of electronically charged particles through a conducting circuit due to the presence of a potential difference. The current at any point in a circuit is the amount of charge flowing per second; its SI unit is the ampere (coulomb per second).

Electricity – a property of matter caused by the movement of electrons. This “movement” is initiated usually by a generator that is fueled by any number of energy resources such as coal, uranium, water (hydropower), or directly converted from solar radiation on photovoltaic cells. Electricity is not energy per se, but the “carrier” of energy that originates in fossil fuel and renewable energy sources.

Electromagnetic Waves – oscillating electric and magnetic fields traveling together through space at a speed of nearly 186,000 mi/300,000 km per second. The (limitless) range of possible wavelengths or frequencies or electromagnetic waves, which can be thought of as making up the electromagnetic spectrum, includes radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.

Embodied Energy – it takes energy to make something. Embodied energy is associated with the production of a good or service or the energy to prepare or make a product.

Energy – energy can be defined as the ability to do work — the ability to exert a force.

Energy Conservation – methods of reducing energy use through insulation, increasing energy efficiency, and changes in patterns of use.

Fluorescent Lamp – This form of lighting constitutes approximately 70% of the electrical light used in North America. Efficacy rates (lumens output / watts input) for fluorescent lamps range from 50-80 lm/w.; much higher than that of incandescent bulbs.

Formaldehyde – A volatile organic compound off-gassed in paints, glue adhesives, and laminates that can cause sickness and contribute to ground-level ozone formation.

Fossil Fuel – fuel, such as coal, oil and natural gas, formed from the fossilized remains of plants that lived hundreds of millions of years ago. Fossil fuels are a nonrenewable resource and will eventually run out. Extraction of coal and oil causes considerable environmental pollution, and burning coal contributes to problems of acid rain and the greenhouse effect.

Fuel Chain – the chain of activities involved in transforming energy into forms more convenient for society. This “chain” may include some or all of the following: fuel exploration, extraction, preparation, transportation, conversion to electricity, distribution and waste disposal.

Geothermal Power – geothermal energy is the natural heat of the earth that is conducted or convected to the earth’s surface through volcanoes and hot springs. By harnessing this energy and using it to power steam turbines, we can convert geothermal energy into electricity that we can use.

Glazing – A transparent covering usually made from glass or plastic used to admit light through a window, door, skylight, or other opening.

Graywater – Water drained from building such as dishwashers, clothes washers, sinks, and showers. Graywater usually requires some degree of treatment before it can be reused because it is likely to contain soap, contaminants from the kitchen, etc., but it does not include wastewater from toilets. Today, we usually mix graywater and blackwater (sewage from toilets). The potential uses for graywater are numerous, including landscape irrigation, carwash, toilet flushing, and pool use.

Grid – the transmission network (or “highway”) over which electricity moves from suppliers to customers.

Habitat – in ecology, the localized environment in which an organism lives. The dominant plant type or physical feature, such as a grassland habitat or rocky seashore habitat often describes habitats.

Heat – a form of internal energy of a material based on the kinetic energy in the motion of its molecules or atoms. Heat energy is transferred by conduction, convection and radiation.

Heat Exchanger – A mechanical system that allows the heat from outgoing exhaust air to be transferred to incoming fresh, air. This is achieved by constant fan-forced ventilation, using either flat-plate, or rotary exchangers.

Hydrocarbons – an extensive group of chemicals that always include the elements hydrogen and carbon. Natural sources of hydrocarbons are the by-products of digestion and decomposition (e.g., rotting, spoiling, and putrefying). Coal, natural gas, oil, sugar, starches, and plastics are all composed of hydrocarbons. The incomplete combustion of hydrocarbons from fossil fuels contributes to our pollution and global warming problems.

Incandescent Lamp – A lamp that produces light by directing electrical current through a metallic medium. The efficiency of lamps is stated as an efficacy rating (lumens/input wattage). For example, a 100-watt lamp that produces 1,740 lumens has an efficacy of 17.4 lumens per watt. The average incandescent lamp typically falls in the 10-25 lumens per watt range. Incandescent lamps are considered to be very inefficient.

