Glossary Of Light Bulb Terms
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A Glossary of Light Bulb Terms and Definitions – An Enlightening Experience

Light bulbs themselves may be quite simple in operation, but there is such a range of options out there that it can get quite bewildering at times. How many times have you found yourself staring up at the shelves in a store holding your shopping list (you probably wrote down ‘more light bulbs’ or similar) and not knowing which ones to pick. Choice is a good thing, but when a sales assistant asks (a bit too loudly for your liking) if they can help, and you mumble, “I just want a light bulb,” it can also be embarrassing.

So here is a helpful glossary of many of the common terms that you’ve seen in the lighting department but been a bit unsure about:

  • A-Lamps (Also known as General Service Bulbs)
    The standard incandescent bulb for most common uses.

  • Accent Lighting
    A type of lighting used to accentuate a given locality or object in the home or office, for example; a doorway or a painting.

  • ADA Brackets
    Fixtures for mounting on a wall that extend no more than 4 inches from the wall to comply with the Americans with Disabilities Act.

  • Adjustables
    Fixtures that can be adjusted or aimed for accent and directional lighting requirements.

  • Alternating-Current (AC)
    An electric current reversing its direction regularly at certain intervals.

  • Ambient Lighting
    Also can be referred to as general lighting. This is the all around, uniform illumination that lights up every home.

  • Amps/Amperes
    The unit used to measure electric current. The amount of current sent through one ohm by one volt. It comes from the name of M.Ampere who was a French scientist.

  • ANSI
    The American National Standards Institute. This is a consumer organization that develops many voluntary guidelines and standards for the electrical and other industry.

  • Aperture
    An opening or gap to allow the free passage of light.

  • Architectural Lighting
    A broad generic term for built-in light fixtures, such as valance lighting, or recessed lighting in ceilings, or in bookshelves or ledges high or low on walls.

  • Argon
    An inert gas from the Earth’s atmosphere that is used inside most light bulbs.

  • Average Rated Life
    The time expressed in hours that half of a given number of test lamps burnt out in. The lifespan of individual bulbs purchased will almost always be slightly above or below this time. (Also expressed as Lamp Life).

  • Baffle
    A part of a fixture used to prevent glare by absorbing or blocking any excess light.

  • Ballast
    A electrical apparatus used to provide current for the start up of HID and fluorescent lamps & light bulbs, and then control the current to regulate their operation.

  • Base
    That part of the light bulb which is placed inside the socket. (Usually these are screw in and made of either aluminum or brass). For HID bulbs they might be ceramic. For compact fluorescent they mostly have either two or four pins. The two pin versions are designed for preheat and have an internal starter. The four pin types (which are dimmable) do not and need a ballast to be used. Slimline fluorescents have only one pin at either end of the lamp. (See also Fluorescent and HID).

  • Beam Spread
    A measure of the angle of the light beam from a lamp with a reflector. This might be narrow spot, narrow flood, or wide flood.

  • Brass
    A metallic alloy made of zinc and copper. Fixtures can be made of this.

  • Bulb
    The commonly used term for an incandescent lamp.

  • Candela or Candlepower
    The unit of measurement of luminous intensity in any given direction from a source of light.

  • Candlepower Distribution
    A representation of how light is spread from a source, whether evenly throughout a curve, or not.

  • Canopy
    A part of a light fixture that conceals an outlet box.

  • Cans
    A term sometimes used to refer to recessed down lights.

  • Cathodes
    An electron emitting electrode. Used in fluorescent lighting systems. (See Fluorescent).

  • Channel
    A selection of fixtures that are jointly operated when using dimmers. Typically they would be of similar function or type. This can also be known as a zone.

  • Chromatics
    The branch of optical science that deals with color and the perception of color.

  • Circuit
    The chosen wiring route for an electric current to traverse.

  • Circuit Breaker
    A safety device that can be fitted to prevent too much current from flowing.

  • Clip
    An attachment that secures a shade to a light bulb.

  • Color Rendering Index (CRI)
    This is very important as it tells you how your light will render, or portray, the accurate color of everything it illuminates. The CRI runs from 1 (for Low Pressure Sodium lamps) to 100 (for the Sun). A CRI somewhere in the 80’s will give you good and true color portrayal.

  • Compact Fluorescent Lamp (CFL)
    These are small long living fluorescent lights that can be used as an alternative to incandescent discount light bulbs. They can also be known as BIAX lamps. (See Fluorescent).

