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HID Grow Lights: Beyond the Fundamentals

July 12, 2016

Because photosynthesis is dependent on light, all plants must have sufficient light to grow. Also, the growth rate of any given species is dependent on both the intensity and duration of the light it receives. However, different species absorb different bandwidths of the light spectrum, and cannabis is no exception. In fact, cannabis plants require light with wavelengths from 400 to 700 nanometers (red to blue); this is called the Photosynthetically Active Radiation (PAR) zone.

The dependence on such a narrow band enables indoor gardeners to use artificial lights to simulate the bandwidths of natural light that plants would normally receive from the sun. However, indoor garden lighting must be both extremely intense and of the correct color to provide plants with both the type and the amount of light they need to produce high quality buds. Therefore, High Intensity Discharge (HID) grow lights—metal halide and high pressure sodium types—tend to be the indoor lighting source of choice among professional growers.

To simulate the changing light cycle of the passing seasons, plants need not only a change in the duration of the light they receive, but a change in the wavelength of light during different stages of growth. As a result, most professional growers use metal halide bulbs for the vegetative stage and high pressure sodium bulbs for the flowering stage.

How HID bulbs work

An HID bulb produces light by passing electricity through vaporized metallic gases contained in a clear ceramic arc tube under very high pressure. The exact mix of chemicals in the arc tube is what determines the color spectrum of the light produced. Metal halide bulbs produce the widest spectrum but are particularly proficient at producing red wavelengths. High pressure sodium bulbs have a narrower spectrum but are particularly proficient at producing blue wavelengths.

Also, it should be noted that all HID bulbs are categorized by the number of watts of electricity that they consume (not the amount of light that they produce). They are commonly available in 30, 50, 70, 100, 150, 175, 200, 250, 310, 400, 600, 1,000 and 1,100 watt models with numerous different sizes, shapes and types of bulbs, as well as different sizes and shapes of reflector hoods.

Because light is emitted from the source and absorbed by plants in the form of photons—which is a measure of light energy—“PAR watts” is an attempt to measure the actual number of photons a plant needs to grow. Blue photons are worth more PAR watts than red photons, so there has been a movement in recent years to rate grow lights in PAR watts instead of the number of watts they require to operate. But so far, scientists have been unable to agree to a standard by which lights can be measured in PAR watts. However, both watts and PAR watts are measures of light intensity rather than the color of the light. Another excellent measure for gauging the suitability of a bulb for growing cannabis is to note its Kelvin temperature rating, which expresses the exact wavelength or color of light that a bulb emits. Bulbs with a Kelvin temperature rating of 3,000 to 6,500 are the best choice for cannabis cultivation.

Advances in HID technology

There have been several advances in HID bulb technology in the last 20 years. Researchers have managed to create new and improved grow bulbs such as pulse start metal halide bulbs, as well as those with higher PAR ratings. In addition, engineers have developed digital ballasts to replace analog ballasts. Digital ballasts are significantly smaller and lighter, and they produce significantly less heat while also managing to use less electricity.

Professional growers in the U.S. tend to prefer 600-watt and 1,000-watt HID grow lights, but it is often a better idea to use two or more smaller lights than one large light. But regardless of which you choose, HID bulbs produce far more lumens of light per watt of electricity consumed than any other source of artificial light and are the most popular choice for cannabis cultivation.


By Bill Bernhardt
Cannabis Cultivation Today articles are for informational purposes only and should not be considered legal guidance or advice on grow practices. You should contact an attorney or a qualified cultivation consultant for specific compliance and cultivation advice.

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