Joule – (symbol J) – named in honor of British physicist James P. Joule (rhymes with pool) who proved in 1843 that a specific amount of work was converted into a specific amount of heat. A joule is now a unit for all forms of energy. One joule of work is done when the force of one NEWTON is exerted on an object moving in the direction of the force, a distance of one meter. It takes about one joule to lift an apple over your head. As the transition from the English system of energy measurement to the international system of units (SI) picks up momentum, we will soon become accustomed to hearing more frequently of kilojoules (Kj) and megajoules (Mj). One kilowatt-hour = 3.6 x 106 joules. One calorie = 4.187 joules.

Kilowatt – a measure of electric energy equal to 1,000 watts. Put another way, it’s the amount of electric energy required to light ten 100-watt light bulbs.

Kilowatt-Hour – a measure of energy use over time. Utility companies typically sell energy as kilowatt-hours (kWh). A kilowatt-hour is 1,000 watts used for one hour. If you purchased a kilowatt-hour from your utility for less than a dime, you could burn ten, 100-watt incandescent light bulbs for one hour. Other electrical appliances such as stoves, heaters and blow dryers, would operate for less time for that same nickel because their energy demands are greater for their operation. (1 kWh = 3.6 x 106J or 36,000,000 J.)

Life Cycle Analysis – A methodology of quantitative assessment that determines the relative environmental “pluses” and “minuses” of a product, over its lifetime, on the topics of resource depletion, manufacture, installation methods, and recyclability and/or reuse.

Light – electromagnetic waves in the visible range, having a wavelength from about 400 nanometers in the extreme violet to about 770 nanometers in the extreme red. Light is considered to exhibit particle and wave properties, and the fundamental particle, or quantum, of light is called the photon. The speed of light (and of all electromagnetic radiation) in a vacuum is approximately 186,000 mi/300,000 km per second, and is a universal constant denoted by c.

Load Profiling – the study of the consumption habits of consumers to estimate the amount of power they use at various times of the day and for which they are billed. Load profiling is an alternative to precise metering.

Luminescence – emission of light from a body when its atoms are excited by means other than raising its temperature. Short-lived luminescence is called fluorescence.

Megawatt – one million watts of power potential. (see watt, joule, and newton)

Megawatt-Hour (MWH) – one million watts used for one hour. If you purchased a megawatt-hour of energy for a nickel per kilowatt-hour, it would cost you 1,000 nickels, or $50.00. Using a kWh your could burn one, 100-watt incandescent for 24 hours a day for about 14 months, or 3 hours a day for over 9 years.

Methane (CH4) – Methane is a simple hydrocarbon composed of one carbon atom surrounded by four hydrogen atoms. It is an odorless, flammable and invisible gas and the primary ingredient in natural gas. Natural gas companies add a strong odorant to the gas for safety so it can be easily detected by smelling. Methane is a relatively clean fuel and it is commonly used to fuel vehicles in many countries, such as New Zealand and Italy.

Natural Gas – mixture of flammable gases found in the Earth’s crust (often in association with petroleum), now one of the world’s three main fossil fuels (with coal and oil). Natural gas is a mixture of hydrocarbons, chiefly methane, with ethane, butane and propane.

Oil – flammable substance, usually soluble in water, and composed chiefly of carbon and hydrogen. Oils may be solids (fats and waxes) or liquids. The three main types are: essential oils, obtained from plants; fixed oils, obtained from animals and plants; and mineral oils, obtained chiefly from the refining of petroleum.

Ozone – O3 highly reactive pale-blue gas with a penetrating odor. Ozone is an allotrope of oxygen made up of three atoms of oxygen. It is formed when ultraviolet radiation or electrical discharge splits the molecule of the stable form of oxygen (O2). It forms a thin layer in the upper atmosphere, which protects life on Earth from ultraviolet rays, a cause of skin cancer. At lower atmosphere levels it is an air pollutant and contributes to the greenhouse effect.

