![Aspect Black 20W Growlight over Bonsai](https://upload.wikimedia.org/wikipedia/commons/8/81/Aspect_Black_20W_Growlight_over_Bonsai.jpg)
A grow light is an
electric light
An electric light, lamp, or light bulb is an electrical component that produces light. It is the most common form of artificial lighting. Lamps usually have a base made of ceramic, metal, glass, or plastic, which secures the lamp in the soc ...
to help plants grow. Grow lights either attempt to provide a
light spectrum
The electromagnetic spectrum is the range of frequency, frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energy, photon energies.
The electromagnetic spectrum covers electromagnetic waves with f ...
similar to that of the sun, or to provide a spectrum that is more tailored to the needs of the plants being cultivated (typically a varying combination of red and blue light, which generally appears
pink
Pink is the color of a namesake flower that is a pale tint of red. It was first used as a color name in the late 17th century. According to surveys in Europe and the United States, pink is the color most often associated with charm, politeness, ...
to
purple
Purple is any of a variety of colors with hue between red and blue. In the RGB color model used in computer and television screens, purples are produced by mixing red and blue light. In the RYB color model historically used by painters, pu ...
to the human eye). Outdoor conditions are mimicked with varying colour temperatures and spectral outputs from the grow light, as well as varying the intensity of the lamps. Depending on the type of plant being cultivated, the stage of cultivation (e.g. the
germination
Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...
/vegetative phase or the flowering/fruiting phase), and the
photoperiod
Photoperiodism is the physiological reaction of organisms to the length of night or a dark period. It occurs in plants and animals. Plant photoperiodism can also be defined as the developmental responses of plants to the relative lengths of light a ...
required by the plants, specific ranges of
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
,
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, measured in lumens per watt in the International System of Units (SI). Depending on context, the power can be either the ...
and
color temperature
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
are desirable for use with specific plants and time periods.
Typical use
Grow lights are used for horticulture, indoor gardening,
plant propagation
Plant propagation is the process by which new plants grow from a variety of sources: seeds, cuttings, and other plant parts. Plant propagation can also refer to the man-made or natural dispersal of seeds.
Propagation typically occurs as a step i ...
and
food
Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is inge ...
production, including indoor
hydroponics
Hydroponics is a type of horticulture and a subset of hydroculture which involves growing plants, usually crops or medicinal plants, without soil, by using water-based mineral nutrient solutions in aqueous solvents. Terrestrial or aquatic plan ...
and
aquatic plants
Aquatic plants are plants that have adapted to living in aquatic environments (saltwater or freshwater). They are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that ...
. Although most grow lights are used on an industrial level, they can also be used in households.
According to the
inverse-square law
In science, an inverse-square law is any scientific law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understo ...
, 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. Reflectors 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.
A range of bulb types can be used as grow lights, such as
incandescent
Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence is ...
s,
fluorescent light
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...
s,
high-intensity discharge lamp
High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tu ...
s (HID), and
light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
s (LED). Today, the most widely used lights for professional use are HIDs and fluorescents. Indoor flower and vegetable growers typically use
high-pressure sodium
A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm.
Two varieties of such lamps exist: low pressure and high pressure. Low-pressure sodium lamps are ...
(HPS/SON) and
metal halide
Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, s ...
(MH) HID lights, but fluorescents an
LEDs are replacing metal halides due to their efficiencyand economy.
Metal halide lights are regularly used for the vegetative phase of plant growth, as they emit larger amounts of blue and ultraviolet radiation. With the introduction of ceramic metal halide lighting and full-spectrum metal halide lighting, they are increasingly being utilized as an exclusive source of light for both vegetative and reproductive growth stages. Blue spectrum light may trigger a greater vegetative response in plants.
High-pressure sodium lights are also used as a single source of light throughout the vegetative and reproductive stages. As well, they may be used as an amendment to
full-spectrum light Full-spectrum light is light that covers the electromagnetic spectrum from infrared to near-ultraviolet, or all wavelengths that are useful to plant or animal life; in particular, sunlight is considered full spectrum, even though the solar spectral ...
ing during the reproductive stage. 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.
