A grow light or plant light is an artificial light source, generally an electric light

Electric light

Electric lights are a convenient and economic form of artificial lighting which provide increased comfort, safety and efficiency. Most electric lighting is powered by centrally-generated electric power, but lighting may also be powered by mobile or standby electric generators or battery systems...

, designed to stimulate plant growth by emitting an electromagnetic spectrum

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

 appropriate for photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. Grow lights are used in applications where there is either no naturally occurring light, or where supplemental light is required. For example, in the winter months when the available hours of daylight may be insufficient for the desired plant growth, grow lights are used to extend the amount of time the plants receive light.

Grow lights either attempt to provide a light spectrum similar to that from the sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, or to provide a spectrum that is more tailored to the needs of the plants being cultivated. Outdoor conditions are mimicked with varying colour temperatures and spectral outputs from the grow light, as well as varying the lumen output (intensity) of the lamps. Depending on the type of plant being cultivated, the stage of cultivation (e.g., the germination

Germination

Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...

/vegetative phase or the flowering/fruiting phase), and the photoperiod required by the plants, specific ranges of spectrum

Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

, luminous efficacy

Luminous efficacy

Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power. Depending on context, the power can be either the radiant flux of the source's output, or it can be the total electric power consumed by the source.Which sense of the term is...

 and colour temperature are desirable for use with specific plants and time periods.

Typical usage

Grow lights are used for indoor gardening, plant propagation

Plant propagation

Plant propagation is the process of creating new plants from a variety of sources: seeds, cuttings, bulbs and other plant parts. Plant propagation can also refer to the artificial or natural dispersal of plants.-Sexual propagation :...

 and food

Food

Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

 production, including indoor hydroponics

Hydroponics

Hydroponics is a method of growing plants using mineral nutrient solutions, in water, without soil. Terrestrial plants may be grown with their roots in the mineral nutrient solution only or in an inert medium, such as perlite, gravel, mineral wool, or coconut husk.Researchers discovered in the 18th...

 and aquatic plants. Although most grow lights are used on an industrial level, they can also be used in households.

According to the inverse square law, the intensity of light radiating from a point source (in this case a bulb) that reaches a surface is inversely proportional to the square of the surface's distance from the source (if an object is twice as far away, it receives only a quarter the light) which is a serious hurdle for indoor growers, and many techniques are employed to use light as efficiently as possible. Reflector

Reflector

-Science:* Reflector, a device that causes reflection * Reflector , used to control lighting contrast* Reflecting telescope* Reflector , the part of an antenna that reflects radio waves...

s are thus often used in the lights to maximize light efficiency. Plants or lights are moved as close together as possible so that they receive equal lighting and that all light coming from the lights falls on the plants rather than on the surrounding area.

Often, the distance between light and plant is as far as 60 cm (23.6 in) (with incandescent lights) up to 10 cm (4 in) (with other lights as compact, large and high-output fluorescent lights). Many home gardeners cover the walls of their grow-room with a reflective material, or alternatively, white paint to maximize efficiency.

A range of bulb types can be used as grow lights, such as incandescents, fluorescent lights, high-intensity discharge lights, and LED

Light-emitting diode

A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...

s. Today, the most widely used lights for professional use are HIDs and fluorescents. Indoor flower and vegetable growers typically use high pressure sodium (HPS/SON) and metal halide (MH) HID lights, but fluorescents are replacing metal halides due to their efficiency and economy.

Metal halide lights are used for the first (or vegetative) phase of growth as they have a bluish light.

Blue spectrum light may trigger a greater vegetative response in plants.

High pressure sodium lights are used for the second (or reproductive) phase of growth as they have a reddish light.

Red spectrum light may trigger a greater flowering response in plants. If high pressure sodium lights are used for the vegetative phase, plants grow slightly more quickly, but will have longer internodes, and may be longer overall.

Also, MH bulbs with added reddish spectrum and HPS bulbs with added bluish spectrum are also available for fuller spectrum and added flexibility during both vegetative and flowering phases.

Light spectra used

Natural daylight has a high color temperature

Color temperature

Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, and other fields. The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of...

 (approx. 6000 K). Visible light color varies according to the weather, and angle of the Sun, and specific quantities (measured in Lumens) of light stimulate photosynthesis. Distance from the sun has little effect on seasonal changes in the quality and quantity of light and the resulting plant behavior during those seasons. The Earth tilts on its axis as it revolves around the sun. During the summer we get nearly direct sunlight and during the winter we get sunlight at a 23.44 degree angle to the equator. This small tilt of the Earth's axis changes the effective thickness of the atmosphere with respect to the distance sunlight has to travel to reach our particular area on Earth. The color spectrum of light that the sun sends us does not change, only the quantity [more during the summer and less on winter] and quality of overall light reaching us. The color rendering index allows comparison of how closely the light matches the natural color of regular sunlight.
Different stages of plant growth require different spectra. The initial vegetative stage requires blue

Blue

Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...

 spectrum of light, whereas the later "flowering" stage is usually done with red–orange spectra. Light bulbs can be manufactured with a specific spectrum range or can be full spectrum, such as the Sylvania GRO-LUX.

