For a more technical and comprehensive list of lighting definitions, visit Nomenclature and Definitions for Illuminating Engineering


A general term for the process by which incident flux is converted to another form of energy, usually and ultimately to heat. (Note: All of the incident flux is accounted for by the processes of reflection, transmission, and absorption.)


This act of focusing the cornea is called accommodation. Accommodation is always a response to an image of the target located on or near the fovea rather than in the periphery. It is used to bring a defocused image into focus or to change focus from one target to another at a different distance. Any condition, either physical or physiological, that handicaps the fovea, such as a low light level, will adversely affect accommodative ability.


The process by which the all or part of the retina becomes accustomed to more or less light than it was exposed to during an immediately preceding period. It results in a change in the sensitivity to light.

Ballast Efficiency Factor (BEF)

The BEF metric is used solely to show compliance with US ballast efficacy regulations. It should not be used as a ballast specification criterion. The BEF is not a true measure of ballast efficiency as its value depends on the following factors:

  • Quantity of lamps operated
  • Type of gas fill in the lamp
  • Lamp tube diameter/size
  • Lamp operating frequency

The BEF does not indicate absolute light level. In general, BEF values are not particularly useful to the specifier even though some ballast manufacturers provide BEF data in their catalogs. The best method of comparing lamp‐ballast systems is by their system efficacy. The U.S. Department of Energy (DOE) issued a final rule amending the existing test procedures for fluorescent lamp ballasts and establishing a new test procedure. The amendments to update a reference to an industry test procedure. The new test procedure changes the efficiency metric to ballast luminous efficiency (BLE), which is measured directly using electrical measurements instead of the photometric measurements employed in the previous test procedure. The calculation of BLE includes a correction factor to account for the reduced lighting efficacy of low frequency lamp operation. The test procedure specifies use of a fluorescent lamp load during testing, allowing ballasts to operate closer to their optimal design points and providing a better descriptor of real ballast performance compared to resistor loads.

Ballast Factor (BF)

It is a relative measure of the light output from a particular lamp‐ballast system and is a characteristic of such a system‐not just of the ballast alone. Ballasts that can operate more than one type of lamp will generally have a different ballast factor for each combination. Ballast factor is the ratio of a lamp’s light output on a given commercial ballast, compared to the lamp’s rated light output as measured on a reference ballast under ANSI test conditions. Ballasts are available with either normal (conforming to ANSI specifications), low or high ballast factors. However, ballast factor is not a measure of energy efficiency. Although a lower ballast factor reduces lamp lumen output, the lamp may also consume proportionally less input power. As such, the careful selection of a lamp‐ballast system with 705 a specific ballast factor allows designers to better minimize energy use by “tuning” the lighting levels in the space. Ballast factor should not be confused with a similar‐sounding metric called the ballast efficacy factor (BEF). The BEF, defined as the relative light output of a particular lamp‐ballast combination under ANSI test conditions (this site is the ballast factor times 100 percent) divided by the measured input power in watts, serves as a relative measure of system efficacy and is used to compliance with federal regulations.


Attribute of a visual sensation according to which an area appears to emit more or less light.

Candela (cd)

The SI unit (metric International System of Units) of luminous intensity. One candela is one lumen per steradian. Note: The fundamental luminous intensity definition in the SI is the candela.


Correlated color temperature indicates the relative color appearance of a white light source, from yellowish- white or “warm” (2700-3000 K) to bluish-white or “cool” (5000+ K).

Ceiling Ratio

The ratio of the luminous flux that reaches the ceiling directly to the upward component from the luminaire.

Chromatic Adaptation

The process by which the chromatic properties of the visual system are modified by the observation of stimuli of various chromaticity’s and luminance’s.

Coefficient of Utilization (CU)

The ratio of luminous flux (lumens) calculated as received on the work plane to the total luminous flux (lumens) emitted by the lamps alone.

Color Appearance

Color appearance is dependent upon the state of chromatic adaptation, the geometric context for the object being viewed, including the background and surrounding surfaces, the absolute luminance levels within the field of view, and other aspects of the optical radiation stimulus and the cognitive attributes of the observer.

Color Rendering Index (CRI)

Measure of the degree of color shift a defined set of objects undergo when comparing their color illuminated by the light source to the color of those same objects when illuminated by a reference source of comparable color temperature.

Color Temperature

The absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source. A blackbody radiator is a an object which, when heated, emits a spectrum of radiant power. Radiant power from a practical source, particularly from an incandescent lamp, is often described by comparison with that from a blackbody radiator.

Compact Fluorescent

The compact fluorescent lamp (CFL) family compromises a wide variety of multi-tube, single-based lamps. Initially designed to physically replace conventional 25 to 100-watt standard incandescent lamps, CFLs conserve up to 75% energy, provide 8-12 times longer lamp life, and approach the color of standard incandescent lamps. Today’s CFL designs include wattages and colors which can replace conventional fluorescent lamps in size-reduced luminaires. Examples include the 32-watt and 42-watt triple-tube lamps, which are available in correlated color temperatures ranging from 2700 K to 6500 K. Some CFLs are manufactured with the lamp and ballast as an integral unit with a (medium) screw base. Others are manufactured without a ballast and are available in 2-pin or 4-pin configurations. Only 4-pin versions are dimmable. Generally the 4-pin versions are used with electronic ballasts (either dimmable or on/off versions). However, 2-pin lamps can also be used on electronic ballasts designed especially for them. Both CFL types plug into appropriate lamp holders that are used in luminaires, and can also be inserted into adapter ballasts, which generally come with a medium screw base for insertion into a standard incandescent socket. Many integrally ballasted and amalgam type plug-in CFLs require a warm-up time when initially energized.


Codes and related documents might seek to control total installed lighting power density, lighting energy use, lighting system component efficiencies, or some combination of these. Some documents might also mandate certain control types for certain space types, such as occupancy sensors, multilevel switching, or daylight harvesting.


Retinal receptors that dominate the retinal response when the luminance level is high and provide the basis for the perception of color.

Correlated Color Temperature (CCT)

The absolute temperature of a blackbody radiator whose chromaticity most nearly resembles that of the light source. CCT is the nearest visual match of source and blackbody chromaticity.

