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A grow light is an artificial electronic light source designed to provide the plant with the light spectrum that it needs for photosynthesis to take place. The main purpose of grow lights is to replicate the light spectrum and intensity that the sun provides on an everyday basis. Grow lights are mostly used for indoor gardening, plant propagation and hydroponic gardening. Although most grow lights are used on an industrial scale, they can also be used in small scale gardening.

HID Lighting

High Intensity Discharge lightning has opened a whole new world for indoor growers, since they have a much higher luminous efficiency than fluorescent and incandescent lamps. They are designed to produce the maximum output of photosynthetically active radiation (PAR) for the amount of power consumed. HID lamps are a type of arc lamp which provides light by means of an electric arc between electrodes housed inside a tube filled with gas and metal salts. Once the arc starts it heats and evaporates the metal salts forming plasma, which increases the light intensity produced by the arc. These systems consist of a lamp, reflector and a power supply. HID lighting made indoor gardening practical, particularly for plants that needs high levels of direct sunlight in their natural environment. The two most common HID systems are metal halide (MH) and high pressure sodium (HPS).


Light intensity is commonly measured in Watts (power) per square foot, or in the total lumen output of a bulb. The ideal watts per square foot ranges from 20 - 50W, 20W being suited for low light plants and 50W for plants that love direct sun light. Many growers increase the light effectiveness by employing a few techniques to use the light as efficiently as possible. Reflectors are often used with grow light systems as well as mylar wall lining and white paint to increase the light dispersal.


Most types of plants grow best when they are exposed to 16-18 hours of light per day.  On flowering plants some growers switch the light period to 16-18 hours in the growing stage, and then to 10-12 hrs light in the flowering period. Seasons can also be manipulated by changing the light periods.

Light Spectrum

Photosynthesis is most active in the blue (5500k to 6500k) and red (2400K to 2600k) wavelengths of light. HID lightning systems are designed to cover these wavelengths, known as the PAR spectrum (photosynthetically active radiation). Different growing stages of a plant requires different light spectrums, in the vegetative stage they require more blue spectrum and in the flowering stage red-orange spectra. Growers prefer to use Metal Halide (Blue) in the vegetative stage and then switch to High Pressure Sodium (red) in the flowering stage. You can also combine the MH and HPS to cover both red and blue light for optimal growth. Below is a chart which shows chlorophyll activity at different light temperatures. Click to Enlarge:

Choosing a Grow Light

When choosing a grow light there are a couple of things one should first consider. 1) Match your room size with equivalent light. The usual equation is 20 to 50 Watts per square foot. 2) Make sure that your light system produces the right light spectrum and intensity for the type of plants you want to grow. For example, vegetables grow at their best in full sunlight, and needs high light levels, so MH and HPS will be recommended. Where plants that grow in shade require lower light levels, thus regular fluorescents can be used. 3) How much heat the lighting system produces and if it’s necessary for you to add extra ventilation or cooling systems.

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