Particulates – Particulates can be suspended solids or liquids that include dust from automobile and truck brake linings, road grit, ash from factory smokestacks, some from home chimneys and aerosols. Particulates reduce visibility and can cause lung and eye damage, especially when combined with other pollutants such as sulfur oxides (SOx) and nitrous oxides (NOx). Many people with respiratory problems are unaware their breathing problems can result from particulate pollution.

Passive Solar System A system that relies on the natural phenomena of energy (radiation, convection, and conduction) for the transfer and storage of heat or “coolness.” Some basic elements of a passive solar system are south-facing glazing for solar collection and thermal mass for absorption, storage and distribution.

Photovoltaic Cells – photovoltaic cells are used to directly convert solar radiation into electricity. Materials called semiconductors, usually made from pure silicon, transfer light energy (photons) into electrical energy in a process known as the photoelectric effect.

Post-Consumer Content – Refers to the percentage of total content of a material that has passed through its end-usage as a consumer item and has been recovered or diverted from the solid waste stream for the purpose of recycling.

Pyrolysis – Decomposition of a chemical by extreme heat.

Quad – a quadrillion BTUs (1015 BTUs). This is an enormous number equivalent to 3.6 x 106 metric tons of coal, or 172,000,000 (1.72 x 106) barrels of oil. A quadrillion is the number one followed by 15 zeros. It would be impossible to count to such a number even if you counted by 1,000s for every second of your life until you were 100 years old. The United States used about 80 quads of energy in 1990.

R-Value – A measure of thermal resistance, indicating how effective a material is as an insulator. R-value is measured in the hours needed for one Btu to flow through one inch of the material when the temperature difference (from one side of the material to the other) is one degree Fahrenheit. Its units are hour-square foot-degree Fahrenheit/Btu-inch.

Radiant Heat – Heat transferred in the form of light energy (including non-visible spectra). Distinct from conductive heat, occurring with the direct contact between two materials.

Radon – A colorless naturally occurring, radioactive, inert gas formed by radioactive decay of radium atoms in soil or rocks. Design strategies help reduce the amount of radon infiltration into a building and remove the gas that does infiltrate. 1

Rainwater Catchment/Harvest – On-site rainwater harvest and storage systems used to offset potable water needs for a building and/or landscape. Systems can take a variety of forms, but usually consist of a surface for collecting precipitation (roof or other impervious surface) and a storage system. Depending on the end use, a variety of filtration and purification systems may also be employed.

Raw Material – the original material as taken from its source, usually the ground. A good example is bauxite ore that is used to make aluminum.

Recycle – to recycle is to put into the cycle again. In other words, to take a product and reuse it when discarded. Recycling saves enormous amounts of energy and raw materials.

Recyclability – Technically, any material can be recycled if the amount of energy or money consumed in the process is not a factor. For our purposes, recyclability means any material that is: in demand, requires less energy to remanufacture than to manufacture from virgin materials and the technology exists to remanufacture it.

Recycled Content The percentage of the total content of a material comprised of post-consumer content and pre-consumer content.

Regional Manufacture – Goods produced within a certain radius of the project site. Using regionally produced goods is considered a sustainable building strategy in that it reduces the transportation impacts associated with the product, it often allows for a better understanding of the production process and increases the likelihood that the product was manufactured in accordance with environmental laws, and it supports regional economies.

Resource – A substance for which there is an identifiable use within society.

Reuse – Using a product or component of waste in its original form more than once; e.g., refilling a glass bottle that has been returned or using a coffee can to hold nuts and bolts. Reuse is a sustainable building strategy in that it reduces the strain on both renewable and nonrenewable resources.

Sick Building Syndrome – Building whose occupants experience acute health and/or comfort effects that appear to be linked to time spent therein, but where no specific illness or cause can be identified. Complaints may be localized in a particular room or zone, or may spread throughout the building.

