Grow Light Glossary
AMPERE (AMP): The unit used to measure the strength
of an electric current.
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 ignitor 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 lighting calculations.
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 repairs.
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 specificastion
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"
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
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
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
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 th e 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 a 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
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 1,000 watts.
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
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 (e.g. Hydrofarm’s Light Track):
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
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 growth.
MERCURY VAPOR LAMPS:
NECK: The narrow, tubular end of the HID bulb,
attached to the threads.
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 daylength.
PHOTOSYNTHESIS: The growth process by which
plants build chemical compounds (carbohydrates) from light energy, water and CO2
PHOTOTROPISM: The gravitation of a plant part
toward a light source.
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 screws into.
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 lamp.
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. The specular inserts included in Hydrofarm’s HID lighting systems
work on this principle.
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
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. offers the only complete line of growlight systems
that is UL-listed.
WATT (W): A unit used to measure electric power.
One watt equals one joule/second.