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 of 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 horticulture, 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.
A range of bulb types can be used as grow lights, such as incandescents, fluorescent lights, high-intensity discharge lamps, 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 sometimes used for the first (or vegetative) phase of growth as they have some blue light; however, they have peak intensity around yellow spectrum.
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.
In recent years LED technology has been introduced into the grow light market. By designing an indoor grow light using diodes, the exact wavelengths necessary for photosynthesis are used to create LED grow lights that are used for both the first (or vegetative) phase and the second (or reproductive) phase of growth. NASA has tested LED grow lights for their high efficiency in growing food in space for extraterrestrial colonization.
Light spectra used
Natural daylight has a high color temperature (approx. 5000 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 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 suffice.
In addition, plants also require both dark and light ("photo"-) periods. Therefore, lights may be turned on or off at set times. 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 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/m), which is approximately 0.093 foot-candle (lm/ft). 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. This makes lux an inappropriate measure of efficiency in a horticultural lighting system. In professional farming, radiometric (watt/metre or microeinstein /second·meter) or photosynthetically active radiation weighted (PAR watt) units are used instead.