In recent years LED technology has been introduced into the grow light market. By designing an indoor grow light using diodes, specific wavelengths of light can be produced. NASA has tested LED grow lights for their high efficiency in growing food in space for
extraterrestrial colonization. Findings showed that plants are affected by light in the red, green and blue parts of the visible light spectrum.
Common types by generation
Fourth generation: Light emitting diodes (LEDs)
LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
light is regarded as the fourth generation of light sources.
LED lights produce the highest
photosynthetically active radiation Photosynthetically active radiation (PAR) designates the spectral range (wave band) 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 o ...
(PAR) of any light.
LED grow lights are usually composed of multiple individual light-emitting diodes in a casing with a
heat sink
A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, th ...
and built-in fans. Most or all LEDs use AC/DC or DC/DC
power supplies
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a res ...
that provide constant
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
through the LEDs, regulating the amount of total power the LEDs can draw and preventing their failure.
Individual LEDs usually provide only a single narrow range of colors, and so different color LEDs are mixed in grow lights in proportions depending on the intended use. It is known from the study of
photomorphogenesis In developmental biology, photomorphogenesis is light-mediated development, where plant growth patterns respond to the light spectrum. This is a completely separate process from photosynthesis where light is used as a source of energy. Phytochromes ...
that green, red, far-red and blue light spectra have an effect on root formation,
plant growth Important structures in plant development are buds, shoots, roots, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living plant a ...
, and flowering, but there are not enough scientific studies or field-tested trials using LED grow lights to recommend specific color ratios for optimal plant growth under LED grow lights. It has been shown that many plants can grow normally if given both red and blue light.
However, many studies indicate that red and blue light (pink to purple depending on the red to blue ratio) only provides the most cost efficient method of growth, plant growth is still better under light supplemented with green.
White LED grow lights provide a full spectrum of light designed to mimic natural light, providing plants a spectrum of red, blue and green. White light is rated on a spectrum in terms of
Color temperature
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
where cooler lights produce more blue photons and warmer lights produce more red photons.
A large number of plant species have been assessed in greenhouse trials to make sure plants have higher quality in biomass and biochemical ingredients even higher or comparable with field conditions. Plant performance of mint, basil, lentil, lettuce, cabbage, parsley, carrot were measured by assessing health and vigor of plants and success in promoting growth. Promoting in profuse flowering of select ornamentals including primula, marigold, stock were also noticed.
In tests conducted by Philips Lighting on to find an optimal light recipe for growing various vegetables in greenhouses, they found that the following aspects of light affects both plant growth (photosynthesis) and plant development (morphology): light intensity, total light over time, light at which moment of the day, light/dark period per day, light quality (spectrum), light direction and light distribution over the plants. However it's noted that in tests between tomatoes, mini cucumbers and bell peppers, the optimal light recipe was not the same for all plants, and varied depending on both the crop and the region, so currently they must optimize LED lighting in greenhouses based on trial and error. They've shown that LED light affects disease resistance, taste and nutritional levels, but as of 2014 they haven't found a practical way to use that information.
The diodes used in initial LED grow light designs were usually 1/3 watt to 1 watt in power. However, higher wattage diodes such as 3 watt and 5 watt diodes are now commonly used in LED grow lights. For highly compacted areas, COB chips between 10 watts and 100 watts can be used. Because of heat dissipation, these chips are often less efficient. Standard LED lighting usually has a
power factor
In electrical engineering, the power factor of an AC power system is defined as the ratio of the '' real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of v ...
of at least 0.90, while good quality LED lights will be around 0.99.
To prevent leaf burn, LED grow lights should be kept between away from plants for lower wattage lamps (under 300 watts) up to away from plants for higher wattage lamps (1000 watts or more).
Historically, LED lighting was very expensive, but costs have greatly reduced over time, and their longevity has made them more popular. LED grow lights are often priced higher, watt-for-watt, than other LED lighting, due to design features that help them to be more energy efficient and last longer. In particular, because LED grow lights are relatively high power, LED grow lights are often equipped with cooling systems, as low temperature improves both the brightness and longevity. LEDs usually last for 10,000 - 50,000 hours until
LM-70 is reached.
