Light sources
Different types of bulbs may be used in grow light, including metal halide, fluorescent, incandescent, high-pressure sodium, and LED lights.
Metal halide (MH)
Metal halide bulbs emit a blue spectrum of light and are a good representation of spring and summer sunlight filled with bright blue skies.
Incandescent
Bare incandescent lights generally have a red-yellowish tone and low color temperature (approx. 2700 K). They are sometimes used to highlight indoor plant groupings but not as a true plant "growing" light. Some incandescent bulbs specifically marketed as "grow lights" come with a blue filter coating which reduces the amount of red light the bulb gives off— this does almost nothing for the plants themselves, however, and only serves to make the bulb produce less light overall by cutting off the red portion of the spectrum. Such "grow lights" have a brief life expectancy of about 750 hours and are extremely energy inefficient, producing much more heat than usable 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 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
LED panel light source used in an experiment on potato plant growth by NASA
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 for indoor plants are attractive because they consume much less electrical power, do not require ballasts, and produce considerably less heat than incandescent or fluorescent lights. This allows LEDs to be placed closer to the plant canopy than other lights. Also, plants under LEDs 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 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. Further research has shown that infrared and ultraviolet diodes give a full spectrum needed for flowering plants to effectively grow and flower.
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.