Cosine law

A law stating 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. The inverse-square law and the cosine law can be combined as E = (I cos θ)/d.

Cosine-cubed law

An extension of the cosine law in which the distance d between the source and surface is replaced by h/cos θ , where h is the perpendicular distance of the source from the plane in which the point is located. It is expressed by E =(I cos3 θ)/h.

Daylight Harvesting

Daylight harvesting is a broadly used term that describes the application of energy saving daylighting strategies to maintain target interior task level illuminances.


Daylighting refers to the art and practice of admitting beam sunlight, diffuse sky light, and reflected light from exterior surfaces into a building to contribute to lighting requirements and energy savings through electric lighting controls.

A view of the outdoors is believed to be important for human psychological and physiological reasons. While daylight can be used to help light a space, extra care should be taken in industrial environments to control the quantity and distribution of the light and its associated heat gain. It should be noted that more illuminance is sometimes needed on interior surfaces near windows to reduce the contrasts between those surfaces and the windows.

Daylight’s dynamic nature makes it a complex light source. The sun’s continuous apparent movement, coupled with changes in atmospheric conditions, causes the solar beam and sky dome luminance distribution to vary in intensity and spectral content. Successful daylighting requires balancing the daylight distribution in the space throughout the entire year, providing sufficient, but not excessive, daylight illumination levels for space activities, while minimizing glare. It also optimizes the building envelope for the geographic location and climate to maximize energy savings from both lighting and HVAC systems.

Direct sunlight can be extremely bright, with intensity of up to 3×1027candela and luminance of about 1.6×109 cd/m². Interior illuminance levels are typically only a few to several percent of outdoor illuminance levels in daytime. The much higher intensities of direct sunlight, when introduced into interior spaces, can produce painful glare and other unwanted effects. Under clear skies, direct sunlight requires very careful management, usually being blocked, diffused, or very carefully transmitted into interior spaces through thoughtful architectural designs. Sunlight generates very high exterior illuminance levels, ranging by solar incident angle from 20,000 lux (about 2,000 fc) for high incident angles, to 100,000 lux (about 10,000 fc) at normal incident.

Daylighting and Controls

For a daylighting system to save energy, daylight must replace electric lighting during daylight hours. This is accomplished by either switching or dimming the electric lighting. Occupant control can provide some savings when flexible personal control is provided through multi-level switching, zoned switching or dimming; however occupants are not focused on minimizing electric lighting energy as daylight conditions change.

Personal control is likely to save energy when the occupant is forced to select an appropriate output setting upon entering a space, rather than have all lights turn on through a single switch. An automatic lighting control system guarantees that lighting energy savings will occur when daylight is present. A photosensor signal can be used to adjust the electric lighting by monitoring either exterior daylight levels, the amount of daylight passing through an aperture, or the combined daylight and electric light within a space.

For proper operation, a photosensor control system must be properly designed and calibrated. This includes establishing a controlled lighting zone that correlates with the daylit area, then selecting, locating, and calibrating the photosensor to accurately sense daylight levels and dim or switch the electric lighting system accordingly. The daylit zone should receive levels of daylight that generate significant savings in electric lighting energy. Higher daylight levels are required with photosensor-based switching as compared to dimming. Occupant overrides are important for achieving user satisfaction, with some advanced systems having the capability of adjusting control based on users’ preferences.

Daylighting Control Commissioning

The commissioning process for a photosensor-based dimming control system typically involves calibrating the control algorithm under a representative daylight condition. Depending on the control algorithm applied, it may also require a nighttime signal. For dimming control, daytime calibration involves placing an illuminance meter at the critical task location, and then the dimmed zone output is adjusted to produce the desired illuminance at this point. Other task locations should then be checked to determine if they require a higher setting from the controlled lighting zone.

The daylight condition used for calibration should meet the following conditions:

The daylight condition should provide a dimmed zone setting slightly above the minimum dimming level. It may be necessary to adjust blinds or shades on the windows to achieve an appropriate daylight level.

The daylight condition selected should provide a relatively high S/E ratio. If horizontal blinds are applied, they should be angled to provide a high photosensor signal (which may be based on the ceiling illuminance at the photosensor) in relation to the critical point illuminances. Calibration under overcast sky conditions should be avoided with side lighting applications since these usually provide low S/E ratios that result in under lit environments under other sky conditions. (S/E being the ratio of the electric lighting and daylighting levels. If the controls aren’t calibrated with a proper S/E ratio when the electric lighting goes off the controls will read too dark of an level and turn them on, when it will read there is now too much light in the space so the lights will go off.)


The introduction of high brightness, low gloss LCD screens that are less susceptible to reflected glare has led to resurgence in the popularity of lensed luminaires. Their higher efficiencies and higher angle distributions can produce brighter, more open visual impressions.

A luminaire with a diffuser spreads the light from its lamps evenly over the diffuser surface. Since the diffuser area is much larger than the area of the lamps, the average luminance (compared to using bare lamps) is reduced. However, when diffusers are used, the luminance at all viewing angles near horizontal may be as high as looking straight up at the luminaire. In a large office, this will result in low Visual Comfort Probability (VCP) as well as possible reflections in specular VDT screens, reducing visibility. In small offices, diffusers may be appropriate if high luminance areas are outside a worker’s peripheral vision, their broad distribution does not create excessive wall luminance, and the partitions are full height and opaque.


Continuous dimming is achieved by reducing the lamp current. Most commercially available dimming ballasts are electronic, though magnetic dimming ballasts may still be encountered in existing construction. The dimming ballast must be able to communicate with the connected control devices, which forms the basis for a controls protocol. Control protocols can be either analog or digital. For more detailed information, refer to IES TM-23-11 Lighting Control Protocols.

Stepped dimming can be achieved in one of two ways: 1. by switching off one or more lamps in a multi-lamp lamp luminaires; 2. with stepped-dim ballasts.

Provided there are enough steps, stepped dimming can approach continuous dimming in final effect.

Direct Component

That portion of the light from a luminaire that arrives at the work-plane without being reflected by room surfaces.