Third generation: High-intensity discharge (HID) lamps
High-intensity discharge lamp
High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tu ...
is regarded as the third generation of light sources.
Fluorescent lighting was formerly the most common type of indoor grow light but
HID lights have surpassed them.
High intensity discharge lamps have a high lumen-per-watt efficiency.
There are several different types of HID lights including mercury vapor, metal halide, high pressure sodium and conversion bulbs. Metal halide and HPS lamps produce a color spectrum that is somewhat comparable to the sun and can be used to grow plants. Mercury vapor lamps were the first type of HIDs and were widely used for street lighting, but when it comes to indoor gardening they produce a relatively poor spectrum for plant growth so they have been mostly replaced by other types of HIDs for growing plants.
All HID grow lights require an
electrical ballast
An electrical ballast is a device placed in series with a load to limit the amount of electric current, current in an electrical network, electrical circuit.
A familiar and widely used example is the inductive ballast used in fluorescent lamp ...
to operate, and each ballast has a particular power rating. Popular HID ratings include 150W, 250W, 400W, 600W and 1000W. 600W HID lights are the most electrically efficient as far as light produced, followed by 1000W. A 600W HPS produces 7% more light (lumen-per-watt) than a 1000W HPS.
Although all HID lamps work on the same principle, the different types of bulbs have different starting and voltage requirements, as well as different operating characteristics and physical shape. Because of this a bulb won't work properly without a matching ballast, even if the bulb will physically screw in. In addition to producing lower levels of light, mismatched bulbs and ballasts will stop working early, or may even burn out immediately.
Metal halide (MH)
Metal halide
Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, s ...
bulbs are a type of HID light that emit light in the blue and violet parts of the light spectrum, which is similar to the light that is available outdoors during spring.
Because their light mimics the color spectrum of the sun, some growers find that plants look more pleasing under a metal halide than other types of HID lights such as the HPS which distort the color of plants. Therefore, it's more common for a metal halide to be used when the plants are on display in the home (for example with ornamental plants) and natural color is preferred. Metal halide bulbs need to be replaced about once a year, compared to HPS lights which last twice as long.
Metal halide lamps are widely used in the horticultural industry and are well-suited to supporting plants in earlier developmental stages by promoting stronger roots, better resistance against disease and more compact growth.
The blue spectrum of light encourages compact, leafy growth and may be better suited to growing vegetative plants with much foliage.
A metal halide bulb produces 60-125 lumens/watt, depending on the wattage of the bulb.
They are now being made for digital ballasts in a pulse start version, which have higher electrical efficiency (up to 110 lumens per watt) and faster warmup.
One common example of a pulse start metal halide is the ceramic metal halide (CMH). Pulse start metal halide bulbs can come in any desired spectrum from cool white (7000 K) to warm white (3000 K) and even ultraviolet-heavy (10,000 K).
Ceramic metal halide (CMH)
Ceramic metal halide (CMH) lamps are a relatively new type of HID lighting, and the technology is referred to by a few names when it comes to grow lights, including ceramic discharge metal halide (CDM),
ceramic arc metal halide.
Ceramic metal halide lights are started with a pulse-starter, just like other "pulse-start" metal halides.
The discharge of a ceramic metal halide bulb is contained in a type of ceramic material known as
polycrystalline alumina (PCA), which is similar to the material used for an HPS. PCA reduces sodium loss, which in turn reduces color shift and variation compared to standard MH bulbs.
Horticultural ceramic metal halide offerings from companies such as Philips have proven to be effective sources of growth light for medium-wattage applications.
Combination MH and HPS ("Dual arc")
Combination HPS/MH lights combine a metal halide and a high-pressure sodium in the same bulb, providing both red and blue spectrums in a single HID lamp. The combination of blue metal halide light and red high-pressure sodium light is an attempt to provide a very wide spectrum within a single lamp. This allows for a single bulb solution throughout the entire life cycle of the plant, from vegetative growth through flowering. There are potential tradeoffs for the convenience of a single bulb in terms of yield. There are however some qualitative benefits that come for the wider light spectrum.
