Lighting Terms

AC (Alternating Current)
Current which flows in one direction and then the other, alternately.

Accent Lighting
Directional lighting to emphasize a particular object or draw attention to a display item.

The process by which the human eye adjusts to a change in light level.

Ambient Lighting
The general lighting present in an area --excluding task lighting and accent lighting but including general lighting and daylight streaming in.

Amperes (Amps)
A measure of electrical current: Amps = Watts/Voltage. For example, say Uncle Sam has put in a 30 Amp breaker for his massive year round Christmas light display that his neighbors are threatening to sue him over. You want to find out how many watts of Savio Lighting’s ultra high-tech LED lights Uncle Sam can put up before it blows the breaker. Since house current is 120Volts: 30A x 120V = 3600W or 3.6 Kilowatts. So if he keeps it under 3600W, his breaker would be safe but we recommend that he should max out at only 2600W (80%) as a precautionary rule of thumb.

Intense luminous discharge formed by the passage of electric current in a gaseous medium across a space between electrodes.

Arc Lamp
A light source containing an arc. Also called a discharge lamp, or an arc discharge lamp.

Arc Length
In High Intensity Discharge lamps this is the distance between the electrode tips, which represents the physical length of the electrical discharge.

This field designates the type of gas or vacuum filling a volume or chamber of the lamp. This chamber might contain a filament or it might refer to the bulb which contains the arc tube.

Inert gas used in incandescent and fluorescent lamp types. In incandescent light sources, argon retards evaporation of the filament.

Average Rated Life
An average rating, in hours, indicating when 50% of a large group of lamps have failed, when operated at nominal lamp voltage and current. Manufacturers use 3 hours per start for fluorescent lamps and 10 hours per start for HID lamps when performing lamp life testing procedures. The life of an LED is defined as the operating time in hours for the lamp to reach L70 which designates 70% lumen maintenance (or 30% reduction in initial light output). Every lamp type has a unique mortality curve that depicts its average rated life.

An auxiliary piece of equipment required to start and to properly control the flow of current to gas discharge light sources such as fluorescent and high intensity discharge (HID) lamps. Typically, magnetic ballasts (also called electromagnetic ballasts) contain copper windings on an iron core while electronic ballasts are smaller and more efficient and contain electronic components.

Ballast Efficacy Factor (BEF)
Defined as ballast factor divided by input watts. The value is used to evaluate various lighting systems based on light output and power input. The BEF can only be used to compare systems operating the same type and quantity of lamps.

Ballast Factor (BF)
This is the percentage of a lamp's rated lumen output that can be expected when operated on a specific, commercially available ballast. Note that the "rated output" is sometimes measured on a reference ballast unlike ones that actually operate the lamp in the field.

For example, a ballast with a ballast factor of 0.93 will result in the lamp's emitting 93% of its rated lumen output. A ballast with a lower BF results in less light output and also generally consumes less power.

Ballast Hum
Sound generated by the vibration of laminations in the iron core of the transformer or inductor present in the ballast.

Ballast Losses
Power or energy dissipated in the ballast as heat and not converted to lamp energy.

Base or Socket
The socket is the receptacle connected to the electrical supply; the base is the end of the lamp that fits into the socket. There are many types of bases used in lamps, screw bases being the most common for incandescent and HID lamps, while bipin bases are common for linear fluorescent lamps.

Base Temperature (Maximum)
The maximum operating temperature permitted for the base in Celsius. Fixture manufacturers need to ensure that these conditions are satisfied in their fixture.

A style of bulb base which uses keyways instead of threads to connect the bulb to the fixture base. The bulb is locked in place by pushing it down and turning it clockwise.

Beam Angle
The angular dimension of the cone of light from reflectorized lamps (such as R and PAR types) encompassing the central part of the beam out to the angle where the intensity is 50% of maximum. The beam angle sometimes called "beam spread" is often part of the ordering code for the reflectorized lamps.

Example: The 50PAR30/HIR/NFL25 is a 50 watt PAR30 narrow flood lamp with a beam angle of 25 degrees.

Beam Lumens
The total lumens present within the portion of the beam contained in the beam angle.

Beam Spread (Approximate)
For reflector type lamps. The total angle of the directed beam (in degrees horizontal or vertical) to where the intensity of the beam falls to 50% or 10% of the maximum candlepower value as indicated.