  • CCT (Correlated Color Temperature)
    This specifies the color that a given bulb itself appears when it is in use. (Whether it looks ‘hot’ ‘warm’ or ‘cold’). It is compared in degrees Kelvin (K), to a source of reference at a given temperature. (Not the color of the glass).

  • Crystal Glass
    Very fine quality glass like that used in chandeliers.

  • Current
    The flow of electricity, measured in amps.

  • Cut-off Angle
    The angle (measured from the vertical) whereat anything used to shield the light from direct view cuts-off that light source from the viewer.

  • Damp Location
    A UL listing for fixtures that are used in locations where there is moisture present. (see UL)

  • Decorative Bulbs (D)
    Attractive looking light bulbs that come in many assorted and unusual shapes.

  • Dichroic Coating
    A film that is used on reflector lamps to prevent as much heat being reflected as light. The thermal energy is instead transmitted through to behind allowing the light beam not to be so hot.

  • Diffuse Lighting
    Light which has been scattered or dispersed so as not to appear to be emanating wholly from its’ source.

  • Diffuser
    Anything used for diffusing light. This might be a paper or fabric shade or etched or Opal (milky looking) glass on the light bulb.

  • Dimmer
    A control switch that reduces the illumination of a lamp by lessening the electricity available to it. These may be incremental, or full range with rotary or slide controls.

  • Direct Current (DC)
    An electric current that flows continuously in only one direction without any alterations.

  • Direct Lighting
    A form of lighting where a minimum of 90% of the light produced is cast in a downwards direction.

  • Directional Lighting
    See Adjustables.

  • Disability Glare
    A glare from a too bright light that results in a high discomfort that affects visibility levels considerably.

  • Discomfort Glare
    This glare is a lesser form of the above. Discomfort is produced but visibility is still kept to an acceptable level.

  • Downlights
    Small recessed lights in a ceiling.

  • Efficacy
    A guide to the efficiency of a light bulb expressed in lumens per watt (LPW). The higher the number, the more light given out for energy used.

  • Energy
    A gauge of how an electrical device works over time, usually shown as kwh (kilowatt hours). Which is 1000 watts used for 1 hour.

  • Etched Glass
    Glass (on a light bulb) treated with acid for the purpose of diffusion. (See diffusion).

  • Extended Life Lamp
    Light bulb with an average rated life of 2500+ hours. (See Average Rated Life).

  • Eyeball
    An adjustable light that is fitted into the ceiling.

  • Filament
    A wire coil made of tungsten that produces light when heated by an electric current.

  • Fill Gas
    The gas inside the light bulb. For incandescent lamps this is usually argon or krypton with sometimes added nitrogen. Halogen is used in addition to this in halogen bulbs.

  • Flood
    The very wide light dispersal from a reflector bulb. FL can be used as an abbreviation.

  • Fluorescent
    A lighting system which works by creating electric arcs inside a gas rich tube to produce ultraviolet light, then converting this to visible flourescent light by its passage through a layer of phosphor on the inside of the glass.

  • Foot-Candle
    This is the common measurement of luminance (level of light) used in the United States. One foot-candle is one lumen on one square-foot of a surface. (See also Lumen and Lux).

  • Frosted Glass
    Glass used for light bulbs that has been roughened or treated with a spray on coating (See IF), for the purpose of diffusing light. (See Diffusion).

  • Full Spectrum
    Light bulbs with this designation accurately imitate natural light and are thought by some to be beneficial to health by reducing stress, depression and headaches, amongst other things.

  • G-Lamp
    Globular shaped light bulbs.

  • General Lighting
    See Ambient Lighting.

  • Gimbal Lighting
    The adjustable ring holding a PAR or MR bulb in place. (See also MR and PAR).

  • Glare
    An excessively bright light that causes discomfort and vision impairment. May be direct or indirect (from a reflection).

  • Gloss
    A glossy finish is a shiny, reflective finish.

  • Halogen Lamp
    A type of bulb which contains halogen gases, usually iodine, or chlorine, bromine, or fluorine, to extend the life of the tungsten filament through a recycling system know as the halogen cycle. Halogen light bulbs are also made of quartz glass, or ’hard glass’ because they have to be hotter to work properly. Halogen bulbs are brighter and produce more lumens per watt (LPW). (See Efficacy and Quartz Lamp).

  • Hard-Back
    A lamp shade with a plastic lining.

  • Heat Ratings
    A safety guide for surface mounted fixtures, which limits the wattage of the bulb to be used.

