General Marijuana Growing
spectrum question in the
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does the spectrum matter?
very detailed question.
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A grow light or plant light is an artificial light source, generally an electric light, designed to stimulate plant growth by emitting an electromagnetic spectrum appropriate for photosynthesis. 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, 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/vegetative phase or the flowering/fruiting phase), and the photoperiod required by the plants, specific ranges of spectrum, luminous efficacy and colour temperature are desirable for use with specific plants and time periods.
 Typical Usage
Grow lights are used for indoor gardening, plant propagation and food production, including indoor hydroponics 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. 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.
Often, the distance between light and plant is as far as 60 cm (24 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 LEDs. 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 (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.
The light spectra of different grow lights
Different stages of plant growth require different spectra. The initial vegetative stage requires blue 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 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:
luminous efficacy (lm/W)Overall
luminous efficiencyCombustioncandle0.30.04%gas mantle1–20.15–0.3%Incandescent100–200 W tungsten incandescent (230 V)13.8–15.22.0–2.2%100–200–500 W tungsten glass halogen (230 V)16.7–17.6–19.82.4–2.6–2.9%5–40–100 W tungsten incandescent (120 V)5–12.6–17.50.7–1.8–2.6%2.6 W tungsten glass halogen (5.2 V)19.22.8%tungsten quartz halogen (12–24 V)243.5%photographic and projection lights355.1%Light-emitting diodewhite LED (raw, without power supply)4.5–150 0.66–22.0%4.1 W LED screw base light (120 V)58.5–82.98.6–12.1%6.9 W LED screw base light (120 V)55.1–81.98.1–12.0%7 W LED PAR20 (120 V)28.64.2%8.7 W LED screw base light (120 V)69.0–93.110.1–13.6%Theoretical limit260.0–300.038.1–43.9%Arc lightxenon arc light30–504.4–7.3%mercury-xenon arc light50–557.3–8.0%FluorescentT12 tube with magnetic ballast609%9–32 W compact fluorescent46–758–11.45%T8 tube with electronic ballast80–10012–15%PL-S 11W U-tube with traditional ballast8212%T5 tube70–104.210–15.63%Spiral tube with electronic ballast114-124.315–18%Gas discharge1400 W sulfur light10015%metal halide light65–1159.5–17%high pressure sodium light85–15012–22%low pressure sodium light100–20015–29%Cathodoluminescenceelectron stimulated luminescence305%Ideal sourcesTruncated 5800 K blackbody25137%Green light at 555 nm (maximum possible luminous efficacy)683.002100% Incandescent Grow Lights
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 Grow Lights
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) lights
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 lighting 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 reflector/enclosures.
 Combination Metal Halide (MH) and HPS lights
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 of light.
 Switchable, Convertible, and Two-Way lights
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 Grow Lights
LED panel light source used in an experiment on plant growth by NASA. Pictured plant is a potato plant.
Recent advancements in LEDs 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 for chlorophyll and carotenoids, and LED grow-lights may use one or more LED colors overlapping these peaks.
Recommendations[by whom?] 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[by whom?], 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[by whom?],[unreliable source?] 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.[unreliable source?]
LED 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.[unreliable source?]
 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) grow in full shade and can grow normally with much lower light levels, thus regular incandescents may already suffice).
In addition, plants also require both dark and light ("photo"-) periods. Therefore, lights may to be timed to turn them on and off at set times. The optimum photo/dark period 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 is an important factor in indoor growing. Illuminance is the amount of light incident on a surface. One lux equals one lumen of light falling on an area of one square meter (lm/m2), which is approx. 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 /second·meter2) or photosynthetically active radiation weighted (PAR watt) units are used instead.