Solar Energy – energy derived from the sun’s radiation. The amount of energy falling on just 0.3861 sq. mil/1 sq. km is about 4,000 megawatts, enough to heat and light a small town. In one second the Sun gives off 13 million times more energy than all the electricity used in the U.S. in one year. Solar heaters have industrial or domestic uses. They usually consist of a black (heat-absorbing) panel containing pipes through which air or water, heated by the sun, is circulated, either by thermal convection or by a pump. Solar energy may also be harnessed indirectly using solar cells (photovoltaic cells) made of panels of semiconductor material (usually silicon), which generate electricity when illuminated by sunlight.

Solar Panels – General term for an assembly of photovoltaic modules. See photovoltaic. Use of solar panels is a sustainable building strategy in that it lessens a building’s reliance on nonrenewable sources of power distributed through the grid system.

Stack Effect – Air that moves upward because it is warmer than the ambient atmosphere.

Stranded Cost – costs that were incurred by utilities to serve their customers with the understanding that state regulatory commissions would allow the costs to be recovered through electric rates. Stranded costs can occur either because particular customers discontinue their use of a service or because such customers are no longer willing to pay the full costs incurred to provide a service. Potentially stranded costs are the result of decisions that were reviewed and approved by government regulators and were made by utilities under the unique regulatory compact with their state and their customers.

Structural Insulated Panel (SIP) – Manufactured panels consisting of a sandwich of rigid insulation between two layers of engineered wood paneling. SIPs can be used for walls, roof, or flooring, and result in a highly insulative structure that is very resistant to air infiltration.

Sulfur Dioxide (SO2) – a corrosive gas produced both by nature and technology in nearly equal amounts. Burning fuels, such as coal and oil, that contain sulfur produces SO2. It is also produced from sea spray, organic decomposition and volcanic eruptions. When combined with water in the air, it produces a weak, corrosive sulfuric acid — an ingredient of “acid rain.”

Sustainable – The Worldwatch Institute defines “sustainable” as “meeting present needs without compromising the ability of future generations to meet their own needs.” For example, wood harvested from a “sustainable forest” means that the wood is derived from a forest managed in a manner so that the trees harvested will be replaced at a rate that matches the rate of removal. In this regard, the forest will continue producing wood for successive generations.

Thermal Mass – A mass (often stone, concrete, or brick) used to store heat and reduce temperature fluctuation in a space, by releasing heat slowly over time.

Toxins – Chemical or natural substances that can cause harmful effects on humans; toxins include heavy metals such as cadmium, lead, and mercury, as well as organic compounds like petroleum products, polychlorinated biphenyls (PCBs), and polynuclear aromatic hydrocarbons (PAHs)

Trombe Wall – Thermal storage system used in passive solar design. A high-mass wall that stores heat from solar gain during the day and slowly radiates the heat back into the living space at night.

U-value – U-value is the overall coefficient of heat transmission. It is a measure of the rate of heat flow through any given combination of materials, air layers, and air spaces. It is equal to the reciprocal of the sum of all resistances (R). In other words, the U-value can be calculated for a particular wall, roof, or floor system by finding the resistances (R-value) of each of its materials, its air layers, and its internal air spaces, then adding all of these resistances and finding the reciprocal. The lower the U-value, the lower the heat loss or the higher the insulating value. The units are Btu/hour/square foot/degree Fahrenheit.

Vapor retarder – a material or coating, impermeable to moisture, designed to impede passage of water or water vapor.

Ventilation – Process by which outside air is conveyed to an indoor space.

Ventilation Control (by Occupants) – The ability of building occupants to control ventilation rates. A strategy for giving control of comfort back to occupants, this can be achieved through access to individual electronic controls or by operable windows in workspaces. Studies show that giving increased control to occupants over their environment results in greater occupant tolerance for variability in the indoor environment.

Ventilation Rate: The rate at which indoor air enters and leaves a building. Expressed as the number of changes of outdoor air per unit of time: air changes per hour (ACH), or the rate at which a volume of outdoor air enters in cubic feet per minute (CFM).