  • HID (High Intensity Discharge) Lamps
    A group of type of lamps that include mercury vapor, high pressure sodium, low pressure sodium and metal halide. They are long lasting and energy efficient.

  • IC Fixture
    Insulated Ceiling Fixture. A fixture that can be directly placed in thermal insulation.

  • IF
    This stands for Inside Frosted. (See Frosted Glass).

  • In-Ground Up Lights
    These are outdoor lights recessed in the ground (for the garden path, or patio, etc.) They can be low-voltage or solar powered.

  • Incandescent Lamp
    A light bulb which uses incandescence to produce light. (See below).

  • Incandescence
    Light produced by the electrical heating of a material.

  • Indirect Lighting
    A form of lighting where a minimum of 90% of emitted light is cast upwards.

  • Infrared (IR)
    An invisible radiation with very long wavelengths which is produced by light bulbs as well as visible radiation (light).

  • Instant Start
    A fluorescent light which can power up immediately without the need to pre-heat the cathodes or use starters. (See Cathodes, Fluorescent and Starters).

  • Integrated Dimming System
    An advanced dimmer that remembers pre-set lighting ‘themes’, and can re-create them by the use of a button on a wall box or remote control.

  • IR Lamp
    An IR lamp is an Infrared Reflecting Halogen Lamp. A special type of halogen lamp which directs excess IR energy on the filament, so as to heat it further without extra use of power. (See also Infrared and filament).

  • Jack
    Plug in attachment found in low voltage light fixtures. (See Low Voltage Fixtures).

  • Kelvin
    An advanced unit of temperature used by scientists. (See CCT).

  • Krypton
    A gas sometimes used in premium bulbs instead of argon.

  • Lacquer
    A clear or colored coat that prevents metal from tarnish and rust.

  • Lamp (Electric)
    A source of man-made or artificial light. The whole of the light bulb.

  • Lamp Holder
    A socket to secure the lamp and connect it to an electric current.

  • Lamp Life
    See Average Rated Life.

  • LCL (Light Center Length)
    The length, usually in inches, from a given point on the base of a lamp to its light center.

  • Light Distribution
    The pattern of light in a room.

  • Light Fixture
    A complete lighting unit containing a lamp, reflector, housing and a connection, a socket and baffles. Also known as a Luminaire.

  • LLF (Light Loss Factor)
    A factor used to calculate the illumination after a certain time according to whatever conditions are present. (Like varying temperature, fluctuating voltage, atmospherics, dust buildup, and maintenance procedure, if any). This used to be referred to as the Maintenance Factor.

  • Line Voltage
    In the U.S.A, this should be 120 volts. (See Volts).

  • Louvers
    A screen of numerous baffles. (See Baffles).

  • Low Voltage
    Lamps that use 6, 12, or 24 volts instead of 120, and require a transformer connected between them and the standard 120 volt power source.

  • Lumens (LM)
    The actual quantity of light produced by a lamp or other source. From the Latin word ’lumen’ which means ’light’. Correctly known as luminous flux. (See Luminous Flux).

  • Luminaire
    A complete light fixture. (See Light Fixture).

  • Luminaire Efficiency
    This is a ratio of light produced by the luminaire as a whole, including whatever shades, baffles and or reflectors that may be fitted, compared to the light produced by the fitted lamp or bulb alone.

  • Luminance
    Reflected light sent in a given direction. The observation of brightness expressed in the measuring unit of light intensity, candelas. (See Candela, Candlepower).

  • Luminance Contrast
    How a given object stands out (or not) from its background due to available light.

  • Luminance Ratio
    A comparison between two or more areas within the viewer’s field of vision.

  • Luminous Flux
    The amount of light flowing over a given area in a period of time. (See Lumens).

  • Lux
    The international standard unit for measuring light levels (as oppose to Foot-Candle in the U.S.A). It is one lumen per square meter. (See Lumens and Foot-Candle).

  • Matte
    A low reflecting dull finish to reduce brightness and glare.

  • Mogul Base
    A lamp base used for high-wattage bulbs. (See Watts).

  • MOL (Maximum Overall Length)
    The entire length of a lamp usually in inches (for the U.S.A). Can be in centimeters.

  • MR Lamp
    A Multi-facet Reflector lamp.

  • National Electric Code (NEC)
    A standard for safe and proper wiring and electrical appliances to be used as a guide for local jurisdictions in disputes.