Any base with two metal pins for electrical contact. This is the typical base for a fluorescent tube of 1 to 4 feet in length. It consists of 2 prong contacts which connect into the fixture. Medium bi-pins are used with type T-8 and T-12 tubular fluorescent lamps, and miniature bi-pins are used for tubular T-5 fluorescent lamps.

A hot body with an incandescent black surface at a certain temperature used as a standard for comparison. Note that a black surface is the best radiator possible. A tungsten filament will emit slightly less radiation than a blackbody at the same temperature.

Black Light
A popular term referring to a light source emitting mostly near UV (320 to 400 nm) and very little visible light.

Whether or not the top of the miniature lamp has a blacktop coating. The coating is used to control unwanted brightness or glare.

A short, thick post with a light at its top, used for grounds and outdoor walkway lighting.

Bottom Exit (BE)
(LFL plug-in ballasts) A configuration with leads or a wire-trap on the bottom or base of the ballast. This type of configuration is usually used when the ballast is mounted onto a junction box plate.

Bottom Exit Studs (BES)
(LFL plug-in ballasts) A configuration with screw studs mounted on the base plate or bottom of the ballast. The screws are 3/8" inches long with a #8-32 thread size (#8-32 nut). They are mounted on a two inch center. The studs are usually used to mount the ballast directly onto a junction box plate.

Bulb Material or Coating
The type of glass (or quartz) used in the glass envelope surrounding the light source. The material can also have coatings applied to achieve particular performances.

Bulb Size
Bulb shape followed by its size (the maximum diameter of the bulb expressed in eighths of an inch). For Compact Fluorescent products, "S", "D", "T", and "Q" are used to represent Single, Double, Triple and Quad sizes. The code also includes a reference such as T4 to represent the size of the tube. Rectangular headlamps are designated as "Rect" and the number of millimeters horizontally.

Candela (cd)
The measure of luminous intensity of a source in a given direction. The term has been retained from the early days of lighting when a standard candle of a fixed size and composition was defined as producing one candela in every direction. A plot of intensity versus direction is called a candela distribution curve and is often provided for reflectorized lamps and for luminaires with a lamp operating in them.

Candlepower (Mean Spherical)
Initial mean spherical candlepower at the design voltage. Mean spherical candlepower is the generally accepted method of rating the total light output of miniature lamps. To convert this rating to lumens, multiply it by 12.57 (4 pi).

Candlepower Distribution Curve
A graphical presentation of the distribution of light intensity of a light source, usually a reflector lamp or luminaire.

Device in ballast that stores electrical energy. Often used for power factor correction and lamp regulation.

Metal filaments that emit electrons in a fluorescent lamp. Negatively charged free electrons emitted by the cathode are attracted to the positive electrode (anode), creating an electric current between the electrodes.

Cathode Resistance
Resistance of the cathode in a Fluorescent lamp. It is measured "cold" before the lamp is turned on (Rc) or "hot" after the lamp is turned on (Rh). The ratio of the hot resistance to the cold resistance is also measured (Rh/Rc).

Center Beam Candlepower (CBCP)
The luminous intensity at the center of the beam of a reflector lamp.

Color Corrected - Daylight/Full Spectrum
Refers to a lamp with a special phosphor or coating to give it a color rendering profile similar to natural daylight. Full spectrum CFL bulbs produce less glare which reduces fatigue and also saves energy. Daylight bulbs reduce eyestrain to enable reading, computing and performing intensive tasks for longer periods of time. Even down to the basics, natural light renders color the best with the CRI of 100.

Color Rendering Index (CRI)
Refers to a lamp’s ability to render an object’s true colors based on a scale of 100.

Incandescent is used as the main rating of 100 CRI. Compact fluorescent lamps are rated at 82-86 CRI, which is considered high quality color rendering. CRI is a more important specification for retail lighting design than it is for office lighting.

Any CRI rating of 80 or higher is considered high and indicates that the source has good color properties. Incandescent lamps and daylight have a CRI of 100, the highest possible CRI. The higher the CRI of the light source, the truer the colors are rendered.

Compact Fluorescent Lamp (CFL)
The general term applied to fluorescent lamps that are single-ended and that have smaller diameter tubes that are bent to form a compact shape. Some CFLs have integral ballasts and medium or candelabra screw bases for easy replacement of incandescent lamps.