Volatile organic compounds (VOCs) – VOCs are chemical compounds common in many building products: solvents in paints and other coatings; wood preservatives; strippers and household cleaners; adhesives in particleboard, fiberboard, and some plywoods; and foam insulation. When released, VOCs can contribute to the formation of smog and can cause respiratory tract problems, headaches, eye irritations, nausea, damage to the liver, kidneys, and central nervous system, and possibly cancer.

Wastewater – The spent or used water from a home, community, farm, or industry that contains dissolved or suspended matter.

Watt – a unit of power defined as a joule of energy per second.

Worm Bin – System for on-site management of food scraps and other organic materials. Similar to a compost bin, a worm bin uses worms to digest organic wastes, in a process known as “vermicomposting”.


lighting glossary

A process by which incident radiant flux is converted to another form of energy, usually (and ultimately) heat.

accent light
Directional lighting designed to emphasize a particular object or to draw attention to a part of the field of view.

The process by which the eye changes focusfrom one distance to another.

The process by which the eye becomes accustomed to varying quantities of light or to light of a different color.

The vertical angular distance of a point in the sky above the horizon. Altitude is measured positively from the horizon to the zenith, from 0 to 90 degrees.

ambient light
Electric and/or natural lighting throughout a space that produces uniform general illumination.

artificial sky
An enclosure that simulates the luminance distribution of a real sky for the purpose of testing physical daylighting models. See hemispherical dome artificial sky and mirror-box artificial sky.

The horizontal angular distance between the vertical plane containing a point in the sky and true south.

A magnetic or electronic device used to control the starting and operation of discharge lamps.

ballast factor
The ratio of lamp lumen output on a particular ballast as compared to that lamp’s (lamps’) rated lumen output on a reference ballast under ANSI test conditions (free, unmoving air at 25° C)

beam component
That component of flux received directly (or by specular reflection or transmission) from a point source (such as the sun or small lamp). It is a direct component.

blinding glare
Glare that is so intense that, for an appreciable length of time after it has been removed, no object can be seen.

The glass outer envelope component of a lamp.

candela (cd)
The SI unit of luminous intensity (formerly called the candle). One candela equals one lumen per steradian-the luminous intensity, in a give direction, of a source that emits monochromatic radiation at a frequency of 540E12 hertz and of which the radiant intensity in that direction is 1/683 watts per steradian.

Refers to the dominant or complementary wavelength and purity aspects of the color taken together, or of the aspects specified by the chromaticity coordinates of the color taken together.

That part of a building rising clear of the roofs or other parts, whose walls contain windows for lighting the interior.

coefficient of utilization (CU)
The ratio of lumens from a luminaire received on the work plane to the total quantity of lumens emitted by the lamps of that luminaire.

color rendering index
A measurement of the amount of color shift that objects undergo when lighted by a light source as compared with the color of those same objects when seen under a reference light source of comparable color temperature. CRI values generally range from 0 to 100.

color temperature
The absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source (see correlated color temperature).

A retinal receptor that dominates the retinal response when the luminance level is high and provides the basis for the perception of color.

The ratio of the luminance of an object to that of its immediate background.

cosine law
The law that the illuminance on any surface varies as the cosine of the angle of incidence. (The angle of incidence is the angle between the normal to the surface and the direction of the incident light.)

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).

dark adaptation
The process by which the retina becomes adapted to a luminance of less than 0.01 footlamberts.

daylight factor (DF)
The ratio of daylight illumination at a given point on a given plane due to the light received directly or indirectly from a sky of assumed or known luminance distribution, to the illumination on a horizontal plane due to an unobstructed hemisphere of this sky, expressed as a percentage. Direct sunlight is excluded for both values of illumination. The daylight factor is the sum of the sky component, the external reflected component, and the internal reflected component. The interior plane is usually horizontal. If the sky condition is the C.I.E. standard overcast condition, then the DF will remain constant, regardless of absolute exterior illuminance. If used in conjunction with other than standard overcast conditions, the sky conditions should be specified. The term is also informally applied to the ratio of horizontal interior to exterior illuminance in the fenestration plane; under clear sky conditions, the DF remains constant only if the fenestration is completely diffusing (such as an ideal opalescent glass).

diffusing (surface)
Those surfaces and glazing that redistribute some of the incident flux by scattering in all directions.

disability glare
Glare resulting in reduced visual performance and visibility . Often accompanied by discomfort glare.

discomfort glare
Glare producing discomfort. Does not necessarily interfere with visual performance or visibility.

