Glossary of Grow Light Terms

Air cooled hood – A sealed, reflective hood connected to an exhaust fan using ducting designed to remove much of the heat put out by an HID lamp from the grow room.

Ampere – The standard unit used for measuring the flow of electric current.

Arc – An electric arc or arc discharge, is an electrical breakdown of a gas (ionization) that once started by an igniter, produces a continuous plasma discharge as a resulting of passing a current through a normally nonconductive media. All HID and fluorescent lamps use this effect to generate light.

Array – a grid of LED emitters on a single chip (integrated circuit).

Average bulb life – An average rating in hours, indicating when 50% of a large group of lamps have failed while being operated at nominal voltage and current.

Ballast – A device which provides the necessary starting voltage and appropriate current to a fluorescent or high intensity discharge (HID) luminaire. Magnetic ballasts feature an iron core. They are heavy, oftentimes hum or buzz and are not very efficient. Digital ballasts are smaller, lighter, and more efficient, albeit at a higher cost.

Base type – These come in a wide variety and refer to the bulb attachment and corresponding sockets. Wattage, circumference, type of bulb and locking mechanism (screw in, pins, etc.) are used to determine the proper designation such as E27, Mogul, etc.

Bulb shape – The code number of a bulb consists of a letter or letters followed by a number. The letter indicates the shape of the bulb and the number relates to the diameter of the bulb in eighths of an inch. There are almost a hundred variations.

Candela – The base unit of luminous intensity roughly equivalent to the light put out by a small candle.

Ceramic metal halide (CMH) – This is a relatively new variant of metal halide that uses an extremely hot ceramic tube to ionize various gasses and salts chosen for a specific spectral output. These bulbs are becoming increasingly popular among indoor growers due to their very high CRI of up to 96 featuring much more red than a standard metal halide.

Color rendering index (CRI) – is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal (100 CRI) or natural light source such as the sun.

Compact fluorescent (CFL) – These are generally spiral wound fluorescents with a built-in ballast and use a standard E27 socket. Popular spectrums are 2700K (warm) and 5000K/6500K (cool) with power ranging from 13W to 200W; the most common being 23W.

Coverage – this is the grow area in square feet that a specific lamp will provide optimal growth. This is not a ‘hard’ specification like many of the terms defined thus most manufacturers will claim a higher number than is practical.

Efficacy – A measure expressed in lumens per Watt representing the efficiency of a lamp/ballast system or luminaire.

Einstein – See Micromole.

Foot-Candle – A unit of measure for the density of light as it reaches a surface. One foot-candle is equal to one lumen per square foot.

Full spectrum – A light source that puts emits radiation in all portions of the visible light spectrum which is roughly in the 400nm to 700nm range (violet to deep red).

Gas lantern routine (GLR) – This is a plant lighting schedule designed to speed up growth and reduce energy costs by shortening the amount of time the plants spend in darkness. The lighting schedule is as follows: 12 hours lights on, 5.5 lights off, 1 hour lights on, 5.5 lights off, then repeat. The 1 hour on in between off period fools the plants to stay in vegetative growth state.

Halogen – An incandescent lamp that has a small amount of a halogen such as iodine or bromine added. While more efficient than a standard incandescent, these bulbs do not compare to HIDs in terms of efficiency.

Heat sink – A component or integral part of a ballast or luminaire used to dissipate heat away from sensitive components such as electronics or LEDs. They are usually made of finned aluminum with copper being the most efficient and most expensive; the greater the surface area, the greater the heat dissipation.

High intensity discharge (HID) – This is a class of lamp based on the use of a high power arc and includes halogens, metal halides (MH) and low (LPS) and high pressure sodium (HPS).

High pressure sodium (HPS) – An HID lamp that uses sodium in an excited state to produce light. These lamps are still the preferred luminaire to flower with due to their very high efficacy (up to 150 LPW). While technically a full spectrum lamp in that light is emitted at all frequencies, the spectrums is quite unbalanced with largest output is in the green/yellow/red spectrum (520nm – 640nm) and very little violet/blue/cyan (400nm – 520nm). Common among small-scale growers are the 150W, 250W and 400W models. Large-scale and commercial growers lean towards the 600W and 1000W units.

Hood – This is a structure for HID lamps that contains a reflector, and may also contain a ballast and/or be glass sealed for safety or to allow for air or water cooling.