High-pressure sodium (HPS)
High-pressure sodium lights are a more efficient type of HID lighting than metal halides. HPS bulbs emit light in the yellow/red visible light as well as small portions of all other visible light. Since HPS grow lights deliver more energy in the red part of the light spectrum, they may promote blooming and fruiting.
They are used as a supplement to natural daylight in greenhouse lighting and metal halide or, as a standalone source of light for indoors/grow chambers.
HPS grow lights are sold in the following sizes: 150W, 250W, 400W, 600W and 1000W.
Of all the sizes, 600W HID lights are the most electrically efficient as far as light produced, followed by 1000W. A 600W HPS produces 7% more light (watt-for-watt) than a 1000W HPS.
![SON-T Master 600W](https://upload.wikimedia.org/wikipedia/commons/9/91/SON-T_Master_600W.jpg)
An HPS bulb produces 60-140 lumens/watt, depending on the wattage of the bulb.
![Grow lights over the pitch at Tottenham Hotspur Stadium](https://upload.wikimedia.org/wikipedia/commons/2/21/Grow_lights_over_the_pitch_at_Tottenham_Hotspur_Stadium.jpg)
Plants grown under HPS lights tend to elongate from the lack of blue/ultraviolet radiation. Modern horticultural HPS lamps have a much better adjusted spectrum for plant growth. The majority of HPS lamps while providing good growth, offer poor
color rendering index
A color rendering index (CRI) is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with a natural or standard light source. Light sources with a high CRI are desirable in ...
(CRI) rendering. As a result, the yellowish light of an HPS can make monitoring plant health indoors more difficult. CRI isn't an issue when HPS lamps are used as supplemental lighting in greenhouses which make use of natural daylight (which offsets the yellow light of the HPS).
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.
Conversion bulbs
Conversion bulbs are manufactured so they work with either a MH or HPS ballast. A grower can run an HPS conversion bulb on a MH ballast, or a MH conversion bulb on a HPS ballast. The difference between the ballasts is an HPS ballast has an igniter which ignites the sodium in an HPS bulb, while a MH ballast does not. Because of this, all electrical ballasts can fire MH bulbs, but only a Switchable or HPS ballast can fire an HPS bulb without a conversion bulb. Usually a metal halide conversion bulb will be used in an HPS ballast since the MH conversion bulbs are more common.
Switchable ballasts
A switchable ballast is an HID ballast can be used with either a metal halide or an HPS bulb of equivalent wattage. So a 600W Switchable ballast would work with either a 600W MH or HPS.
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.
Second generation: Fluorescent
Fluorescent lamp
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...
is regarded as the second generation of light sources.
Fluorescent lights come in many form factors, including long, thin bulbs as well as smaller spiral shaped bulbs (compact fluorescent lights). Fluorescent lights are available in color temperatures ranging from 2700 K to 10,000 K. The
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, measured in lumens per watt in the International System of Units (SI). Depending on context, the power can be either the ...
ranges from 30 lm/W to 90 lm/W. The two main types of fluorescent lights used for growing plants are the tube-style lights and compact fluorescent lights.
Tube-style fluorescent lights
Fluorescent grow lights are not as intense as HID lights and are usually used for growing
vegetables
Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the flowers, fruits, stems, ...
and herbs indoors, or for starting seedlings to get a jump start on spring plantings. A ballast is needed to run these types of fluorescent lights.
Standard fluorescent lighting comes in multiple form factors, including the T5, T8 and T12. The brightest version is the T5. The T8 and T12 are less powerful and are more suited to plants with lower light needs. High-output fluorescent lights produce twice as much light as standard fluorescent lights. A high-output fluorescent fixture has a very thin profile, making it useful in vertically limited areas.
Fluorescents have an average usable life span of up to 20,000 hours. A fluorescent grow light produces 33-100 lumens/watt, depending on the form factor and wattage.