  • NEMA (National Electrical Manufacturers Association)
    An organization for promoting harmony and agreement within the industry, and deciding on the designation of new products.

  • Neodymium
    A metal used in the production of full spectrum bulbs. (See Full Spectrum).

  • Neon Lamp
    Culturally famous high voltage, low pressure bulbs used in outdoor signage.

  • Nominal Watts
    The power rating of light bulbs and other lamps. (See Watts).

  • Opal Glass
    A milky looking glass used to diffuse light. (See Diffuse Lighting).

  • Opaque
    Any material through which light does not penetrate.

  • Over Voltage
    It will shorten the operating life of a bulb to use it above its proper voltage. (See Volts).

  • PAR Lamps
    This stands for Parabolic Aluminized Reflector. These are extremely bright bulbs that can be used for the accurate controlling of light levels. Since the parabola shaped reflector is excellent at reflecting and focusing all the light produced in a beam in a chosen direction.

  • Pendant Lights
    These are suspended from the ceiling and regularly seen in many homes.

  • Power Factor
    This is a measure of efficiency of electrical devices.

  • Prism
    This refracts light, or bends it. A light beam enters through one of the prisms’ multiple, see-through straight sides, and leaves via another. (Light is sent off in different directions). It is not a reflector because the light changes direction inside the prism, rather than bouncing off the outside surface. (See Reflector).

  • Quality Of Lighting
    Simply; a term used for matching the type of lamps chosen for the task required.

  • Quartz Lamp
    A halogen bulb with glass made of quartz. This should never be touched with bare skin as an oily mark will appear that will cause the structure to weaken. If touched, clean the glass with an alcohol such as methylated spirits or a turpentine.

  • R Lamp or Reflector Lamps
    Reflector lamp are made of ‘soft’ glass as oppose to the ‘hard’ glass of a PAR lamp. This distinction concerns the glass structure and ability to deal with higher temperatures. They also differ in usually having their reflector source as a aluminum or silvery coating on the bulb itself.

  • Rated Lamp Life
    See Average Rated Life.

  • Recessed Fixture
    A light fixture mounted into the ceiling.

  • Recessed Lighting
    Here the fixture is installed above the ceiling, with only an opening for the light visible from below.

  • Reflectance
    The amount of light in percentage reflected by a given surface. (The rest of the light is either absorbed by the material, or transmitted through it).

  • Reflector
    A device for reflecting light in a chosen direction from its surface. Depending on the brightness desired, these may be either diffuse, glossy, matte, or specular. (See Diffused Lighting, Gloss, Matte and Specular).

  • Relamping
    A maintenance project either at work or home to clean all light fixtures and replace bulbs with new ones.

  • Rough Service Lamp
    Incandescent light bulbs with extra support to the filament for better withstanding shaking and vibrations.

  • Satin Finish
    A lightly textured ‘brushed’ finish to metal or glass.

  • Scalloped
    The fan or shell shaped pattern on some fixtures.

  • Scene
    Pre-set settings for dimmers. (See Dimmer).

  • Sconce
    A wall mounted fixture in the shape of a candelabra. Popular for use with decorative bulbs as they are exposed (to allow them to be admired).

  • Shade
    An opaque or translucent covering to lessen the light from a bulb when viewed from certain angles, and redirect it to others. (See Opaque and Translucent).

  • Shade Measurement
    Always should be in this order:
    - Across at the top
    - Across at the bottom
    - The slanting of the sides
    - The height

  • Shielding
    A group term for diffusers, baffles, louvers and shades.

  • Soft Shade (Also known as a Lined Shade)
    A lamp shade that has a liner of a fabric.

  • Solid Brass
    These are fixtures that are made of brass only, not a simulation or brass-plate. Some confusion exists here as the fixtures are probably hollow and not solid at all! The word ‘solid’ applies only to the brass itself. This comes from an old expression which referred to something as being ‘solid’, when it was honest and trustworthy. So in this case, meaning; it is what it says it is - brass!

  • SPD (Spectral Power Distribution)
    An illustration of how the power output of a given bulb changes in each different wavelength across the spectrum.

  • Specular
    A finish to a surface that is highly polished and mirror-like.

  • Spider
    The name of the cross frame by the top of a lamp shade.

  • Spot Lamp
    A reflectorized bulb with a narrow beam of light emitted.

  • Starters
    An electrical mechanism used together with a ballast for starting a fluorescent or HID lamp. (See Ballast, Fluorescent and HID).

  • Step Lights
    These are lights recessed into a wall along a pathway or into stairs on a stairwell.