Direct glare

Is glare resulting from high luminances or insufficiently shielded light sources in the field of view. It usually is associated with bright areas, such as luminaires, ceilings, and windows that are outside the visual task or region being viewed. A direct glare source may also affect performance by distracting attention.

Direct Ratio

The ratio of the luminous flux that reaches the floor of a room cavity directly to the downward component from the luminaire.

Disability Glare

Is caused by a veiling luminance superimposed on the retinal image within the eye, which reduces visual performance or visibility, and is often accompanied by discomfort. Reducing illuminance at workers’ eyes and/or raising the source of the disability glare can alleviate the problem.

Discomfort Glare

Produces visual discomfort without necessarily interfering with visual performance or visibility. It occurs when luminous objects (or reflections of luminous objects) have significantly higher luminance than the balance of the person’s field of view. Size, luminance and angular displacement from the line of sight are all factors. Even a source that is directly overhead, if bright enough, can cause discomfort glare.

Discomfort Glare Rating (DGR)

A numerical assessment of the capacity of a number of sources of luminance, such as luminaires, in a given visual environment for producing discomfort. It is the net effect of the individual values of index of sensation M, for all luminous areas in the field of view.


Disposal of luminaires varies based on the specific components within the luminaire. Federal and state laws govern disposal of these products, (such as ballasts, fluorescent lamps, HID lamps, and batteries) following federal regulations such as: the Resource Conservation and Recovery Act (RCRA); the Toxic Substance Control Act (TSCA); the Comprehensive Environmental Response, and Compensation and Liability Act (CERCLA, or Superfund); and the Universal Waste Rule (UWR). While many federal regulations pre-empt state laws, in the case of the Universal Waste Rule, the federal guidelines are a baseline for states to adopt or exceed. Some states have enacted standards that are more stringent than these federal rules and regulations. And for some products, many states have passed their own laws that are broad enough to encompass future technologies like LED. All mercury containing lighting products are regulated by the U.S. EPA/UWR, including lamps that contain mercury, cadmium, or lead. The US EPA hosts a municipal solid waste state data website that has information on state regulations and contacts.

Downward Component

That portion of the luminous flux from a luminaire that is emitted at angles below the horizontal.


The cost of the lighting system is, of course, an important area to be considered. There are direct costs related to the actual cost of the installation and operation of the lighting system which must be carefully weighed and balanced against factors related to quality of the lighting for a given application. Economic analysis gives insight into the question of when a lighting system under consideration will “pay off.” It can help the lighting designer make decisions regarding energy conservation. Most importantly, it provides methods for gauging the profitability of a capital investment in a lighting system. Many metrics and techniques for answering these questions have been proposed over the years. These methods can be classified into two categories: first-level analysis methods, and second-level analysis methods.

First-level methods (such as simple payback) are attractive due to their simplicity and can be used for quick estimates involving short payback periods. Second-level analysis allows the comparison of all economic events in the life of a lighting system (including initial cost, maintenance, energy cost, and salvage value). These factors are converted into their value today, or present value using the principle of time equivalence. The benefits and savings are totaled and compared with the sum of the costs and disadvantages. If the first sum is greater, the system should be purchased. If the second is greater, it would be unprofitable to purchase the system. Of the second-level methods, Life cycle Cost/ Benefit Analysis (LCCBA) has emerged as the most robust method (is only robust if our information on the lesser known factors such as employee satisfaction and productivity is complete), and the one that is accepted by experts in managerial economics from all industries.

Accordingly, LCCBA is the economic analysis method recommended by IES.

Electrodeless Lamps (Induction)

Induction fluorescent lamps are basically low pressure gas discharge fluorescent lamps that operate without the need of electrodes. Because there are no electrodes to fail, the lamps have lifetimes of up to 100,000 hours but this depends on thermal management and removal of heat from the generator or driver.

Emergency Lighting

Emergency lighting helps ensure the safety of a building’s occupants when the normal lighting system fails. The emergency lighting system should permit orderly, accident free building egress during an emergency situation. For those people remaining inside, the emergency lighting should provide a feeling of safety and comfort until the general lighting is restored.

The emergency electric lighting systems in present use are:

Separate luminaires and wiring powered by a standby or emergency generator with an independent driving source (e.g., a diesel engine, etc.). This system provides backup power to the normal lighting system and may provide the necessary illumination if the normal lighting fails.

Common luminaires supplied by the regular utility and by a secondary source that is activated automatically when the primary source fails. The secondary source may be centrally located batteries or inverter equipment that are automatically recharged after use, or it may be an on-premises auxiliary generator that provides the necessary power within ten seconds after the primary source fails.

Unit equipment supplied by the same regular utility branch circuits that serve other lighting in the spaces being protected. Each unit is activated automatically and independently when the utility or building electrical systems fails because each has its own battery. These batteries are recharged after power is restored.

Each state or jurisdiction is responsible for creating and adopting its own building code. The National Fire Protection Association (NFPA) develops model codes that jurisdictions typically adopt, with or without amendments. NFPA 70, the National Electrical Code (NEC), establishes minimum standards for the installation of lighting-related equipment and electrical safety as well as power for emergency egress lighting. NFPA 101, the Life Safety Code, defines allowable paths of egress and minimum lighting during egress conditions.


The ratio of radiance in a given direction (for directional emittance) or radiant exitance (for hemispherical emittance) of a sample of a thermal radiator to that of a blackbody radiator at the same temperature.

Energy Management Checklist

The checklist is designed to be initialed and dated by the designer and client to signify subject review and agreement regarding an energy-conscious lighting program.

Equivalent veiling luminance

The luminance of the reflected image of a bright surface that is superimposed on a test object to measure the veiling effect equivalent to that produced by stray light in the eye from a disability glare source. The disability glare source is turned off when the reflected image is turned on.

Exit Signs

Exit pathways and exit access doors must be clearly identified with approved exit signs easily visible from any direction of egress travel. Light sources may be inside or outside of signs, but they must operate for no less than 90 minutes during power interruptions. Even at locations where the exit is not clearly visible, the route to the exit must be identified in the same manner as the exit itself. Proper directional arrows must be provided on all route-to-exit signs. Exit sign placement shall be such that no point in the egress pathway is more than 100 feet from the nearest visible exit sign. Exit signs should be located and lighted so they are visible during the presence of smoke. Each state or jurisdiction is responsible for creating an adopting its own building code. NFPA 101, the Life Safety Code, defines placement and visibility of exit signs.