A measure of the luminous efficiency of a radiant flux, expressed in lumens per watt as the quotient of the total luminous flux by the total radiant flux. For daylighting, this is the quotient of visible flux incident on a surface to radiant flux on that surface. For electric sources, this is the quotient of the total luminous flux emitted by the total lamp power input.

The ratio of radiance (for directional emissivity) or radiant exitance (for hemispherical emissivity) of an element of surface on a temperature radiator to that of a blackbody at the same temperature. By Kirchoff’s Law, for a given wavelength of the electromagnetic spectrum, emissivity of a surface equals its absorptivity (and is the reciprocal of its reflectivity).

The density of light reflecting from a surface at a point, measured in lumens per square foot (formerly “footlamberts”). It is determined by multiplying the footcandles striking a diffuse reflecting surface times the reflectance of that surface.

Any opening or arrangement of openings (normally filled with glazing media) for the admission of daylight, including any devices in the immediate proximity of the opening that affect distribution (such as baffles, louvers, draperies, overhangs, light-shelves, jambs, sills, and other light-diffusing materials).

Informal substitute term for luminaire.

A discharge lamp in which a phosphor coating transforms ultraviolet energy into visible light.

The time rate of flow. For example, volume per hour is the flux of a fluid.

A standard measurement of illuminance, representing the amount of illuminance on a surface one foot square on which there is a uniformly distributed flux of one lumen.

footlambert (fl)
A unit of luminance equal to 0.3183010 candela per square foot, or to the uniform luminance of a perfectly diffusing surface emitting or reflecting light at a rate of one lumen per square foot, or to the average luminance of a surface emitting or reflecting light at that rate. An unobstructed sky of one footlambert uniform luminance contributes one footcandle of illuminance on a horizontal plane.

A small region at the center of the retina, subtending about two degrees and forming the site of the most distinct vision and greatest color discrimination.

See direct glare, disability glare, discomfort glare, reflected glare.

glare index
A method of predicting the presence of discomfort glare due to daylighting. Factors affecting the glare index include the size and relative position of fenestration, sky luminance, and interior luminance. Most widely used in Europe, the glare index is similar to the index of sensation and the discomfort glare rating, which are used in North America for electric lighting applications.

The attribute of a color that allows it to be classified as red, yellow, blue, and so on.

International Association of Lighting Designers

Illuminating Engineering Society of North America

The density of incident luminous flux on a surface; illuminance is the standard metric for lighting levels, and is measured in lux (lx) or footcandles (fc).

The emission of visible electromagnetic radiation due to the thermal excitation of atoms or molecules.

indirect sources
Surfaces which, after being illuminated by other sources (direct sources such as the sun, sky, or electric light, or other indirect sources), have measurable luminance and, in turn, become sources themselves.

infrared radiation
Radiation with wavelengths too long to be perceived by the human eye (that is, longer than 0.77 microns) and less than 1,000 microns. Room IR is infrared radiation in the 7.7-8.0 micron region and typical of that radiated from surfaces near room temperature.

inverse-square law
The law stating that the illuminance at a point on a surface varies directly with the intensity of a point source, and inversely as the square of the distance between that source and that surface.

irradiance (E)
The density of radiant flux incident on a surface.

isolux (isofootcandle) line
A line plotted on any appropriate set of coordinates to show all the points on a surface where the daylight illuminance is the same. A series of such lines for various illuminance values is called an isolux (isofootcandle) diagram.