The light is used in conjunction with a reflector to control and intensify the light emissions and will include an electrical ballast

Electrical ballast

An electrical ballast is a device intended to limit the amount of current in an electric circuit. A familiar and widely used example is the inductive ballast used in fluorescent lamps, to limit the current through the tube, which would otherwise rise to destructive levels due to the tube's...

 to convert mains AC to DC, setting the voltage and amps appropriately to power the light.

Luminous efficiency of various light sources

The following table lists luminous efficacy of a source and efficiency for various light sources:

Category	Type	Overall luminous efficacy (lm/W)	Overall luminous efficiency
Combustion	candle Candle A candle is a solid block or cylinder of wax with an embedded wick, which is lit to provide light, and sometimes heat.Today, most candles are made from paraffin. Candles can also be made from beeswax, soy, other plant waxes, and tallow...	0.3	0.04%
	gas mantle Gas mantle An incandescent gas mantle, gas mantle, or Welsbach mantle is a device for generating bright white light when heated by a flame. The name refers to its original heat source, existing gas lights, which filled the streets of Europe and North America in the late 19th century, mantle referring to the...	1–2	0.15–0.3%
Incandescent Incandescent light bulb The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...	100–200 W tungsten incandescent (230 V)	13.8–15.2	2.0–2.2%
	100–200–500 W tungsten glass halogen (230 V)	16.7–17.6–19.8	2.4–2.6–2.9%
	5–40–100 W tungsten incandescent (120 V)	5–12.6–17.5	0.7–1.8–2.6%
	2.6 W tungsten glass halogen (5.2 V)	19.2	2.8%
	tungsten quartz halogen (12–24 V)	24	3.5%
	photographic and projection lights	35	5.1%
Light-emitting diode Light-emitting diode A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...	white LED (raw, without power supply)	4.5–150	–%
	4.1 W LED screw base Edison screw The Edison screw fitting is a system of connectors used for light bulbs, developed by Thomas Edison and licensed starting in 1909 under the Mazda trademark. Most have a right-hand threading, so that it goes in when turned clockwise and comes out when turned counterclockwise, like a hardware screw...  light (120 V)	–	–%
	6.9 W LED screw base light (120 V)	–	–%
	7 W LED PAR20 Parabolic aluminized reflector light A parabolic aluminized reflector lamp is a type of electric lamp that is widely used in commercial, residential, and transportation illumination. Usage includes locomotive headlamps, aircraft landing lights, and residential and commercial recessed lights...  (120 V)		%
	8.7 W LED screw base light (120 V)	–	–%
	Theoretical limit	–	–%
Arc light Arc Light Arc Light is the debut novel by Eric L. Harry, a techno-thriller about limited nuclear war published in 1994 and written in 1991-2.As China and Russia clash in Siberia in June 1999, nuclear missiles strike the United States. The U.S. retaliates against Russia, and World War III begins...	xenon arc light	30–50	4.4–7.3%
	mercury Mercury (element) Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum... -xenon Xenon Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...  arc light	50–55	7.3–8.0%
Fluorescent	T12 tube with magnetic ballast	60	9%
	9–32 W compact fluorescent	46–75	8–11.45%
	T8 tube with electronic ballast	80–100	12–15%
	PL-S 11W U-tube with traditional ballast	82	12%
	T5 tube	70–104.2	10–15.63%
	Spiral tube with electronic ballast	114-124.3	15–18%
Gas discharge	1400 W sulfur light	100	15%
	metal halide light	65–115	9.5–17%
	high pressure sodium light	85–150	12–22%
	low pressure sodium light	100–200	15–29%
Cathodoluminescence Electron stimulated luminescence Electron Stimulated Luminescence is light produced by accelerated electrons hitting a phosphor surface in a process known as cathodoluminescence....	electron stimulated luminescence Electron stimulated luminescence Electron Stimulated Luminescence is light produced by accelerated electrons hitting a phosphor surface in a process known as cathodoluminescence....	30	5%
Ideal sources	Truncated 5800 K blackbody	251	37%
	Green light at 555 nm (maximum possible luminous efficacy)	683.002	100%

Incandescent

Incandescent grow lights have a red-yellowish tone and low color temperature (approx. 2700 K). They are used to highlight indoor plant groupings and not as a true plant 'growing' light (although they may be labeled as such). Incandescent growing lights have an average life span of 750 hours. In addition, they are less energy efficient than fluorescent or high-intensity discharge lights, converting much of the electricity consumed into heat (rather than light).