Compact fluorescent lights (CFLs)
![Compact-Fluorescent-Bulb](https://upload.wikimedia.org/wikipedia/commons/c/c3/Compact-Fluorescent-Bulb.jpg)
Compact Fluorescent lights (CFLs) are smaller versions of fluorescent lights that were originally designed as pre-heat lamps, but are now available in rapid-start form. CFLs have largely replaced
incandescent light bulb
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxida ...
s in households because they last longer and are much more electrically efficient.
In some cases, CFLs are also used as grow lights. Like standard fluorescent lights, they are useful for propagation and situations where relatively low light levels are needed.
While standard CFLs in small sizes can be used to grow plants, there are also now CFL lamps made specifically for growing plants. Often these larger compact fluorescent bulbs are sold with specially designed reflectors that direct light to plants, much like HID lights. Common CFL grow lamp sizes include 125W, 200W, 250W and 300W.
Unlike HID lights, CFLs fit in a standard mogul light socket and don't need a separate ballast.
Compact fluorescent bulbs are available in warm/red (2700 K), full spectrum or daylight (5000 K) and cool/blue (6500 K) versions. Warm red spectrum is recommended for flowering, and cool blue spectrum is recommended for vegetative growth.
Usable life span for compact fluorescent grow lights is about 10,000 hours.
A CFL produces 44-80 lumens/watt, depending on the wattage of the bulb.
Examples of lumens and lumens/watt for different size CFLs:
Cold cathode fluorescent light (CCFL)
A cold cathode is a
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
that is not electrically heated by a
filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by
thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mate ...
alone. It is used in
gas-discharge lamps, such as
neon lamp
A neon lamp (also neon glow lamp) is a miniature gas discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and Penning mixture, other gases at a low pressure and two electrodes (an anode and a cold ...
s,
discharge tube
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric d ...
s, and some types of
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
. The other type of cathode is a
hot cathode
In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element. ...
, which is heated by electric current passing through a
filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its
operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
by other methods, such as the current passing from the cathode into the gas.
First generation: Incandescent light bulbs
Incandescent light bulb
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxida ...
is regarded as the first generation of light sources.
Light requirements of plants
The quantity, quality, and duration of light regulate plant growth and development. In general, if a plant does not get enough light, it will become stunted, have reduced pigmentation, or begin shade-avoidance response. A plant that does not receive the right quality of light may exhibit
physiological differences when compared to the same plants grown under optimal lighting conditions.
Grow light quantity and quality has been technologically limited in the past.
High pressure sodium
A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm.
Two varieties of such lamps exist: low pressure and high pressure. Low-pressure sodium lamps are ...
(HPS) and
metal halide
Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, s ...
(MH) were and are still common supplemental lighting options for greenhouses and some
sole-source operations. Older
LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
grow lights composed solely of blue and red LEDs due to both their efficiency at converting electricity to photons and efficiency at driving photosynthesis. As LEDs become less expensive and more efficient, an interest in studying light quality has risen in the field of plant science.
Light Quantity
Light quantity refers to the amount of light a plant requires each day for optimal growth. Historically, light quantity was expressed in units of W m
−2,
lumens, or
lux. While these units are useful in energy calculations, W m
−2, or in human lighting (lumens and lux), plant scientists now prefer to measure the
photosynthetic photon flux density (PPFD), in units of μmol m
−2s
−1. PPFD is an explicit measure of the quantity of photons hitting a surface per square meter per second, a more accurate way to measure how plants interact with photons.
Another useful way to measure light quantity is through the
daily light integral, or DLI. The DLI takes into account the PPFD and the total number of hours a plant is exposed to that PPFD to get the total quantity of photons per day, in units of mol m
−2d
−1. The equation for converting PPFD to DLI, assuming constant PPFD, is below.
DLI (mol m−2d−1) =0.0036 * PPFD (μmol m−2s−1) *Hours of Light
![HortiPower Smart Spectrum Meter](https://upload.wikimedia.org/wikipedia/commons/0/08/HortiPower_Smart_Spectrum_Meter.png)
The light quantity requirements for crops vary, in general the light requirement for a specific crop is greater for crops that are fruiting and flowering and is less for crops that remain vegetative. Leafy greens such as
lettuce
Lettuce (''Lactuca sativa'') is an annual plant of the family Asteraceae. It is most often grown as a leaf vegetable, but sometimes for its stem and seeds. Lettuce is most often used for salads, although it is also seen in other kinds of food, ...
