Glossary

Ampere, A
A unit expressing the rate of flow of electric current.

Average rate life
An average rating in hours, indication 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 high intensity discharge (HID) lamps when performing lamp life testing procedure every lamp type has a unique mortality curve that depicts its average rated life.

Ballast
A device used with an electric-discharge lamp to obtain the necessary circuit conditions (voltage, current and waveform) for starting and operating: all fluorescent and high intensity discharge (HID) light sources require a ballast for proper operation.

Ballast factor, BF
The measured ability of a particular ballast to produce light from the
lamp(s) it powers: ballast factor is derived by dividing the lumen output of a particular lamp/ballast combination by the lumen output of the same lamp(s) on a reference ballast.

Candela, cd
The unit of measure indicating the luminous intensity (candlepower) of a light source in a specific direction; any given light source will have many different intensities, depending upon the direction considered.

Candlepower distribution
A curve that repesents the variation in luminous intensity (expressed in candelas) in a plane through the light center of a lamp luminaire: each lamp or lamp/luminaire combination has a unique set of candle power distributions that indicate how light will be spread.

Center beam candlepower, CBCP
The intensity of light produced at the center of a reflector lamp, expressed in candelas.

Color rendering index, CRI
The measure of a light source's ability to render the color of objects "correctly" as compared with a reference source with comparable color temperature. The highest number on this index is 100.

Correlated color temperature, CCT
A specification of the color appearance of a lamp relating its color to that of a reference source heated to a particular temperature, measured in degrees kelvin, expressed in Kelvin(K): CCT generally measures the "warmth" or "coolness" of light source appearance.

Current, I
A measure of the flow of electricity, expressed in amperes(a).

Efficacy
Efficiency of a light source expressed in lumens per watt (LPW or Lm/w).

Energy
A measure of work done by an electrical system over a given period of time often expressed in kilowatt-hours (kWh).

Foot-candle, fc
A unit of illuminance equal to 1 lumen per square foot.

Frequency
The number of times per second that an alternating current system reverses for positive to negative and back to positive, expressed in cycles per second or hertz, Hz)

Glare
Excessive brightness that maybe caused by either direct or indirect viewing of a light source.

Harmonic
An electrical frequency that is an integer multiple of the fundamental frequency; for example, if 60 Hz is the fundamental frequency, then 120 Hz is the second harmonic and 180 Hz is the third harmonic: some electronic devices, such as ballast or power supplies, can cause harmonic distortion, directly affecting power quality.

Illuminance
Light arriving at a surface, expressed in lumens per unit area; 1 lumen per foot equals 1 Foot-candle, while 1 lumen per square meter equals 1 lux.

Light
Radiant energy that is capable of producing a visual sensation.

Light center length, LCL
The distance from a specified reference point on a lamp base to its light center, typical expressed in inches.

Lumen depreciation
The decrease in lumen output of a light source over time; every lamp type has a unique lumen depreciation curve (sometimes called lumen maintenance curve) depicting the pattern of decreasing light output.

Lumen, Lm
A unit of luminous flux: the overall light output of a luminous source is measured in lumens.

Luminaire
A light fixture; the complete lighting unit, including lamp, reflector, ballast, socket, wiring, diffuser, and housing.

Luminance, L
Light reflected in a particular direction; the photometric quantity most closely associated with brightness perception, measured in units of luminous intensity (candelas) per unit area ( square feet or square meters).

Lux, lx
A unit of illuminance equal to 1 lumen per square meter.

Maximum overall length, MOL
The total length of a lamp, from top of bulb to bottom of base, typically expressed in volt-amperes (VA).

Power
The rate at which energy is taken from an electrical system or dissipated by a load, expressed in inches.

Power factor
A measure of the effectiveness with which an electrical device converts volt-amperes to watts; devices with power factors > 9 are "high power factor" devices.

Reflectance
The percentage of light reflected back from a surface, the difference having been absorbed or transmitted by the surface.

Resistance, R
A measure of resistance to flow of current, expressed in ohms.

Spectral power distribution, SPD
A curve illustrating the distribution of power produced by the lamp, at each wavelength across the spectrum.

Voltage, E
A measure of electrical potential, expressed in volts (V).

Wavelength
Distance between two successive points of a periodic wave; the wavelengths of light are typically expressed in nanometers (nm), or billionths of a meter.