Helpful Lighting Terms & Definitions
Directional lighting to emphasize a particular object or to draw attention to a part of the field of view.
Alternating Current (AC)
Flow of electricity which cycles or alternates direction many times per second. The number of cycles per second is referred to as frequency. Most common frequency used in this country is 60 Hertz (cycles per second).
Background or fill light in a space.
(amps or A)
The unit of measurement of electric current.
The illumination provided for scenery in off-stage areas visible to the audience.
An opaque or translucent element that serves to shield a light source from direct view at certain angles, or serves to absorb unwanted light.
An auxiliary device consisting of induction windings wound around a metal core and sometimes includes a capacitor for power correction. It is used with fluorescent and HID lamps to provide the necessary starting voltage and to limit the current during operation.
The unit of measurement of luminous intensity of a light source in a given direction.
Luminous intensity expressed in candelas.
Class "P" Ballast
Contains a thermal protective device which deactivates the ballast when the case reaches a certain critical temperature. The device resets automatically when the case temperature drops to a lower temperature.
The measurement of color.
Color Rendering Index (CRI)
Measure of the degree of color shift objects undergo when illuminated by the light source as compared with the color of those same objects when illuminated by a reference source of comparable color temperature.
The absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source.
Parabolic reflector that directs light downward thereby eliminating brightness at high angles.
The difference in brightness (luminance) of an object and its background.
Cool Beam Lamps
Incandescent PAR lamps that use a special coating (dichroic interference filter) on the reflectorized potion of the bulb to allow heat to pass out the back while reflecting only visible energy to the task, thereby providing a "cool beam" of light.
Lighting comprising sources sheilded by a ledge or horizontal recess, and distributing light over the ceiling and upper wall.
Outdoor luminaires that restrict all light output to below 85Â° from vertical.
Digital Addressable Lighting Interface (DALI)
An open communications protocol used by multiple control and ballast manufacturers for digital control.
Special fluorescent lamp ballast, which when used with a dimmer control, permits varying light output.
Direct Current (DC)
Flow of electricity continuously in one direction from positive to negative.
Lighting involving luminaires that distribute 90 to 100% of emmited light in the general direction of the surface to be illuminated. The term usually refers to light emitted in a downward direction.
A lamp in which light (or radiant energy near the visible spectrum) is produced by the passage of an electric current through a vapor or a gas.
Lighting system designed to provide minimum illumination required for safety, during power failures.
"ER" (Elliptical Reflector)
Lamp whose reflector focuses the light about 2" ahead of the bulb, reducing light loss when used in deep baffle downlights.
Extended Life Lamps
Incandescent lamps that have an average rated life of 2500 or more hours and reduced light output compared to standard general service lamps of the same wattage.
Illumination added to reduce shadows or contrast range.
A system designed for lighting a scene or object to a luminance greater than its surroundings. It may be for utility, advertising or decorative purposes.
A low-pressure mercury electric-discharge lamp in which a phosphor coating transforms some of the ultraviolet energy generated by the discharge into light.
The unit of illuminance when the foot is taken as the unit of length. It is the illuminance on a surface one square foot in area on which there is a uniformly distributed flux of one lumen.
Foot Lambert (fl)
A unit of luminance of perfectly diffusing surface emitting or reflecting light at the rate of one lumen per square foot.
A set of striplights at the front edge of the stage plateform used to soften face shadows cast by overhead luminaires and to add general toning lighting from below.
High Intensity Discharge (HID) Lamp
A discharge lamp in which the light producing arc is stabilized by wall temperature, and the arc tube has a bulb wall loading in excess of three watts per square centimeter. HID lamps include groups of lamps known as mercury, metal halide, and high pressure sodium.
High Output Fluorescent Lamp
Operates at 800 or more milliamperes for higher light output than standard fluorescent lamp (430MA).
High Pressure Sodium (HPS) Lamp
High intensity discharge (HID) lamp in which light is produced by radiation from sodium vapor. Includes clear and diffuse-coated lamps.
The self-emission of radiant energy in the visible spectrum due to the thermal excitation of atoms or molecules.
A lamp in which light is produced by a filament heated to incandescence by an electric current.
Instant Start Fluorescent Lamp
A fluorescent lamp designed for starting by a high voltage without preheating of the electrodes.
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 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 fc=cp/d2.
Unit of measurement for color temperature. The Kelvin scale starts from absolute zero, which is -273° Celsius.
Unit of electrical power consumed over a period of time. KWH = watts/1000 x hours used.
An artificial source of light (also portable luminaire equipped with a cord and plug).
The ratio of lumens produced by a lamp to the watts consumed. Expressed as lumens per watt (LPW).