Exitance is the luminous flux density leaving a surface at a point, expressed in lumens per unit area (lumens per square foot or meter). It can be related to how luminous the emitting surface is or how “bright” it appears.

Externally Reflected Daylight

While the sun and sky are the primary sources of daylight, externally reflected light from the ground and adjacent structures or objects also contributes luminous flux to daylight apertures. For a vertical window on a flat site, the ground encompasses the lower half of the field of view. Like skylight, ground light is usually diffuse, with its luminance a function of the ground reflectance, the sky conditions, and shadowing and reflections provided by surrounding objects.

Light reflected from the ground provides an important daylight contribution, since it is directed through vertical apertures to the ceiling and walls. The fraction of the total incident daylight on a vertical facade that arrives from the ground can range from below 10% to as high as 70-80% at a ground reflectance of 20%. The lowest fractions occur when direct sunlight strikes the facade, while the highest occur on a facade facing away from the sun on a clear day, when the sky is deep blue and the ground is sunlit. Under an overcast sky, the ground contribution is generally around 20%.

Ground reflectance can vary significantly. Light-colored ground surfaces such as sand and snow will result in higher ground contributions.

Objects such as trees, neighboring buildings, and other portions of the same building can limit the view of the ground or sky seen from a daylight aperture. In these situations, daylight from portions of the sky or ground is replaced by light reflected from the obstructing object, which may either increase or decrease the daylight delivered to a building interior.

Eye Adaptation

The process by which all or part of the retina becomes accustomed to more or less light than it was exposed to during an immediately preceding period. It results in a change in the sensitivity to light. (Note: Adaptation is also used to refer to the final state of the process, as reaching a condition of adaptation to this or that level of luminance.)

Factors Related to Direct Cost of Light.

Those factors having a direct impact on estimates of the cost of light produced by any specific lighting system include:

  • Cost of luminaires
  • Lamp cost
  • Auxiliary equipment costs
  • Labor costs (of installation)
  • Luminous efficacy
  • Cost of electricity, include use and demand charges
  • Efficiency of auxiliary equipment
  • Useful life of lamps and auxiliary equipment
  • Replacement cost (labor plus materials)
  • Operating hours per year
  • Starting frequency
  • Cleaning scheduled
  • Maintenance program
  • Amortization rates
  • Interest rates
  • Taxes
  • Insurance
  • Environmental costs

Fluorescent Ballast

The ballast controls the starting of the lamp, the electrical conditions during operation (e.g. power factor, harmonics), and is a key component of system efficacy. The current limiting component of a ballast can be a resistor, capacitor, inductor (a.k.a. ‘choke’), or an electronic circuit.

High frequency electronic ballasts should be employed for new specifications because they have several important advantages over the magnetic types: improved lamp and system efficacy of approximately 10%, no flicker or stroboscopic effects, integrated starting circuitry, increased lamp life, excellent ability to regulate lamp lumen output, integrated power factor (PF) correction, quiet operation, comparatively light weight, many options for input voltage, and some can be used with direct current (DC).

Low power factor ballasts are more common with compact fluorescent systems than for 4-ft and 8-ft fluorescent systems. Some utilities require high power factor equipment or have established penalty clauses in their rate schedule for installations with low power factor.


A small region at the center of the retina, subtending about two degrees which contains cones but no rods, and forms the site of most distinct vision.

Full Wattage Linear T12 Lamps

Under the terms of the Energy Policy Act of 1992 (EPACT) many of these lamps can no longer be manufactured due to their relative low efficacy and/or poor color characteristics.


The sensation produced by luminance within the visual field that is sufficiently greater than that to which the eyes are adapted. Glare may cause annoyance, discomfort or loss of visual performance and visibility.

Halogen Lamps

A gas-filled tungsten filament incandescent lamp containing halogens or halogen compounds and utilizing the halogen regenerative cycle to prevent blackening of the lamp envelope during life.

Hanging (a.k.a. Pendant)

A luminaire that is hung from a ceiling by supports.

High Output Lamps

High output fluorescent lamps are a high-current rapid start lamp operating at approximately 800 milli-amperes (mA). This family of lamps is commonly applied where the standard lamp does not provide sufficient light output per lamp length. T12 (and newer T8) lamps are available in up to 2440 mm (96 in.) lengths and are also particularly suitable for outdoor applications. They use a recessed double contact base. T12 high output lamps are also affected by EPACT legislation. Reduced wattage versions of T12/HO lamps meet current legislative requirements. The newer T8/HO tri-phosphor lamps often retrofit T12/HO with a ballast change to save energy.

High Pressure Sodium (HPS) lamp

A high intensity discharge (HID) lamp in which light is produced by radiation from sodium vapor operating at a partial pressure about 1.33 x 104 Pa (100 Torr). Includes clear and diffuse-coated lamps.

High-Intensity Discharge (HID) Lamp

The term high-intensity discharge lamp describes a wide variety of light sources. The HID family includes high-pressure sodium, mercury, metal halide, and ceramic metal halide lamps. HID lamps are among the most efficacious light sources. They are characterized by compact size, long life, and full temperature range starting and operation. HID sources are normally designed with inner arc tubes, hard glass outer bulbs, and single ended screw bases or bi-pin bases with the more compact versions. The inner arc tube contains an arc discharge operating at a significantly higher pressure than fluorescent lamps. All HID sources must be operated with a current-limiting ballast.


Standardized testing procedures through IES LM-79, LM-80 and TM-21 make product quality more identifiable. Additionally, as with HPT8, voluntary testing and standards programs such as CALiPER, ENERGY STAR and DesignLights™ Consortium and the Municipal Solid State Lighting Consortium offer designers and consumers clearing houses for finding good quality products on everything from holiday light strings to downlights to pole mounted area lighting.


The areal density of the luminous flux incident at a point on a surface.

In the Ceiling or the Wall

A luminaire that is mounted above the ceiling (or behind a wall or other surface) with the opening of the luminaire level with the surface.

