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 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 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 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 foot.

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 source.

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 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, flicker, etc.

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 growth.

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 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 (carbon dioxide).

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.

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 other.

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.