Cool White
A term loosely used to denote a color temperature of around 4100 K. The Cool White (CW) designation is used specifically for T12 and other fluorescent lamps using halophosphors and having a CRI of 62.
Correlated Color Temperature (CCT)
A numerical measurement of the color appearance of a light source measured in degrees Kelvin (K). It also refers to the way color groups are perceived (psychological impact of lighting) with warm colors at the red/orange/yellow end of the spectrum and cool colors at the blue end.

The most typical Kelvin degree lamps used in task lighting
KelvinAssociated EffectsMoods Appropriate Applications
2700°Friendly, personal, intimateHomes, libraries, restaurants
3000°Soft, warm pleasing lightHomes, hotel rooms and lobbies, restaurants, retail stores
3500°Friendly, inviting, non-threateningExecutive offices, public reception areas, supermarkets
4100°Neat, clean, efficientOffice, classrooms, mass merchandisers, showrooms
5000°Bright, alertGraphic industry, hospitals
6500°Bright, coolJewelry stores, beauty salons, galleries, museums, printing

A measure of the rate of flow of electricity, expressed in amperes (A).

Dichroic Reflector (or Filter)
A reflector (or filter) that reflects one region of the spectrum while allowing the other region(s) to pass through. A reflector lamp with a dichroic reflector will have a "cool beam" i.e. most of the heat has been removed from the beam by allowing it to pass through the reflector while the light has been reflected.

Whether or not the lamp lumens can be varied while maintaining reliability.

Dimmer, Dimming Control
A device used to lower the light output of a source, usually by reducing the wattage it is being operated at. Dimming controls are increasing in popularity as energy conserving devices.

Diode - LED
A two-terminal semiconductor device having a p-n (positive-negative) junction which mainly lets energy travel in one direction. Specialized diodes are used to produce light - LED (Light Emitting Diode). When a diode is switched on, electrons are able to recombine with holes within the device, releasing energy in the form of photons. This is electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is usually small and fused optical components are used to shape its radiation pattern and boost reflection.

Discharge Lamp
A light source that produces light by passing a current between electrodes through a vapor or gas. Includes fluorescent and high intensity discharge lamps.

An LED driver is a self-contained power supply that has outputs which match the electrical characteristics of the LEDs. Drivers should be current-regulated and may also offer dimming by means of pulse width modulation (PWM) circuits.

A measurement of how effective the light source is in converting electrical energy to lumens of visible light. Expressed in lumens-per-watt (LPW) this measure gives more weight to the yellow region of the spectrum and less weight to the blue and red region where the eye is not as sensitive.

The efficiency of a light source is simply the fraction of electrical energy converted to light, i.e. watts of visible light produced for each watt of electrical power with no concern about the wavelength where the energy is being radiated. For example, a 100 watt incandescent lamp converts 7% of the electrical energy into light; discharge lamps convert 25% to 40% into light.

The efficiency of a luminaire or fixture is the percentage of the lamp lumens that actually comes out of the fixture

Electrical Discharge
A condition under which a gas becomes electrically conducting and becomes capable of transmitting current, usually accompanied by the emission of visible and other radiation. An electric spark in air is an example of an electrical discharge, as is a welder's arc  and a lightning bolt.

Electromagnetic Ballast (Magnetic Ballast)
A ballast used with discharge lamps that consists primarily of transformer-like copper windings on a steel or iron core.
Energy Policy Act (EPACT)
Energy legislation passed by Congress in 1992 and updated periodically, mandating labeling and minimum energy efficiency requirements for many commonly used incandescent and fluorescent lamps.

Field Angle
The angular dimension of the cone of light from reflectorized lamps (such as R and PAR types) encompassing the central part of the beam out to the angle where the intensity is 10% of maximum.

Wire used in incandescent lamps, usually made of tungsten and often coiled, that emits light when heated by an electrical current.The code number of a filament consists of a letter(s) to indicate how the wire is coiled. A number followed by the letter indicates the arrangement of the filament on the supports. The letter “C” is commonly used indicating “Coil” type filament.

Used to refer to the beam pattern of a reflector lamp, which disperses the light over a wide beam angle, typically 20 degrees or more. ("Flood" as opposed to "spot")

A luminaire used to light a scene or object to a level much brighter than its surroundings. Usually floodlights can be aimed at the object or area of interest.

Fluorescent Lamp
A high efficiency lamp utilizing an electric discharge through inert gas and low pressure mercury vapor to produce ultraviolet (UV) energy. The UV excites phosphor materials applied as a thin layer on the inside of a glass tube which makes up the structure of the lamp. The phosphors transform the UV to visible light.

