Many professional growers use a combination of high intensity discharge (HID) metal halide and high pressure sodium bulbs to simulate the different bands of the light spectrum that plants would normally receive from the sun. Cannabis uses only a very narrow spectral range, and it needs different bandwidths of light at different stages of growth. However, there has been a movement in recent years to replace HID bulbs with LEDs (light-emitting diodes).
The physics of light
It’s important to understand that all light is emitted from its source and assimilated by plants in the form of photons, which can be visualized as particles of light. The process of photosynthesis is both triggered and powered by the plant’s absorption of photons. Cannabis uses from 400 to 700 nanometers of the visible light spectrum, which is called the Photosynthetically Active Radiation (PAR) zone and ranges in color from red to blue.
Light intensity, which is a measure of the amount of light per unit area, is of paramount importance when designing a grow room. Intensity will determine size, density and quality of the buds. The Inverse Square Law states I = L/D², in which I stands for intensity, L for light output or lumens, and D for distance. This law describes the relationship between the amount of light or intensity emitted from a point source and the distance to any given point. According to this law, the intensity that reaches a point two feet from a given intensity bulb is only one-fourth the amount that reaches a point just one foot away from the same bulb. So, the buds on lower branches are seldom of equal size and quality to those produced on the upper branches.
HID bulbs vs. LEDs
To maintain intensity at greater distances, professional growers use either 400-, 600-or 1,000-watt HID bulbs. Although they have a high intensity, HID bulbs also produce a significant amount of heat, which must be removed from the grow room to prevent the plants from overheating. In addition, HID bulbs are expensive to operate. But they offer the option of using a single fixture to hold either a metal halide or a high pressure sodium bulb. Bulbs can be outfitted with one of several different types of reflectors so that the area covered by the light can be adjusted to suit individual needs. The 600-watt bulb is by far the most popular type among growers. It produces more light than a 400 watt bulb and can be placed closer to the top of the plants than a 1,000-watt bulb (which produces more heat).
LEDs are available for emitting red and blue spectrum, which are meant to mimic the specific wavelengths that cannabis uses during vegetative and flowering stages, respectively. But there are some important differences between HID bulbs and LEDs:
It should be noted that LED technology has made major advances from single- to multi-wavelength types, as well as increased intensity for enhanced plant growth. So, LEDs are presently seen as the technology of the future. In the meantime, HID bulbs with a digital ballast remain the lighting of choice among professional growers. They produce less heat and are cheaper to operate than those with analog ballasts, although they are more expensive to buy.
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