Glossary - Lighting
AMPERE (AMP) - The unit used to measure the strength of an
ARC - The luminous discharge of electricity between two
electrodes in HID lighting.
ARC DISCHARGE - A transfer of electricity across two
electrodes (anode and cathode), characterized by high electrode current
densities and a low voltage drop at the electrode.
ARC TUBE - The enclosure which contains the luminous gases
and also houses the arc.
BALLAST - An auxiliary piece of equipment designed to start
and to properly control the flow of power to gas discharge light sources such
as fluorescent and high intensity discharge lamps. In metal halide systems, it
is composed of the transformer, capacitor and connecting wiring; sodium systems
require an igniter in addition to the transformer and capacitor.
BU - An industry code indicating that the bulb is to be
operated only in a base up position.
BULB - The glass outer envelope component of an HID lamp
which protects the arc tube.
BULB WALL TEMPERATURE - The temperature at the bulb wall of
a lamp, which effects lumen output and input wattage and which is important in
CANDELA (CD) - A unit of luminous intensity in a given
direction, equal to one lumen per steradian.
CANDLEPOWER (CP) - The luminous intensity of a light source,
as expressed in candelas.
CANDLEPOWER DISTRIBUTION CURVE - A curve that represents the
varying distribution of luminous intensity of a lamp or luminaire.
CAPACITOR - An electronic device that can store electrical
charge. The capacitor is one of the main components of an HID lighting ballast.
Because they can store a very strong electrical charge, capacitors can be very
dangerous to someone who is unaware of this fact and opens a ballast in order
to examine or repair it. If one does not know how to safely discharge the
stored electricity, one should allow a trained technician to do any ballast
COLD START TIME - The length of time required to bring an
HID lamp to 90% light output from a cold condition.
COLOR TEMPERATURE or KELVIN TEMPERATURE - The unit of
measurement to express the color (spectrum) of light emitted by a lamp; the
absolute temperature of a blackbody radiator having a chromaticity equal to
that of the light source (see correlated color temperature).
CONVERSION BULB - A bulb of a certain spectrum type (e.g.
sodium) specially designed to operate while used in the fixture/ballast of a
different type (e.g. metal halide). The most popular conversion bulbs by far
are sodium conversion bulbs, which allow one to have the sodium spectrum while
still using a metal halide system.
CORRELATED COLOR TEMPERATURE (CCT) - A specification of the
color appearance of a light source, relating its color to that of a blackbody
radiator, as measured in Kelvins (K). CCT is a general measure of a lamp's
"coolness" or "warmness."
DOME - The portion of an HID outer bulb located opposite
base (the neck and threads).
DOME SUPPORT - The spring-like brackets which mount the arc
tube within the outer envelope (bulb).
DISCHARGE LAMP - A lamp that produces light by discharging
an electric arc through a mixture of gases and gaseous metals.
ELECTRODES - Filaments located at either end of a discharge
lamp that maintain an electrical arc between them. See arc discharge.
FIXTURE - The electrical fitting used to contain the
electric components of a lighting system.
FLUORESCENT LAMP - A discharge lamp in which a phosphor
coating transforms ultraviolet energy into visible light. Fluorescent lamps are
good for starting seedlings and rooting cuttings, but do not have enough
intensity to sustain aggressive growth in plants in the later stages of life,
and are not efficient enough in their conversion of electrical power to lumens
of light output.
FOOTCANDLE - A standard measurement of light intensity,
representing the amount of illuminance on a surface one foot square on which
there is a uniformly distributed flux of one lumen. More simply, one footcandle
of illuminance is equal to the light emitted by one candle at a distance of one
FREQUENCY - The number of waves or cycles of electromagnetic
radiation per second, usually measured in Hertz (Hz).
HALOGEN LAMP - A short name for the tungsten-halogen lamp.
Halogen lamps are high pressure incandescent lamps containing halogen gases
such as iodine or bromine which allow the filaments to be operated at higher
temperatures and higher efficacies. While excellent for home lighting and
similar applications, halogen lamps are not effective or efficient as grow
lights due to their limited spectrum and high operating temperatures.
HID - The popular acronym for High Intensity Discharge.
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 various gases and metal salts operating at relatively
high pressures and temperatures.
HIGH-PRESSURE SODIUM LAMP - High-pressure sodium lamps
operate by igniting sodium, mercury and xenon gases within a sealed ceramic arc
tube. Sodium lamps emit light energy in the yellow/red/orange regions of the
spectrum; the red spectrum stimulates flowering and fruit production. Many
indoor gardeners switch to sodium lamps when it is time to induce flowering or
fruiting of their plants.
HOOD - The reflective cover used in conjunction with an HID
lamp. The more reflectivity a hood can provide, the more effective it is.
HOR - An industry code indicating that the bulb is to be
operated in a horizontal position.
HOT SPOT - The area immediately under an HID lamp where the
light intensity is strongest. Hot spots cause uneven growth, but can be
remedied by using light movers.
HOT START TIME - The length of time required to bring an HID
lamp to 90% light output after a short power interruption.
IGNITOR - A component of the ballast necessary for the
starting of the bulb in sodium systems.
ILLUMINANCE - The density of incident luminous flux on a
surface; illuminance is the standard metric for lighting levels, and is
measured in lux (lx) or footcandles (fc).
ILLUMINATION - The act of illuminating or state of being
illuminated. This term is often used incorrectly in place of the term
illuminance to denote the density of luminous flux on a surface.