Incandescent filament lamp

A lamp in which light is produced by a filament heated to incandescence by an electric current. Note: Normally, the filament is of coiled or coiled coil (doubly coiled) tungsten wire. However, it may be uncoiled wire, a flat strip, or of material other than tungsten.

Indirect Component

That portion of the luminous flux from a luminaire that arrives at the work-plane after being reflected by room surfaces.

Initial Luminous Exitance

This term can be used in two different ways. In flux transfer it is the density of luminous flux leaving a surface within an enclosure before inter reflections occur

Note: For light sources this is the luminous exitance. For non-self-luminous surfaces it is the reflected luminous exitance of the flux received directly from sources within the enclosure or from daylight. In lighting calculations it is the total exitance at time zero before depreciation (light loss) occurs.

Interreflection (also called interflection)

The multiple reflection of light by the various room surfaces before it reaches the work-plane or other specified surface of a room.

Inverse-square law

A law stating that the illuminance E at a point on a surface varies directly with the intensity I of a point source, and inversely as the square of the distance d between the source and the point. If the surface at the point is normal to the direction of the incident light, the law is expressed by E = I/d.

Note: For sources of finite size having uniform luminance this gives results that are accurate within one percent when d is at least five times the maximum dimension of the source as viewed from the point on the surface. Even though practical interior luminaires do not have uniform luminance, this distance d is frequently used as the minimum for photometry of such luminaires, when the magnitude of the measurement error is not critical.

Lamps a.k.a. Light Bulbs

A generic term for a man-made source created to produce optical radiation. By extension, the term is also used to denote sources that radiate in regions of the spectrum adjacent to the visible. Note: Through popular usage, a portable luminaire consisting of a lamp with shade, reflector, enclosing globe, housing, or other accessories is also called a “lamp.” In such cases, in order to distinguish between the assembled unit and the light source within it, the latter is often called a “bulb” or “tube,” if it is electrically powered.


Light Emitting Diodes (LEDs) are solid state electronic devices for generating light. In recent years, the architectural lighting applications for these devices have seen rapid growth as improvements in luminous efficacy and chromaticity have made them into viable alternatives for some applications. Limitations in color rendering and light color appearance for “white” LEDs still restrict their full acceptance into point source applications where incandescent or low-voltage incandescent have previously been used. Issues with heat management and controls integration also require design consideration.

LED light engines have a typical rated life of 35,000 to 50,000 hours of operation at which point the light output of the LED will have decreased to 70% of its initial value. At the end of rated life, the LEDs will likely continue to operate for an extended period time with a continuing decline in light output. The rated life of LEDs can be affected by the quality of the electric power and the ambient temperature of their environment. Poor quality power can negatively affect the electronic components of the LED system. High ambient temperatures can make it difficult for the LEDs to shed heat to maintain appropriate operating criteria.

The lumen depreciation of LED light sources must be accounted for in the initial design. The design layout must over light the space based on the expected lumen depreciation or the luminaires must either be dimmed or maintain a constant light output with increasing power consumption over the life of the system. Successful dimming of LEDs requires careful coordination between the LED vendors and the controls vendors to match up the required operational conditions. The increased power consumption of constant output LEDs can be significantly higher than the initial power consumption and significantly impact the lighting power density of the project.


A luminaire with a lens may incorporate a series of small prisms that control the photometric distribution of the light to reduce the luminance of the luminaire in the near-horizontal viewing angles between 45° and 90° from vertical. Depending on the exact characteristics of a specific lens, glare from the luminaire may be reduced in large open spaces. However, with most lenses, the reduction is not sufficient to prevent luminaire reflections in specular VDT screens or to provide an acceptable VCP.

Light Loss Factor (LLF)

(Formerly called maintenance factor) The ratio of illuminance (or exitance or luminance) for a given area to the value that would occur if lamps operated at their (initial) rated lumens and if no system variation or depreciation had occurred. Components of this factor may be either initial or maintained.

Note: The light loss factor is used in lighting calculations as an allowance for lamp(s) or luminaire(s) operating at other than rated conditions (initial) and for the depreciation of lamps, light control elements, and room surfaces to values below the initial or design conditions, so that a minimum desired level of illuminance may be maintained in service. Light loss factors address losses that result in direct changes to lamp lumens, emitted luminaire lumens, or the interreflected light delivered to the space.

Lighting System Selection and Design

In designing an electric lighting system for a daylit space, the designer must consider how to best integrate electric light with daylighting. Integration includes selection and layout of a complete system, including lamps, ballasts, luminaires and controls. In cases where a task-ambient approach is selected, daylight may supply the room ambient lighting.

Some useful recommendations related to the integration of an electric lighting system with daylighting are the following:

  • Select an electric light distribution that best integrates with the daylight delivery system and the room geometry. In an office environment, an indirect system will be less noticeable when dimmed, and helps brighten room surfaces within the non-daylit zone. A downlight system provides more localized lighting that may better differentiate lighting control zones, but will result in a much darker ceiling outside the daylight zone.
  • Balance luminances across the space. In large daylit spaces, lighting the interior wall that faces a window helps to balance room surface brightness. Indirect lighting for the ceiling serves a similar function.
  • Provide a luminaire layout and control zones that are coordinated with the daylight zone. This holds whether or not an automated lighting control system is applied. A row of luminaires along the windows can be manually switched off during daylight hours when provided with separate zone control.
  • Select a lamp color temperature that integrates well with daylight, while serving the space needs. Daylight is generally very cool, with color temperatures of 5000K and above. 5000 K lamps, however, may be unacceptably cool for non-daylight hours in a building interior. 3500 and 4100 K lamps provide acceptable color temperature when combined with daylighting. 3000 K will be noticeably warmer than daylight, but may be selected when a warm color temperature is desired at night. Daylight at the beginning and end of the day is somewhat warm and many believe that the interior should respond in a similar manner.

Line voltage

A voltage supplied by the electric grid. In US residential buildings, this refers to 120-volt alternating current (AC) power.