  • Surface Mounted
    This is another term for ceiling mounted light fixtures.

  • Swing Arm
    A horizontal arm for the multiple positioning of a wall mounted or table light.

  • Switch Leg
    This is how the wiring between a light fixture and its control or dimmer switch is known.

  • T-Lamps
    A lamp that is tubular in shape.

  • Task Lighting
    Lighting that is designed for a specific task. Examples of this are security lighting, which has to be extra bright and reliable, and sometimes comes with an infrared sensor or motion detector attached. Or another example being a reading light, which should be bright enough to prevent eyestrain but not too glary, to avoid the user from having to squint.

  • Three-Way Lamp
    An incandescent light bulb with two filaments inside it for three levels of light from a sequential switch. The smaller filament is used for the low light level, the larger for the medium setting, and both are used simultaneously when the switch is set to high.

  • Three-Way Switch
    Sometimes also known as a ‘three-pole switch’ this is a system that operates a light fixture from two separate locations. This could be the top and bottom of a stairway for example, or next to the bed and the door in the bedroom.

  • Tilt
    The angle of inclination in an upwards direction of a source of light from the horizontal level starting position.

  • Timer
    A gadget that will turn lights on or off at pre-set times. Either through its own clock or a countdown.

  • Tinted Glass
    Glass that had a color (any color) pigment added during manufacture rather than being applied after the glass had cooled from its molten state.

  • Toggle
    A common control switch for electric lamps.

  • Torchiere
    A lamp for the floor that sends almost all its emitted light upwards. (See Indirect Lighting).

  • Translucent
    A translucent material allows some light that hits it to pass through and emerge from the other side.

  • Transmission
    The passage of light through an object.

  • Transparent
    A transparent material allows all light that strikes it to pass through it and emerge from the other side.

  • Types (Of Bulb)
    The designation used for bulbs is that of a letter or letters to indicate the shape, and then a number to show the size. In the United States, flashlight, halogen & LED light bulbs are measured in eighths of an inch around their maximum diameter.

  • UL (Underwriters Laboratory)
    A wholly independent organization that apply strict tests to electrical manufacturer’s products. When and if lightbulbs pass the Underwriters Laboratory tests, the makers can designate them as ‘UL Listed’.

  • U-Lamp
    A fluorescent lamp with two ends that is shaped like the letter ‘u’.

  • Utility Lighting
    Often partly made from plastic these are basic and durable unadorned fixtures for everyday usage.

  • Urn
    A wall bracket fixture shaped liked an urn for sending light upwards.

  • Valance Lighting
    This is where the fixture or fixtures are installed behind a horizontal shielding running along a wall or above a window. It is a form of Architectural Lighting.

  • Vanity Lighting
    A light fixture installed above or alongside a mirror.

  • Vapor-Tight Luminaire
    A light fixture that is resistant to entry by vapors or gases to its chamber. Often this applies to water vapor that as steam might cause problems in a kitchen or bathroom.

  • Volts
    The unit of electrical force or pressure that creates current. Named after Alessandro Volta, an Italian scientist who invented the electric battery in 1800.

  • Voltage Drop
    This is the difference in current along a circuit caused by the resistance of the lights and the wire. When using low voltage wiring it can cause lights at the far end from the transformer to be noticeably dimmer than the ones nearer.

  • Wall Brackets
    Light fixtures designed for vertical surfaces such as walls. They are also referred to as Wall Mount Fixtures.

  • Wall Grazing
    A method of lighting where light is aimed down a wall to produce dramatic shadowing.

  • Wall Lighting
    A lighting method where light is ‘bounced’ off walls to give the appearance of more spacious rooms.

  • Wall Washing
    A technique used to bathe a wall in light to give it added emphasis.

  • Watts
    A unit of electrical power. Named after James Watt, a British inventor. 746 watts = 1 horsepower.

  • Wattage
    The amount of electricity used by a light bulb to produce light.

  • Xenon
    A rare gas used in specialized lamps.

  • Yard Lighting
    A general term used for outdoor and garden light fixtures.

  • Zone
    See Channel.

About The Author

Matt Jacks is a successful freelance writer providing tips and advice for consumers purchasing halogen light bulbs, comparing or reviewing remote control cars, boats & planes and even helpdesk software & applications. His numerous articles offer moneysaving tips and valuable insight on typically confusing topics.

This "Glossary of Light Bulb Terms & Definitions" reprinted with permission.

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