Horizontal – The orientation of an HID bulb with the long side being parallel to the floor. For a round bulb, the socket would be perpendicular to the floor.

Illuminance – The luminous flux incident on unit area of a surface expressed in lumens per unit area.

Initial Lumens – The lumens produced by a lamp after a burn-in period (usually 100 hours).

Induction – A type of lamp similar to a standard fluorescent except that the sealed gasses are stimulated to produce light by induced current.

Input Watts – The total wattage required by both the ballast and the lamp in a luminaire.

Instant start – A type of ballast that starts by applying high voltage across the lamp with no preheating of the cathode.

Kelvin temperature – The unit of measurement used to express the color spectrum (or average temperature) of light emitted by a lamp. Light with a lower Kelvin rating such as 2700K will have a yellowish tint, while light with a higher Kelvin rating such as 6500K, will have a bluish tint.

KiloWatt – 1000 Watts.

KiloWatt hour – 1000 Watts used continuously for one hour.

LED driver – An electronic device which converts input power into a constant or fixed current source. It protects LEDs from voltage fluctuations as they heat up and/or age.

Lens – A glass or plastic element used in luminaries to seal a fixture or control or focus the exiting light; sometimes used with LEDs to vary the viewing angle.

Life expectancy – The average number of hours a light will last before it drops below 50% of initial lumens. HIDs generally have a life expectancy of 10,000 to 20,000 hours with LEDs lasting up to 50,000 hours.

Light bleaching – When the radiation hitting a leaf is too intense, the chloroplasts will turn white so as to not process any more light and are permanently damaged.

Light emitting diode (LED) – These are semi-conductor devices that put out light when current is run through a substrate. Different substrate materials are used to give us the various color outputs. These may either be monochromatic (single color) narrow band emitters or white (full spectrum). Due to the relatively low heat, long life and ever-increasing efficiency, they are gaining popularity as grow lights.

Light saturation – There is a limit to how much radiation a leaf can process. Saturation is the point at which increasing light intensity will no longer produce an increase in photosynthesis. Going beyond that point results in a burned condition known as light bleaching.

Lumen – the amount of radiant flux as perceived by the human eye. This measurement is not very useful for determining a good grow light as plants ‘see’ light differently. See: Radiant Flux.

Lumens per Watt – The amount of radiation in lumens converted for each Watt of input energy consumed. See: Luminous efficacy.

Luminous efficacy – The conversion rate of input energy into usable light expressed as a percentage.

Luminaire – A complete lighting fixture including bulb, ballast, reflector and mounting attachments.

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

Maintenance Lumens – This is a measurement comparing the amount of light produced from a light source when it is brand new to the amount of light output at a specific time in the future. For instance, if an HPS produced 150,000 lumens of light initially and now produces 100,000 lumens of light after 20,000 hours, then it would have lumen maintenance of 66% at 20,000 hours.

Mean lumens – The average lumen output of a lamp over its rated life. Mean lumen values for fluorescent and HID lamps are typically measured at 40% of their rated lives.

Mercury vapor – The oldest and least efficient member of the HID family, these lamps work by arcing electricity through mercury vapor. Low efficacy combined with an improper color spectrum for horticultural applications, makes mercury vapor lamps a poor choice for a grow light.

Metal halide (MH) – A form of HID lamp that produce light by running an arc current through vaporized mercury and metal halide gasses. These have an efficacy of between 75 to 105 LPW and are quite popular for ‘vegging’ marijuana due to their very high blue content.

Micromole (µmol) – A millionth of a mole (interchangeable with a microeinstein). It is a measurement of the amount of PAR photons striking a square meter per second.

Parabolic reflector – A lighting distribution control device that is designed to redirect the light from a HID lamp in a specific direction.

Phosphor – Substances which emit light after being bombarded by electrons. Phosphors are used to coat the inside of fluorescent or induction lamps. Rare-earth elements are chosen as phosphors to emit color in a specific color band.

Photoperiod -The relative periods of light and dark within a 24 hour period, also referred to as day-length. To keep a marijuana plant in the vegetative state, lights are generally run 18 hours on and 6 hours off or 24 hours on. Switching to a 12 on/12 off light/dark cycle is used to induce flowering.

Photoinhibition – The inhibition of photosynthesis by excess light.