LM-80 and TM-21

Standardized testing procedures through IES LM-79, LM-80 and TM-21 make product quality more identifiable. Additionally, as with HPT8, voluntary testing and standards programs such as CALiPER, ENERGY STAR and DesignLights™ Consortium and the Municipal Solid State Lighting Consortium offer designers and consumers clearing houses for finding good quality products on everything from holiday light strings to downlights to pole mounted area lighting.

Load Shedding

A facility operator- or automatic control strategy in which illuminances are lowered based on time of day when demand charges are highest or in response to energy price spikes or utility emergency event.

Low voltage

Some electrical devices are designed to work with voltage lower than that supplied by the electrical system. Such devices use a transformer or power supply to convert 120v AC power to the voltage and current needed by the device.

Low Voltage Transformers

Low-voltage incandescent lamps operate at 12V or 24V and typically require transformers to adjust the line voltage down from 120V, 277V, or 347V. The transformers can use either magnetic coils or solid state electronics to adjust the voltage. Depending on the application, the style of luminaire and the type of transformer used, transformers are sized for either a single lamp or for a group of multiple lamps. The secondary voltage provided by the transformer must be verified for proper operation, some transformers are intentionally designed to provide more or less than the expected design voltage to account for line voltage variations or voltage drop through long lengths of wiring. Power consumption of the transformer is also something to be noted and accounted for both with the lights operating and with the lamps off but with primary magnetizing current still flowing to the transformer.


A unit of luminous flux. Radio-metrically, it is determined from the radiant. Photometrically, it is the luminous flux emitted within a unit solid angle (one steradian) by a point source having a uniform luminous intensity of one candela.

Luminaire efficacy

Light output of a luminaire, divided by total wattage to the power supply, given in lumens per watt (lm/W). Luminaire efficacy accounts for all driver, thermal, and luminaire optical losses.


Luminance is the amount of light coming from a surface or point, and it is measured in candelas per square meter (cd/m2). Some objects are self-luminous, that is, they generate light; lamps and computer screens, for example. Other objects simply reflect light from other sources, office walls, for example. In the latter case, the surface luminance is a function of the surface illuminance and the reflective properties of the surface. For example, a dark gray and a light gray surface can be lit with the same illuminance yet will have very different luminances. This also exemplifies why room surface finishes should also be considered as an integral part of the lighting design.

The human eye responds to luminance, not illuminance. The predominance of illuminance in recommendations is partly due to the fact that illuminance is easier to measure and calculate.

Luminance Contrast

Every visual task has some combination of light and dark areas that the human visual system must discern in order to “see” it. This is called contrast.

It could be a black character printed on a white page, or the shadow line of a nose against a cheek in facial features, or a shadowy area of dirt on a floor. Contrast may be affected by source/task/eye geometry, disability glare, shadows and the surface characteristics of objects being viewed.

Luminance Ratio

The ratio between the luminances of any two areas in the visual field.

Luminous efficacy

Light output of a light source, divided by nominal wattage, given in lumens per watt (lm/W). Does not include driver, thermal, or luminaire optical losses.

Maximizing Energy Efficiency through Equipment Selections

Lighting systems (lamps and ballasts) and luminaire distributions continue to improve, providing additional strategies for reduced energy consumption.

One notable example is within the fluorescent product segment. Standard T8 lamps with commonly used “standard” electronic ballasts are being replaced by third generation “high performance” T8 systems (Consortium for Energy Efficiency specifications) and highly efficient T5 systems, providing system efficacies above 90 lumens per watt. The “NEMA Premium” program tests ballasts for meeting the same qualifying specifications CEE HPT8 and allows such products that pass to be labeled “NEMA Premium”.

There are notable examples in most other categories. Ceramic metal halide sources can last over twice as long and produce twice the center beam candle power while using half the power of incandescent accent lighting. Compact fluorescent lamps have all but replaced incandescent lighting for general down lighting solutions. Solid state lighting, better known as LED, is rising to meet our lighting challenges. Standardized testing procedures through IES LM-79, LM-80 and TM-21 make product quality more identifiable. Additionally, as with HPT8, voluntary testing and standards programs such as CALiPER, ENERGY STAR and DesignLights™ Consortium and the Municipal Solid State Lighting Consortium offer designers and consumers clearing houses for finding good quality products on everything from holiday light strings to downlights to pole mounted area lighting.

Maximizing Energy Efficiency through System Design

One of the best ways to reduce lighting power is to install lighting only where it is needed. There are many aspects to quality lighting, but the designer should always be aware that lighting systems do not necessarily need to rely on permanently installed ceiling systems to generate all lighting. For instance, task / ambient systems provide permanent ceiling or wall mounted lighting systems providing a general “ambient” level of light and rely on work station “task” lighting to supplement the ambient light producing the resultant recommended levels only where they are needed. It is generally accepted that room surfaces using light finishes can also produce more pleasant environments. Carefully integrating daylight into the space not only improves indoor environmental quality, but may provide the opportunity to eliminate permanently installed lighting or at least reduce the amount of permanently installed lighting or the amount of time electric lighting is needed by the use of automatic controls.

Mercury lamp

A high intensity discharge (HID) lamp in which the major portion of the light is xiii Obsolete USA term produced by radiation from mercury operating at a partial pressure in excess of 105 Pa (approximately one atmosphere). Includes clear, phosphor-coated(mercury-fluorescent), and self-ballasted lamps.

Mesopic Vision

Vision with fully adapted eyes at luminance conditions between those of photopic and scotopic vision, i.e. between about 3.4 cd/m2 and 0.034 cd/m2.

Metal Halide Lamp

A high intensity discharge lamp (HID) in which the major portion of the light is produced by radiation of metal halides and their products of dissociation – possibly in combination with metallic vapors such as mercury. Includes clear and phosphor-coated lamps, quartz metal halide and ceramic metal halide.

Multilevel Switching

Multilevel switching is required by some energy codes, and involves switching of different lamps within luminaires or different luminaires within a layout to provide two or more illuminance levels within a space. In day lit spaces, this provides energy savings when a user determines that full light output is not necessary and activates only a portion of the available hardware.

Occupancy Sensors

Electric lighting that remains on in spaces which are unoccupied is usually a waste of energy. Occupancy sensors incorporate motion sensors that detect occupancy and can switch lighting on when someone enters a space and off when the space is vacated.