Fluorescent

Today, fluorescent lights are available in any desired color temperature in the range from 2700 K to 7800 K. Standard fluorescents are usually used for growing vegetables and herbs indoors or for starting seedlings to get a jump start on spring plantings. Standard fluorescents produce twice as many lumens per watt of energy consumed as incandescents and have an average usable life span of up to 20,000 hours. Cool white fluorescent lights are sometimes used as grow lights. These offer slightly lower performance, a white light, and lower purchase cost.

High-output Fluorescent lights produce twice as much light as standard fluorescent lights. A HO fluorescent fixture has a very thin profile, making it extremely useful in vertically limited areas. High-output fluorescents produce about 5,000 lumens per 54 watt bulb and are available in warm (2700 K) and cool (6500 K) versions. Usable life span for high-output fluorescent lights is about 10,000 hours.

Compact Fluorescent lights are smaller versions of fluorescent lights used for propagation, as well as for growing larger plants. Compact fluorescents work in specially designed reflectors that direct light to plants, much like HID lights. Compact fluorescent bulbs are also available in warm/red (2700 K), full spectrum or daylight (5000 K) and cool/blue (6500 K) versions. Usable life span for compact fluorescent grow lights is about 10,000 hours.

High-output fluorescent/high-intensity discharge hybrids combine cool operation with the penetration of high intensity discharge technology. The primary advantages to these fixtures is their blend of light colors and broad even coverage and reduced electric requirements.

High-pressure sodium (HPS)

High-pressure sodium lights yield yellow lighting (2200 K) and have a very poor color rendering index of 22. They are used for the second (or reproductive) phase of the growth. If high-pressure sodium lights are used for the vegetative phase, plants will usually grow slightly more quickly. The major drawback to growing under high-pressure sodium alone is that the plants tend to be taller and leggier, with a longer internodal length than plants grown under metal halide bulbs. High-pressure sodium lights enhance the fruiting and flowering process in plants. Plants use the orange/red spectrum HPS in their reproductive processes, which produces larger harvests of higher quality herbs, vegetables, fruits or flowers. Sometimes the plants grown under these lights do not appear healthy due to the poor color rendering of high-pressure sodium, which makes the plants look pale, washed out or nitrogen starved.

High-pressure sodium lights have a long usable bulb life and six times more light output per watt of energy consumed than a standard incandescent grow light. Due to their high efficiency and the fact that plants grown in greenhouses get all the blue light they need naturally, these lights are the preferred supplemental greenhouse lights. But, in the higher latitudes, there are periods of the year where sunlight is scarce, and additional sources of light are indicated for proper growth. HPS lights may cause distinctive infrared and optical signatures, which can attract insects or other species of pests; these may in turn threaten the plants being grown. High-pressure sodium lights emit a lot of heat, which can cause leggier growth, although this can be controlled by using special air-cooled bulb reflectors or enclosures.

Combination metal halide (MH) and HPS

Combination HPS/MH lights combine a metal halide bulb and a high pressure sodium bulb in the same reflector, either with a single integrated ballast assembly or two separate ballast assemblies. The combination of blue metal halide light and red high pressure sodium light is said by manufacturers to create an ideal spectral blend and extremely high outputs, but in reality it is a compromise on both situations. These types of lights cost more than a standard light and have a shorter life span. Also because they use two smaller lights rather than one larger light the distance that the light penetrates is significantly shorter, in comparison to a regular hid bulb, due to the inverse-square law

Inverse-square law

In physics, an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity....

 of light.

Switchable, convertible, and two-way

Switchable, two-way and convertible lights burn either a metal halide bulb or an equivalent wattage high pressure sodium bulb in the same fixture, but not at the same time. Growers use these fixtures for propagating and vegetatively growing plants under the metal halide, then switching to a high pressure sodium bulb for the fruiting or flowering stage of plant growth. To change between the lights, only the bulb needs changing and a switch needs to be set to the appropriate setting. These are commonly known as conversion bulbs and usually a metal halide conversion bulb will be used in an HPS ballast since the MH conversion bulbs are more common.

LED

Recent advancements in LED

Light-emitting diode

A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...

s allow production of relatively inexpensive, bright, and long-lasting grow lights that emit only the wavelengths of light corresponding to the absorption peaks of a plant's typical photochemical processes. Compared to other types of grow lights, LEDs are attractive to indoor growers since they consume much less electrical power, do not require ballasts, and produce considerably less heat. This allows LEDs to be placed closer to the plant canopy than other lights. Also, plants transpire less, as a result of the reduction in heat, and thus the time between watering cycles is longer.