,
spinach
Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to central and western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common edible vegetable consumed either f ...
, and
kale
Kale (), or leaf cabbage, belongs to a group of cabbage (''Brassica oleracea'') cultivars grown for their edible leaves, although some are used as ornamentals. Kale plants have green or purple leaves, and the central leaves do not form a head ...
are typically considered low-light crops, requiring a DLI between 12 and 17 mol m
−2d
−1.
Tomato
The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...
es,
cucumber
Cucumber (''Cucumis sativus'') is a widely-cultivated Vine#Horticultural climbing plants, creeping vine plant in the Cucurbitaceae family that bears usually cylindrical Fruit, fruits, which are used as culinary vegetables. s, and
peppers
Pepper or peppers may refer to:
Food and spice
* Piperaceae or the pepper family, a large family of flowering plant
** Black pepper
* ''Capsicum'' or pepper, a genus of flowering plants in the nightshade family Solanaceae
** Bell pepper
** Chili ...
require between 20-30 mol m
−2d
−1.
Cannabis
''Cannabis'' () is a genus of flowering plants in the family Cannabaceae. The number of species within the genus is disputed. Three species may be recognized: ''Cannabis sativa'', '' C. indica'', and '' C. ruderalis''. Alternatively ...
has one of the highest light requirements of cultivated plants, requiring a DLI of up to 40 mol m
−2d
−1.
Light Quality
![Chlorophyll ab spectra-en](https://upload.wikimedia.org/wikipedia/commons/2/23/Chlorophyll_ab_spectra-en.svg)
Light quality refers to the spectral
distribution of light given to a plant. Light quality is grouped into colors based on wavelength; 320-400 nanometers (nm) is
UVA
UVA most often refers to:
* Ultraviolet A, a type of ultraviolet radiation
* University of Virginia, a public research university in Charlottesville, Virginia, United States
Uva or UVA may also refer to:
Places
* Uva, Missouri, an unincorpora ...
, 400-500 nm is
blue
Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when obs ...
, 500-600 nm is
green
Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 Nanometre, nm. In subtractive color systems, used in painting and color printing, it is created by ...
, 600-700 nm is
red
Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
, and 700-750 nm is
far red, sometimes referred to as
near-infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
. Light quality can also be expressed as ratios, e.g. 3:2 red:blue ratio, or sometimes as their peak irradiance, e.g. 450 nm blue light and 660 nm red light.
Photomorphogenesis In developmental biology, photomorphogenesis is light-mediated development, where plant growth patterns respond to the light spectrum. This is a completely separate process from photosynthesis where light is used as a source of energy. Phytochromes ...
is the term for light-mediated plant responses to light spectrum. Plants are able to sense parts of the
electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
through a network of
photoreceptors including
phytochrome
Phytochromes are a class of photoreceptor in plants, bacteria and fungi used to detect light. They are sensitive to light in the red and far-red region of the visible spectrum and can be classed as either Type I, which are activated by far-re ...
s,
cryptochrome
Cryptochromes (from the Greek κρυπτός χρώμα, "hidden colour") are a class of flavoproteins found in plants and animals that are sensitive to blue light. They are involved in the circadian rhythms and the sensing of magnetic fields i ...
s,
phototropin
Phototropins are photoreceptor proteins (more specifically, flavoproteins) that mediate phototropism responses in higher plants. Phototropins can be found throughout the leaves of a plant. Along with cryptochromes and phytochromes they allow plants ...
, and
zeiltupe. Each receptor is able to sense different parts of the electromagnetic spectrum. Information about the light spectrum can affect seed germination, the signal to transition from vegetative to flowering, and the production of
secondary metabolite
Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norm ...
s such as
anthocyanin
Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical compo ...
s.