An electrically energized source of light, commonly called a bulb or tube.

lamp lumen depreciation
The decrease over time of lamp lumen output, caused by bulb wall blackening, phosphor exhaustion, filament depreciation and other factors.

Radiant energy that is capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum (light) extends from about 0.38 to 0.77 microns.

light loss factor (LLF)
A factor used in calculating the illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, lamp depreciation (of electric luminaries), dirt accumulation on luminaire and room surfaces, maintenance procedures and atmosphere conditions. Formerly called maintenance factor.

light shelf
A horizontal shelf positioned (usually above eye level) to reflect daylight onto the ceiling and to shield direct flare from the sky.

The quantity of luminous flux emitted within a unit solid angle (one steradian) by a point source with one candella intensity in all directions.

lumen method
A method of estimating the interior illuminance due to window daylighting at three locations within a room. Based on empirical studies, the use of this method is primarily limited to North America.

A complete lighting unit, consisting of a lamp or lamps together with the components required to distribute the light, position the lamps, and connect the lamps to a power supply. Often referred to as a “fixture.”

luminaire dirt depreciation
A multiplier used in lighting calculations to account for the reduction in illuminance produced by the accumulation of dirt on a luminaire.

The luminous intensity of a surface in a given direction per unit area of that surface as viewed from that direction; often incorrectly referred to as “brightness.”

lux (lx)
The SI unit of illuminance equal to one lumen per square meter.

matte surface
Surface from which the reflection is predominantly diffuse, with or without a negligible specular component.

A raised section of roof that includes a vertically (or near-vertically) glazed aperture for the purpose of daylighting illumination.

near infrared (solar infrared)
The region of the electromagnetic spectrum between 0.77 to 1.4 microns. Most of the infrared solar radiation falls into this region. This near infrared (or solar IR) region is transmitted, absorbed, and reflected in a similar manner to visible light by most glazing and nonmetallic building materials.

overcast sky
A sky luminance distribution three times brighter near (C.I.E. Standard Condition) the zenith than at the horizon, as defined by a formula proposed by Moon and Spencer in 1942 and adopted by the Commission International de l’Eclairge in 1955

A horizontal building projection, usually above a window, for the purpose of shading.

A device that measures the amount of incident light present in a space.

peripheral vision
The seeing of objects displaced from the primary line of site and outside of the central visual field.

point method
A method of estimating the illuminance at various locations in a building using photometric data.

The opening in the iris of the eye that admits light.

radiant energy (radiation)
Energy traveling in the form of electromagnetic waves. Measured in units of energy such as joules, ergs, or kilowatt-hours.

The process by which the direction of light changes as it passes obliquely from one medium to another in which its speed is different.

The ratio of reflected flux to incident flux.

A light-sensitive membrane lining the posterior part of the inside of the eye.

Retinal receptors that respond to low levels of luminance but cannot distinguish hues. Not present in the center of the fovea region.

room cavity ratio
In lighting calculations, a measure of room proportion as determined by dimensions of length, width, and height.

A relatively horizontal glazed roof aperture for the admission of daylight.

specular angle
The angle of mirror reflection (angle of incidence equals angle of reflectance).

specular reflection
The process by which incident light is redirected at the specular (mirror) angle.

specular transmission
The process by which incident flux passes through a surface or medium without scattering.

task light
Light that is directed to a specific surface or area to provide illumination for visual tasks.

ultraviolet radiation (uv)
Any radiant energy within the wavelength range of 0.001 to 0.38 microns.

veiling reflection
Specular reflection superimposed upon diffuse reflection from an object that partially or totally obscures the details to be seen by reducing the contrast. Controlled by distributing the source over a larger area, relocating the source out of the reflected field of view, changing the task surface specular reflectance or tilt, or relocating the observer.

visual acuity
A measure of the ability to distinguish fine details.

visual comfort probability (VCP)
The rating of a lighting system expressed as a percentage of the people who, when viewing from a specified location and in a specified direction, will be expected to find it comfortable in terms of discomfort glare.


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