Footcandle (fc)
A unit of illuminance or light falling onto a surface measured as the light level on a surface one foot from a standard candle. This can be thought of as the amount of light that falls on a surface. The foot-candle is equal to one lumen per square foot.

Visual discomfort caused by excessive brightness is called discomfort glare. If task performance is affected it is called disability glare. Glare can be direct glare or indirect (reflected) glare

Halogen Cycle
A regenerative cycle of tungsten and halogen atoms, which prevents blackening of the lamp envelope during the life of the lamp.

Halogen Lamp
A halogen lamp is an incandescent lamp with a filament that is surrounded by halogen gases, such as iodine or bromine. Halogen gases allow the filaments to be operated at higher temperatures and higher efficacies. The halogen participates in a tungsten transport cycle, returning tungsten to the filament and prolonging lamp life.

Heat Sink
A heat sink (or heatsink) is an environment or object that absorbs and dissipates heat from another object.

High Intensity Discharge (HID) Lamp
A general term for mercury, metal halide and high-pressure sodium lamps. HID lamps contain compact arc tubes which enclose mercury and various gases with other chemicals and operate at relatively high pressures and temperatures.

High Output Fluorescent (HO)
High output fluorescent lamps designed to be used with an 800 milliampere ballast. Able to operate at low temperatures (down to 0ºF) and still produce high light levels.

High Power LED
An LED that can be powered at hundreds of milliamps (mA) in contrast to “traditional” indicator LEDs (SMD, 8mm, 5mm, etc.) which can only be powered at a few milliamps. A High Power LED produces high lumen output and can be set in an array to form a powerful LED lamp.

High-Pressure Sodium (HPS) Lamp
HPS lamps are high intensity discharge light sources that product light by an electrical discharge though sodium vapor operating at relatively high pressures and temperatures.

Hot Restart Time
Time it takes for a High Intensity Discharge lamp to reach 90% of light output after going from on to off to on.

Light arriving at a surface, expressed in lumens per unit area; 1 lumen per square foot equals 1 footcandle, while 1 lumen per square meter equals 1 lux. It is a measure of the intensity of light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Prior, illuminance was often called brightness. However, this leads to confusion with other uses of the word.

Incandescent Lamp
Works by incandescence (a general term for heat-driven light emissions). An electric current passes through a thin filament, heating it to a temperature that produces light. The enclosing glass bulb contains either a vacuum or an inert gas to prevent oxidation of the hot filament.

Indirect Lighting
The method of lighting a space by directing the light from luminaires upwards towards the ceiling. The light scattered off the ceiling produces a soft, diffuse illumination for the entire area.

Induction Lighting
Gases can be excited directly by radio-frequency or microwaves from a coil that creates induced electromagnetic fields. This is called induction lighting and it differs from a conventional discharge, which uses electrodes to carry current into the arc. Induction lamps have no electrodes inside the chamber and generally, therefore, have longer life than standard lamps

Initial Lumens
The measured luminous output of a new light source.

Instant Start
A type of fluorescent lamp-ballast circuit designed to start fluorescent lamps as soon as the power is applied. Originally, instant-start circuits were developed to eliminate separate mechanical starter devices. Slim fluorescents lamps operate only on instant circuits.

Integrated Circuit (IC)
IC-based CFLs operate by controlling the voltage and current by adjusting the output frequency which provides stable operation of the CFL. Controlling the current produces less stress on the cathode and the electronic components, which results in long life, smoother dimming, and less noise.

A lens is an optical device that refracts light, converging or diverging the beam.

Light Pollution
Light that is directed to areas where it is not needed, and thereby interferes with some visual act. Light pollution directed or reflected into the sky creates a "dome" of wasted light and makes it difficult to see stars above cities.

Lumen (lm)
A measure of luminous flux or quantity of light emitted by a source. Casually, the lumen can be thought of as a measure of the total amount of visible light in some defined beam or angle, or emitted from some source.

Lumen Depreciation
The gradual decline in light output from a light source over time due to filament deterioration and bulb darkening.

Lumen Maintenance
A measurement of how a lamp maintains its light output over time.

Lumen Per Watt (LPW)
A measure of the efficacy (efficiency) of a light source. The number is achieved by dividing lumens produced by watts consumed.

Luminous Efficacy
The light output (lumens) of a light source divided by the total power input (watts) to that source. It is expressed in lumens per watt.