Lamp Lumen Depreciation (LLD)
Multiplier factor in illumination calculations for reduction in the light output of a lamp over a period of time.
Radiant energy that is capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 770 nm.
Light emitting diode - a semiconductor devise that emits visible light of a certain color.
Used in luminaires to redirect light into useful zones.
Lighting designed to provide illuminance over a relatively small area or confined space without providing any significant general surrounding lighting.
A series of baffles used to shield a source from view at certain angles or to absorb unwanted light. The baffles usually are arranged in a geometric pattern.
Long Life Lamps
See Extended Life Lamps.
Low Pressure Sodium Lamp
A discharge lamp in which light is produced by radiation of sodium vapor at low pressure producing a single wavelength of visible energy, i.e. yellow.
Low Voltage Lamps
Incandescent lamps that operate at 6 to 12 volts.
The unit of luminous flux. 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.
A complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps and to connect the lamps to the power supply.
Luminaire Direct Depreciation (LDD)
The multiplier to be used in illuminance provided by clean, new luminaires to the reduced illuminance that they will provide due to direct collection on the luminaires at the time at which it is anticipated that cleaning procedures will be instituted.
The ratio of luminous flux (lumens) emitted by a luminaire to that emitted by the lamp or lamps used.
The amount of light reflected or transmitted by an object.
The metric unit of illuminance. One lux is one lumen per square meter (lm/m2).
Maintenance Factor (MF)
A factor used in calculating illuminance after a given period of time and under given conditions. It takes into account temperature and voltage variations, dirt accumulation on luminaire and room surfaces, lamp depreciation, maintenance procedures and atmosphere conditions.
A high intensity discharge (HID) lamp in which the major portion of the light is produced by radiation from mercury. Includes, clear, phosphor-coated and self-ballasted lamps.
Metal Halide Lamp
A high intensity discharge (HID) lamp in which the major portion of the light is produced by radiation of metal halides and their products of dissociation-possible in combination with metallic vapors such as mercury. Includes clear and phosphor coated lamps.
Overall Length (OAL)
Maximum overall length of a light fixture.
Parabolic aluminized reflector lamps which offer excellent beam control, come in a variety of beam patterns from very narrow spot to wide flood and can be used outdoors unprotected because they are made of "hard" glass that can withstand adverse weather.
A grid of baffles which redirect light downward and provide very low luminaire brightness.
The measurement of light quantities.
Point Method Lighting Calculation
A lighting design procedure for predetermining the illuminance at various locations in lighting installations, by use of luminaire photometric data.
The process by which the transverse vibrations of light waves are oriented in a specific plane. Polarization may be obtained by using either transmitting or reflecting media.
Ratio of: Watts (volts x amperes) Power factor in lighting is primarily applicable to ballasts. Since volts and watts are usually fixed, amperes (or current) will go up as power factor goes down. This necessitates the use of larger wire sizes to carry the increased amount of current needed with Lowe Power Factor (L.P.F) ballasts. The addition of a capacitor to an L.P.F. ballast converts it to a H.P.F. ballast.
Preheat Fluorescent Lamp
A fluorescent lamp designed for operation in a circuit requiring a manual or automatic starting switch to preheat the electrode in order to start the arc.
Reflectorized lamps available in spot (clear face) and flood (frosted face).
Rapid Start Fluorescent Lamp
A fluorescent lamp designed for operation with a ballast that provides a low-voltage winding for preheating the electrodes and initiating the arc without a starting switch or the application of high voltage.
A device used to redirect the light flow from a source by bouncing it off the surface.
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.
Rough Service Lamps
Incandescent lamps designed with extra filament supports to withstand bumps, shocks and vibrations with some loss in lumen output.
Self-ballasted Mercury Lamps
Any mercury lamp of which the current-limiting device is an integral part.
Ratio of the distance between luminaire centers to the mounting height above the work-plane for uniform illumination.
Spectral Energy Distribution (SED) Curves
A plot of the level of energy at each wavelength of a light source.
Surface Mounted Luminaire
A luminaire that is mounted directly on a ceiling.
Suspended (Pendant) Luminaire
A luminaire that is hung from a ceiling by supports.
Lighting directed to a specific surface or area that provides illumination for visual tasks.
Incandescent lamps that have two separately switched filaments permitting a choice of three levels or light such as 30/70/100, 50/100/150 or 100/200/300 watts. They can only be used in a base down position.
A device to raise or lower electric voltage.
The passage of light through a material.
A gas filled tungsten incandescent lamp containing a certain proportion of halogens.
Wall Wash Lighting
A smooth even distribution of light over a wall.