This glossary will help you understand terms commonly used in the world of lighting.
AVERAGE LIFE - The life expectancy of a lamp, based on laboratory
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.
BASE - The end of the lamp that inserts into the lamp socket.
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.
BURNING POSITION - The position in which a lamp is designed to be
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.
COLOUR TEMPERATURE or KELVIN TEMPERATURE - The unit of measurement to express the colour (spectrum) of light emitted by a lamp.
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
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.
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 light output.
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 very poor spectrum
(extreme far red) and high operating temperatures.
(HID) HIGH-INTENSITY DISCHARGE 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.
(HPS) 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 (in this case relative to bulb and not reflective
material) - 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 or air-cooling the encased hood.
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.
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 light output 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 source.
KELVIN TEMPERATURE (K) - The unit of measurement to express
the colour (spectrum) of light emitted by a lamp; the absolute
temperature of a blackbody radiator having a chromaticity equal to that
of the light source. 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 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 DEPRECIATION (LD) - The decrease over time of lamp
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, flicker, etc.
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 and reduce hotspots.
LUMEN - A measurement of light output; relative to human perception
which 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.
LUMINA IRE - 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."
(MH) 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.
(MV) MERCURY VAPOUR LAMPS - The oldest member of the HID
family, mercury vapour lamps work by arcing electricity through mercury
vapour. While more efficient than incandescent, halogen and fluorescent
lamps, mercury vapour lamps are the least effective of the entire HID
family. This, combined with an improper colour spectrum for
horticultural applications, makes mercury vapour lamps a poor choice
for a grow light.
MOG - Mogul base.
MOL - Maximum overall length of a lamp, from the tip of the base to the top of the bulb.
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 day length.
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.
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.
SPECULA 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 secular inserts included in Hydrofarm’s HID
lighting systems work on this principle.
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 other.
ULTRAVIOLET (UV) LIGHT - Light with very short wavelengths, out of the visible spectrum.
WATT (W) - A unit used to measure electric power. One watt equals one joule/second.