A complete lighting unit consisting of a lamp (or lamps), ballast (or ballasts) as required together with the parts designed to distribute the light, position and protect the lamps and connect them to the power supply. A luminaire is often referred to as a fixture.

A measure of "surface brightness" when an observer is looking in the direction of the surface. It is measured in candelas per square meter (or per square foot) and was formerly referred to as "photometric brightness."

Lux (lx)
A unit of illuminance or light falling onto a surface. One lux is equal to one lumen per square meter.

Mercury Lamp
A high-intensity discharge light source operating at a relatively high pressure (about 1 atmosphere) and temperature in which most of the light is produced by radiation from excited mercury vapor. Phosphor coatings on some lamp types add additional light and improve color rendering.

Metal Halide Lamp
A high intensity discharge light source in which the light is produced by the radiation from mercury, plus halides of metals such as sodium, scandium, indium and dysprosium. Some lamp types may also utilize phosphor coatings.

Open Fixture Rated
Lamps that are approved for burning in open fixtures (as opposed to enclosed fixtures which have an acrylic lens or plate glass enclosure).

Operating Position or Burn Position
Mercury and High Pressure Sodium lamps may be operated in any burn position and will still maintain their rated performance specifications. Metal Halide and Low Pressure Sodium lamps, however, are optimized for performance in specific burn positions, or may be restricted to certain burn positions for safety reasons.

U = Universal burning position
HBU = Horizontal -15º to Base Up
HBD = Horizontal +15º to Base Down
HOR = Horizontal ±15º
H45 = Horizontal to -45º only
VBU = Vertical Base Up ±15º
VBD = Vertical Base Down ±15º
If no special burn position is noted, the burn position is universal.

Organic LED
Based on organic (carbon based) materials. In contrast to LEDs, which are small point sources, OLEDs are made in sheets which provide a diffuse area light source. OLED technology is developing rapidly and is increasingly used in display applications such as cell phones and PDA screens. However, OLEDs are still some years away from becoming a practical general illumination source. Additional advancements are needed in light output, color, efficiency, cost, and lifetime.

PAR Lamp
PAR is an acronym for parabolic aluminized reflector. A PAR lamp, which may utilize either an incandescent filament, a halogen filament tube or a HID arc tube, is a precision pressed-glass reflector lamp. PAR lamps rely on both the internal reflector and prisms in the lens for control of the light beam.

Phosphor conversion
A method used to generate white light with LEDs. A blue or near-ultraviolet LED is coated with a yellow or multichromatic phosphor, resulting in white light.

Reflector Lamp (R)
A light source with a built-in reflecting surface. Sometimes, the term is used to refer specifically to blown bulbs like the R and ER lamps; at other times, it includes all reflectorized lamps like PAR and MR.

Red, green, and blue, the three primary colors of light. When the primaries are mixed, the resulting light appears white to the human eye. Mixing the light from red, green, and blue LEDs is one way to produce white light. The other approach is known as phosphor conversion.

Solid State Lighting (SSL)
A technology that uses semi-conducting materials to convert electricity into light. SSL is an umbrella term encompassing both light-emitting diodes (LEDs) and organic light emitting diodes (OLEDs).

A colloquial term referring to a reflector lamp with a tight beam of light, typically around 10 degrees or less. It comes from the fact that such a lamp produces a narrow spot of light as opposed to a wide flood of light.

Task Lighting
Supplemental lighting provided to assist in performing a localized task, e.g. a table lamp for reading or an inspection lamp for fabric inspection.

Valance Lighting
Lighting from light sources on a wall typically above eye level, shielded by horizontal panels. The light may be upward or downward directed.

Veiling Reflection
Effective reduction in contrast between task and its background caused by the reflection of light rays; sometimes called "reflected glare." You might have dealt with veiling reflections when you have to tilt a shiny magazine to avoid glare so as to read it, or struggled with reading a computer monitor because of the reflection of a window or a light fixture.

Visual Comfort Probability (VCP)
For a given lighting scheme, VCP is a ratio expressed as a percent of people who, when viewing from a specific location and in a specified direction, find the system acceptable in terms of glare.

A measure of "electrical pressure" between two points. The higher the voltage, the more current will be pushed through a resistor connected across the points. The volt specification of an incandescent lamp is the electrical "pressure" required to drive it at its designed point. The "voltage" of a ballast (e.g. 277 V) refers to the line voltage it must be connected to.

A measurement of the electromotive force in an electrical circuit or device expressed in volts. Voltage can be thought of as being analogous to the pressure in a waterline.