INCANDESCENT LAMP - A light source which generates light
utilizing a thin filament wire (usually of tungsten) heated to white heat by an
electric current passing through it. Incandescent lamps are the most familiar
type of light source, with countless application in homes, stores and other
commercial settings. Light is produced by passing electric current through a
thin wire filament, usually made of tungsten. Incandescent lamps are totally
ineffective as grow lights; they have very limited spectrum, are very
inefficient in their conversion of electrical power to lumens of light output
(lumen-to-watt ratio). They also put off far too much heat per watt to use in
horticulture, even if the above-mentioned problems did not exist.
INTENSITY - A term referring to the magnitude of light
energy per unit; light intensity diminishes evenly as you get further from the
KELVIN TEMPERATURE (K) - The unit of measurement to express
the color (spectrum) of light emitted by a lamp; the absolute temperature of a
blackbody radiator having a chromaticity equal to that of the light source (see
correlated color temperature). A standard clear metal halide HID lamp has an
average Kelvin temperature rating of 4,000K.
KILOWATT (kW) - A unit of electric power usage equal to
KILOWATT HOUR (kWh) - A measurement of electric energy. A
kilowatt hour is equal to 1,000 watts of power used over a period of one hour.
LAMP - An electrically energized source of light, commonly
called a bulb or tube.
LAMP LIFE - A measure of lamp performance, as measured in
median hours of burning time under ANSI test conditions.
LAMP LUMEN DEPRECIATION (LLD) - The decrease over time of
lamp lumen output, caused by bulb wall blackening, phosphor exhaustion,
filament depreciation, and other factors.
LAMP STARTING - Generic term used to describe a discharge
lamp's starting characteristics in terms of time to come to full output,
LIGHT - Radiant energy which can be sensed or seen by the
human eye. The term generally applied to the visible energy from a source.
Light is usually measured in lumens or candlepower. When light strikes a
surface, it is either absorbed, reflected or transmitted. Visible light is
measured in lumens.
LIGHT MOVER - A motorized device which moves an HID lamp
back and forth across the ceiling of a grow room to provide more even
distribution of the light.
LUMEN - A measurement of light output; refers to the amount
of light emitted by one candle that falls on one square foot of surface located
at a distance of one foot from the candle.
LUMINAIRE - A complete lighting unit, consisting of a lamp
or lamps together with the components required to distribute the light,
position the lamps, and connect the lamps to a power supply. Often referred to
as a "fixture."
LUX - A standard unit of illuminance. One lux is equal to
one lumen per square meter.
METAL HALIDE LAMP - A high-intensity-discharge lamp in which
the light is produced by arcing electricity through a mixture of metal halides.
The light produced by metal halide lamps is in the white-blue spectrum, which
encourages vegetative growth and "bushiness" while discouraging
upward growth. This is the bulb to use in the first, vegetative phase of plant
MERCURY VAPOR LAMPS - The oldest member of the HID family,
mercury vapor lamps work by arcing electricity through mercury vapor. While
more efficient than incandescent, halogen and fluorescent lamps, mercury vapor
lamps have the least efficient lumen-to-watt ratio of the entire HID family.
This, combined with an improper color spectrum for horticultural applications,
makes mercury vapor lamps a poor choice for a grow light.
NECK - The narrow, tubular end of the HID bulb, attached to
PARABOLIC REFLECTOR - A lighting distribution control device
that is designed to redirect the light from an HID lamp in a specific
direction. In most applications, the parabolic device directs light down and
away from the direct glare zone.
PHOTOPERIOD - The relative periods of light and dark periods
within a 24-period. Also referred to as day length.
PHOTOSYNTHESIS - The growth process by which plants build
chemical compounds (carbohydrates) from light energy, water and CO2 (carbon
PHOTOTROPISM - The gravitation of a plant part toward a
REFLECTOR - The term sometimes used to refer to the
reflective hood of an HID lamp.
REFLECTIVITY - The measure of the reflective quality of a
surface; the relative ability of a given surface to reflect light away from it
without absorbing, diffusing or otherwise compromising the light's quality,
intensity and spectrum.
SOCKET - The threaded, wired receptacle that an HID bulb
SODIUM LAMP (HIGH-PRESSURE SODIUM LAMP) - High-pressure
sodium lamps operate by igniting sodium, mercury and xenon gases within a
sealed ceramic arc tube. Sodium lamps emit light energy in the
yellow/red/orange regions of the spectrum; the red spectrum stimulates
flowering and fruit production. Many indoor gardeners switch to sodium lamps
when it is time to induce flowering or fruiting of their plants.
SON-AGRO - A sodium bulb which, according to the
manufacturer, produces 30% more blue light than standard sodium bulbs. The
430-watt SON AGRO also emits 6% more light than the standard 400-watt sodium
SPECULAR REFLECTION - The redirection of incident light
without diffusion at an angle that is equal to and in the same plane as the
angle of incidence.
STERADIAN - A unit solid angle on the surface of a sphere
equal to the square of the sphere's radius.
TRANSFORMER - The component in the ballast that transforms
electric current from one voltage to another.
U (for UNIVERSAL) - An industry code indicating that the
bulb can be operated in any position: horizontal, vertical (base up) or any
ULTRAVIOLET (UV) LIGHT - Light with very short wavelengths,
out of the visible spectrum.
UNDERWRITERS LABORATORIES (UL) - A private organization
which tests and lists electrical (and other) equipment for electrical and fire
safety according to recognized UL and other standards. A UL listing is not an
indication of overall performance.
WATT (W) - A unit used to measure electric power. One watt
equals one joule/second.