Occupancy sensors that automatically turn lights on the half load (requiring manual operation to go to full load) actually saves energy over those that are manual on only.


As of this writing, an emerging lamp technology that is comprised of molecules of carbon and hydrogen, providing a large area source that is thin, driven with low-voltage DC current power supplies. Relatively low light output and expensive, although progress is rapid.

On The Ceiling or Wall

A luminaire that is mounted directly on a wall or on the ceiling.

On/Off Switching

On/off switching is commonly performed at the entrance to a space, as required by code, and at other locations of convenience to the users. Switching is generally used in spaces where lighting system operation is intended to be either on or off, or where switching individual groups of lights can provide sufficient adjustment of work plane illuminance and general or localized luminance distributions.

Parabolic Louvers

Luminaires shielded with a grid of parabolic louvers having a specular (mirror-like) finish can control luminance precisely. The grid comprises an array of cells, each with their walls in the form of parabolic reflectors. Cell size ranges from 1.27 cm by 1.27 cm (0.5 in. by 0.5 in.) to almost 30.48 cm by 30.48 cm (12 in. by 12 in.). The smaller cell types are usually injection-molded plastic vacuum-metallized with aluminum. The larger cells are usually fabricated from aluminum sheets, anodized prior to forming. A specular finish permits cells to control luminaire light output precisely so that almost no light is emitted above the cutoff angle. When this is the case, the louver may look darker than the ceiling. A semi–specular or mostly diffuse louver finish may be used to brighten the appearance of the luminaire. However, the higher luminance of more diffuse finishes may show up as reflections in VDT screens.


The measurement of quantities associated with light, often used to refer to the intensity (candlepower) distribution curve of a lamp or luminaire.

Photopic Vision

Vision mediated essentially or exclusively by the cones, generally associated with adaptation to a luminance of at least 3.4 cd/m2.

Planned Indoor Lighting Maintenance

Maintenance is the link between predictive design and actual performance over the long term. In the design of new systems, the owner’s maintenance practices, which include relamping and luminaire cleaning, determine the light loss factors that are applied in lighting system design calculations. These in turn may affect the number of lamps or luminaires required in the space. Proper maintenance can therefore help to achieve a low energy design. A lighting system that is regularly cleaned will result in less luminaire dirt depreciation and require fewer luminaires, which reduces the system’s initial cost, installed lighting power, and lighting energy consumption. Similarly, group relamping can help maintain higher lamp lumen depreciation factors.

In some cases, it may be necessary to educate the owner or facility operator on the importance of good maintenance practices, and the impact these practices have on the installed lighting power and the associated initial and operating costs.


Modern lighting systems may contain small amounts of mercury, lead, cadmium, other heavy metals, PCBs, corrosives, and other toxic substances. These substances may be damaging to human health and the environment if they are released from the products in which they are contained. In the past, ballasts containing polychlorinated biphenyls (PCBs) and lamps containing mercury were disposed of in the normal building solid waste stream. These toxic substances were buried in landfills – free to react with many substances – or incinerated to create energy. They could leach slowly into the groundwater or be emitted into the air through the landfill’s ventilation system, or be released directly into the atmosphere with a combustion facility’s emissions.

It is important to check state and local regulations to ensure proper compliance. In addition, the use of registered and certified recycling companies can limit liability, insure compliance with all applicable rules and regulations, and simplify record keeping – see

Reduced Wattage Linear T12 Lamps

The 1992 EPACT still permits the use of antiquated reduced wattage T12 lamps such as the 34-watt, 48 inch lamps, or the so-called energy-saving lamps. These lamps save up to 15 percent energy on (older) existing 40-watt electromagnetic T12 ballasts, at the expense of a corresponding reduction in lumen output. Reduced wattage versions of several types of T12 lamps are still available to directly replace their full wattage T12 counterparts except in those applications where the lamp temperature is too cold or the ballast is unsuitable, such as some electronic and/or “shoplight” fixtures.


The ratio of the reflected flux to the incident flux.

“Reflected Direct” Luminaires

A class of luminaires, similar in performance to lensed or diffused luminaires, makes use of light reflecting off the inner surface of a recessed housing from concealed or obscured sources. Typically, these luminaires exhibit perforated metal baskets backed with diffuse acrylic to hide the lamps from direct view and direct light onto the upper reflector surface. The upper reflector is commonly a smooth surface with a matte, high-reflectance finish, however, textured aluminum reflector variations are also available. This optical system results in photometric distributions generally similar to those of perfectly diffuse emitters. Other variations manipulate the lamp shields with louvers or lenses for higher luminaire efficiency, but with more glare potential.

Reflected Glare


A general tem for the process by which the incident flux leaves a (stationary) surface or medium from the incident side, without change in frequency.


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


Retinal receptors that continue to respond at low levels of luminance, even below the threshold for cones. At these levels there is no basis for perceiving differences in hue and saturation. No rods are found in the fovea.

Room Reflectance

The ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal perfectly diffused, completely reflecting standard surface in exactly the same way as the sample.

Scheduling/Control zones

Control zones (sometimes referred to as control channels) are groups of luminaires that are switched or dimmed together, and should be arranged to provide flexibility, appropriate light distributions for functional and aesthetic effects, and energy savings. Load schedules, which consider source and/or ballast type and lighting power within each zone are used to size the lighting circuits and lighting control equipment. For zones that are to be dimmed, control zones consist of luminaires and/or lamps of the same type for consistent, reliable dimming.

The layout of control zones is generally based on groupings of lighting equipment according to one or more of the following:

  • By the architectural feature, area, or task being illuminated
  • By access to daylight to allow for energy savings in day lit zones
  • By operating schedule for both manual and time schedule control
  • To provide multilevel control in spaces through switching or dimming
  • By equipment type

Scotopic Vision

Vision mediated essentially or exclusively by the rods, generally associated with adaptation to a luminance below about 0.034 cd/m2.


Shadows can interfere with task visibility by placing detail in darkness or they can enhance definition of three-dimensional details. Point sources (e.g., incandescent or high intensity discharge lamps) create more defined shadows than fluorescent lamps, which produce diffuse shadows.