There are multiple absorption peaks

Photosynthetically active radiation

Photosynthetically active radiation, often abbreviated PAR, designates the spectral range of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to...

 for chlorophyll and carotenoids, and LED grow-lights may use one or more LED colors overlapping these peaks.

Recommendations for optimal LED designs vary widely. According to one source, to maximize plant growth and health using available and affordable LEDs, U.S. patent #6921182 from July 2005 claims that "the proportion of twelve red 660 nm LEDs, plus six orange 612 nm LEDs and one blue 470 nm LED was optimal", such that the ratio of blue light to red & orange light is 6-8%.

It is also often published that for vegetative growth, blue LEDs are preferred, where the light has a wavelength somewhere in the mid-400 nm (nanometers). For growing fruits or flowers, a greater proportion of deep-red LEDs is considered preferable, with light very near 660 nm, the exact number this wavelength being much more critical than for the blue LED.

Early LED grow lights used hundreds of fractional-watt LEDs and were often not bright enough and/or efficient enough to be effective replacements for HID lights. Newer advanced LED grow lights may use high-brightness multiple-watt LEDs, with growing results similar to HID lights.

Grow light LEDs are increasing in power consumption resulting in increased effectiveness of the technology. LEDs used in previous designs were 1 watt in power, however 3 watt and even 5 watt LEDs are now commonly used in LED grow lights. LED grow lights are now being produced which exceed 600 watts.

Light requirements of plants

The plants' specific needs determine which lighting is most appropriate for optimum growth; artificial light must mimic the natural light to which the plant is best adapted. The bigger the plant gets the more light it requires; if there is not enough light, a plant will not grow, regardless of other conditions.

For example, vegetables grow best in full sunlight, and to flourish indoors they need equally high light levels; thus fluorescent lights or MH-lights are best. Foliage plants (e.g., Philodendron

Philodendron

Philodendron is a large genus of flowering plants in the Araceae family, consisting of close to 900 or more species according to TROPICOS . Other sources quote different numbers of species. According to S.J. Mayo there are about 350-400 formally recognized species whereas according to Croat there...

) grow in full shade and can grow normally with much lower light levels, thus regular incandescents may suffice.

In addition, plants also require both dark and light ("photo"-) periods. Therefore, lights may be be turned on or off at set times

Time switch

A time switch is a timer that plugs into an electric socket, between the socket itself and a power plug. This physical arrangement allows the connected device to automatically receive power for a desired duration of time...

. The optimum photo/dark period ratio depends on the species and variety of plant, as some prefer long days and short nights and others prefer the opposite or intermediate "day lengths".

Illuminance, or luminous flux density, measured in lux

Lux

The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...

 is an important factor in indoor growing. Illuminance is the amount of light incident on a surface. One lux equals one lumen

Lumen (unit)

The lumen is the SI derived unit of luminous flux, a measure of the total "amount" of visible light emitted by a source. Luminous flux differs from power in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux...

 of light falling on an area of one square meter (lm/m²), which is approximately 0.093 foot-candle (lm/ft²). A brightly lit office would be illuminated at about 400 lux.

Lux are photometric units, in that different wavelengths of light are weighted by the eye's response to them. In professional farming, radiometric (watt/metre² or microeinstein

Einstein (unit)

An einstein is a unit used in irradiance and in photochemistry. One einstein is defined as one mole of photons, regardless of their frequency. Therefore, the number of photons in an einstein is Avogadro's number, 6.022×1023. Irradiance might be measured in einsteins per square metre per second, if...

 /second·meter²) or photosynthetically active radiation

Photosynthetically active radiation

Photosynthetically active radiation, often abbreviated PAR, designates the spectral range of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to...

 weighted (PAR watt) units are used instead.

See also

• Chlorophyll
  Chlorophyll
  Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
  
  
• Indoor plant cultivation
• Vertical farming
  Vertical farming
  Vertical farming is a concept that argues that it is economically and environmentally viable to cultivate plant or animal life within skyscrapers, or on vertically inclined surfaces...
  
  
• Biochar
  Biochar
  Biochar or terra preta is charcoal created by pyrolysis of biomass. Biochar is under investigation as an approach to carbon sequestration via bio-energy with carbon capture and storage. Biochar thus has the potential to help mitigate climate change, via carbon sequestration...
  
  
• Photosynthetically active radiation
  Photosynthetically active radiation
  Photosynthetically active radiation, often abbreviated PAR, designates the spectral range of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to...