Photosynthetically active radiation (PAR) – Light wavelengths between the 400nm to 700nm range that correspond to the wave band absorbed by photosynthetic pigments.

Photon – A discrete physical unit of radiant energy.

Photoperiod – The amount of time per day that a plant is exposed to light or darkness. This may be used to control various aspects of sexual or vegetative reproductive development, including flowering.

Photosynthesis – The conversion of light energy to chemical energy by photosynthetic pigments using H2O and CO2, and producing carbohydrates.

Photosynthetic Photon Flux Density (PPFD) – Photosynthetically active radiation expressed as the number of photons (in micromoles) striking a square meter every second. This is one of the best measurements for determining the evenness of your lamp over the entire garden as well as the density of the radiation.

Phototropins 1 and 2 – The plant photoreceptors for the blue-light signaling pathway that induces phototropic tracking. See: Phototropism.

Phototropism – The self-orientation of the leaves of a plant (tracking) toward a light source, mostly respondent to the blue part of the spectrum.

Phytochrome – A plant growth-regulation photoreceptor protein that absorbs primarily red light and far-red light, and blue light to a lesser degree.

Photopic lumens — A type of light measured in lumens that is generally detected by common light meters and accounts for part of the human eye’s perception of brightness.

PLL – A type of externally ballasted T5 fluorescent with one or more u-bends or folds. This allows the efficacy of a linear tube in a smaller area.

Power compact fluorescent – See PLL.

Power factor – A measure of the effectiveness with which a ballast, power supply or LED driver converts volt-amperes to Watts with 0.85 to 0.95 being typical. Energy not converted is lost as heat.

Programmed Rapid Start (PS) – A method of starting fluorescent lamps associated with electronic ballasts, where low voltage is applied to the cathode prior to lamp ignition.

Radiant flux – The total amount of radiation put out by a light source taking into account infrared, ultraviolet, and visible light.

Rapid start – A method of starting typically associated with magnetic ballasts wherein a low filament voltage is applied to preheat the cathodes.

Reflector – A polished surface in a housing to collect and focus light that normally would be wasted.

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.

Restrike – Refers to the restarting of a previously operating lamp shortly after turnoff. Metal halide lamps typically require a minimum of 4-15 minutes to restart after turn-off.

Self-ballasted lamps – These are fixtures or bulbs with the ballast built in. It is done for convenience at the cost of efficiency. CFLs are a prime example.

Specular – A highly polished or mirrored aluminum surface with a reflectivity rating ranging from 84% to 98%.

Socket – The threaded, wired receptacle that a bulb screws into.

T5/T8/T12 fluorescent bulbs – “T” stands for tubular, while the second number stands for the number of eights of an inch in diameter. Therefore a T8 lamp would be a Tubular 8/8”, or 1” diameter lamp. T5s (the thinnest bulbs) are the most popular choice for indoor horticulture due to high efficacy and a wide choice of spectral options.

T5 biax bulb: See PLL.

Ultraviolet (UV) – Radiation in the band of light lower in wavelength than that of the human visual range just below the violet spectrum or between 100nm-400nm. UV-C (100nm-280nm) is detrimental to all known life. There is great debate as to whether or not UV-B (280nm-315nm) stimulates higher concentrations of THC in trichomes. UV-A (315nm-400nm) can be used by plants without damage, albeit at a very low absorption rate and thus, is not considered important in a horticultural lamp.

Universal – an HID lamp that can be safely placed in either a horizontal or vertical orientation without shortening the life of the bulb.

Vertical – The placement of an HID bulb with the longer side of the body perpendicular to the floor; often used in a stadium or coliseum-type grow configuration. For a round, single-ended bulb, the globe would be facing the floor.

Viewing angle – the light dispersion pattern of a bulb.HID, incandescent, CFL, and fluorescent all have a viewing angle of 360 degrees; whereas LEDs may have a viewing angle anywhere from 15 degrees to 140 degrees with 45 degrees to 120 degrees being the norm for horticultural purposes.

Water cooled hood – A sealed, reflective hood connected to a chilled recirculating reservoir and water pump outside of the grow room designed to remove much of the heat put out by a high power HID lamp. Due to its density, water cooling is much more effective than air cooling and allows the lamps to be put much closer to your plants.

Watt – A standard unit of measurement equivalent to one joule per second and equal to the power in a circuit in which a current of one ampere flows across a potential difference of one volt.