Shadows cast by the structure of the task may reveal detail, or may mask what needs to be seen. High reflectance surroundings help fill in and modify shadows, as do luminaires with 10 percent or more up light when the ceiling cavity reflectance is over 50 percent.

The presence of shadows may be desirable, and the interplay of highlight and shadow helps to define the form of many visual tasks, as well as the architectural environment. Lighting vertical surfaces to at least half the horizontal illuminance level often brings the ratio of highlight to shadow into a tolerable range for three-dimensional tasks. Some shadow will still be present, which helps to model the task and reveal form. Since each visual task has an optimum range of modeling, a careful evaluation of critical visual tasks should be made to determine the effects of various ratios of horizontal vs. vertical illuminance on visibility.

Slimline Lamps

Slimline lamps are similar to the T12 lamps in their energy loading, but they use a single pin base (instead of the double or bi-pin base) and are instant start lamps, not requiring a lamp starter. These lamps are available in several lengths up to 2440 mm (96 in.) in T6, T8, and T12 diameters, the latter of which is by far the most prevalent.

Subjective Brightness

The subjective attribute of any light sensation giving rise to the perception of luminous magnitude, including the whole scale of qualities of being bright, light, brilliant, dim, or dark.

Note: The term brightness often is used when referring to the measurable luminance. While the context usually makes it clear as to which meaning is intended, the preferable term for the photometric quantity is luminance, thus reserving brightness for the subjective sensation.

T5 High Output

T5/HO lamps provide significantly greater light output than their T5 counterparts. Although they identically resemble T5 lamps in appearance and dimension, they are not electrically interchangeable with T5 lamps. Electronic ballasts that operate T5/HO lamps are required. Special care needs to go into fixture designs that employ T5/HO lamps, as otherwise significant glare potentially even greater than that of T5 may result, especially when used at low mounting heights or in non-industrial spaces. Reduced wattage versions of the standard 54W lamp have recently been developed which do not require a ballast change.

T5 Lamps

Further developments in lamp technology have resulted in the development of high efficacy T5 straight tube lamps employing tri-phosphor technology. Smaller, more compact luminaires are possible using these lamps. Standard and high output (higher power) versions are available. T5 lamp design has promoted development of luminaires that are more efficient than those using T8 and T12 lamps. T5 lamps, whether linear or twin tube, require properly designed fixtures to minimize source glare and visual discomfort.

T8 Lamps

The availability of higher-efficacy phosphors and different gas fill pressures allowed the development of T8 lamps. They have become the preferred choice in the specification of new installations of linear fluorescent lamps and offer over 20 percent increase in efficacy over 40-watt T12 lamps. When the system is powered by electronic ballasts, system efficacy improves further still. T8 lamps are available in lengths similar to T12 with compatible bases and sockets, but require a different, unique ballast. Therefore, in retrofit situations, the ballast must be replaced. Several wattages of 4-ft bi-pin T8 lamps are available including the North American standard 32-watts. There are also reduced wattage 4-ft T8 lamps available in 30, 28, 25 and 23 watts, which in many cases can directly replace the 32-watt T8 with no ballast change required.

The Lighting Fixture

A luminaire is a complete lighting system consisting of a lamp(s) and ballast(s) (when applicable) together with the parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply.

Thermal Energy Effects

Every watt of lighting power introduces 3.6 kJoules (3.41 BTUs) per hour of heat into the space. If the building is in cooling mode, air conditioning energy (usually provided by electricity) will be required to remove this heat. If the building is in heating mode, the heat from lighting offsets the heating provided by the building’s main heating system (usually not electric). Most office buildings in North America experience both heating and cooling regimes over the course of a year, so the overall effect of lighting on thermal loads is building and climate dependent.


A general term for the process by which incident flux leaves a surface or medium on a side other than the incident side, without a change in frequency.

Note: Transmission through a medium is often a combination of regular and diffuse transmission.


The ratio of the transmitted flu to the incident flux. It should be noted that transmittance refers to the ratio of flux emerging to flux incident; therefore, reflections at the surface as well as absorption within the material operate to reduce the transmittance.


Even if a regular grid pattern of identical luminaires is used, the light distribution of luminaires is such that perfect uniformity across work surfaces can never be produced. The ratio of average illuminance to minimum illuminance over the practical work surface, (i.e., excluding corners and edges) for many applications should not be lower than 1.4. Illuminance variations across an entire space may be larger, and should produce luminances that respect the luminance ratio limits.

Upward Component

That portion of the luminous flux from a luminaire that is emitted at angles above the horizontal.

Vacancy Sensors

A sensor that requires the occupant to switch lighting on, which has the potential to save additional energy, particularly in daylit spaces. The sensors are configured to only turn lighting off when a sensor detects no activity within a space.

Veiling Brightness

A brightness superimposed on the retinal image that reduces its contrast. It is this ceiling effect produced by bright sources or areas in the visual field that results in decreased visual performance and visibility.

Veiling Reflection

Regular reflections that are superimposed upon diffuse reflections from an object that partially or totally obscure the details to be seen by reducing the contrast. This sometimes is called reflected glare. Another kind of veiling reflection occurs when one looks through a plate of glass. A reflected image of a bright element or surface may be seen superimposed on what is viewed through the glass plate.

Very High Output Lamps

The 1500 mA fluorescent lamp is also of rapid start design and has the highest current density commonly available. It is physically, but not electrically interchangeable with the 800 mA lamp and is used when a lower current lamp will not meet light output requirements. These lamps are also affected by EPACT legislation. Reduced wattage versions are available meeting legislative requirements. Lumen maintenance is inherently poor for both the T12 & the T10 versions.


The quality or state of being perceivable by the eye.


The plane on which a visual task is usually done, and on which the illuminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 0.76 meter (30 inches) above the floor.

Zonal Cavity Coefficients

Coefficients used in interior lighting design to determine illuminance and exitance (or luminance) values from room and luminaire parameters.

Leave a Reply

Free Lighting Language Guide
LightLogic_Light Language Ebook

Get your complete guide on lighting terms used by professionals. You’ll have everything you need to help you become an expert.