Photoperiodism
In addition, many plants also require both dark and light periods, an effect known as
photoperiodism
Photoperiodism is the physiological reaction of organisms to the length of night or a dark period. It occurs in plants and animals. Plant photoperiodism can also be defined as the developmental responses of plants to the relative lengths of light a ...
, to trigger flowering. Therefore, lights may be turned on or off at set
times
Time is the continued sequence of existence and events, and a fundamental quantity of measuring systems.
Time or times may also refer to:
Temporal measurement
* Time in physics, defined by its measurement
* Time standard, civil time specific ...
. 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".
Much emphasis is placed on photoperiod when discussing plant development. However, it is the number of hours of darkness that affects a plant’s response to day length.
In general, a “short-day” is one in which the photoperiod is no more than 12 hours. A “long-day” is one in which the photoperiod is no less than 14 hours. Short-day plants are those that flower when the day length is less than a critical duration. Long-day plants are those that only flower when the photoperiod is greater than a critical duration. Day-neutral plants are those that flower regardless of photoperiod.
Plants that flower in response to photoperiod may have a facultative or obligate response. A facultative response means that a plant will eventually flower regardless of photoperiod, but will flower faster if grown under a particular photoperiod. An obligate response means that the plant will only flower if grown under a certain photoperiod.
Photosynthetically Active Radiation (PAR)
![Photosynthesis yield photon flux spectral weighting](https://upload.wikimedia.org/wikipedia/commons/f/f8/Photosynthesis_yield_photon_flux_spectral_weighting.svg)
Lux and lumens are commonly used to measure light levels, but they are
photometric units which measure the intensity of light as perceived by the human eye.
The spectral levels of light that can be used by plants for photosynthesis is similar to, but not the same as, what is measured by lumens. Therefore, when it comes to measuring the amount of light available to plants for
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
, biologists often measure the amount of
photosynthetically active radiation Photosynthetically active radiation (PAR) designates the spectral range (wave band) 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 o ...
(PAR) received by a plant.
PAR designates the spectral range of solar radiation from 400 to 700
nanometer
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
s, which generally corresponds to the spectral range that photosynthetic organisms are able to use in the process of
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
.
The irradiance of PAR can be expressed in units of energy flux (W/m
2), which is relevant in energy-balance considerations for photosynthetic
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s. However, photosynthesis is a quantum process and the chemical reactions of photosynthesis are more dependent on the ''number'' of photons than the amount of energy contained in the photons.
Therefore, plant biologists often quantify PAR using the number of photons in the 400-700 nm range received by a surface for a specified amount of time, or the Photosynthetic Photon Flux Density (PPFD).
This is normally measured using mol m
−2s
−1, but the value relevant for plant growth is the
Daily light integral (DLI), the PPFD integrated over 24 hours. Most plant species will grow well with a DLI of 5-15 mol m
−2 day
−1. Shade-tolerant species can grow with DLI values of 1-3 mol m
−2 day
−1, light-demanding species easily handle 30-50 mol m
−2 day
−1.
Society and culture
See also
*
Andrei Famintsyn
Andrei Sergeyevich Famintsyn (russian: Андрей Серге́евич Фаминцын; June 29 ( O.S. June 17), 1835 in Moscow – December 8, 1918 in Petrograd) was a Russian botanist, public figure, and academician of the Petersburg Academy ...
-
Russia
Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
n
botanist
Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek wo ...
, first to use artificial light for plant growing and research (1868)
*
Daily light integral
*
PI curve
The PI (or photosynthesis-irradiance) curve is a graphical representation of the empirical relationship between solar irradiance and photosynthesis. A derivation of the Michaelis–Menten curve, it shows the generally positive correlation between ...
*
Chlorophyll
Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
*
High Pressure Sodium Lamp
*
Fluorescent Lights
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...
*
Compact Fluorescent Lights
A compact fluorescent lamp (CFL), also called compact fluorescent light, energy-saving light and compact fluorescent tube, is a fluorescent lamp designed to replace an incandescent light bulb; some types fit into light fixtures designed for inca ...
References
{{Artificial light sources
Gardening aids
Lighting
Cannabis cultivation