A grow light or plant light is an artificial light source, designed to stimulate plant growth by emitting an electromagnetic spectrum appropriate for photosynthesis. Grow lights are used in absence of natural lights or to extend the time the plants receive light and accelerate growth.
Grow lights either 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 lights. 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.
The intensity of light radiating from a point source 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.
Blue colour and ultraviolet lights are regularly used for the first (or vegetative) phase of growth. Blue spectrum light may trigger a greater vegetative response in plants. Red colour light trigger a greater flowering response in plants. If red spectrum 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/Induction technology has been introduced into the grow light market. The exact wavelengths necessary for photosynthesis are used to create LED/Induction grow lights that are used for both the first (or vegetative) phase and the second (or reproductive) phase of growth.
Different stages of plant growth require different spectra. The initial vegetative stage requires a blue spectrum of light, whereas the later "flowering" stage is usually promoted with red–orange spectra.
The ability of a plant to absorb light varies with species and environment; however the general measurement for the light quality as it affects plants, is the PAR value, or Photosynthetically Active Radiation. This measures the useful light energy received by the plant and the spectra measurements favor the blue and red portions of the light, while ignoring in part, the green and yellow portions which plants generally do not benefit from.