Intensity, spectrum and focus are three key essentials of high-quality lighting, both natural and artificial. When these three core elements are combined into a comprehensive indoor-lighting system, it allows a grower to focus attention to other areas of plant growth like disease and bug prevention, plant nutrition and environmental control. However, before explaining the essential lighting elements, one must first understand how high-intensity discharge (HID) grow lamps create light.

One of the best ways to begin learning how lamps create light is to compare HID lamps to something more familiar: the incandescent light bulb. In incandescent bulbs, electricity freely flows into the lamp (bulb) through the base and up to the filament. As the energy tries to flow across the filament, it slows down due to the resistance caused by the filament material and construction. This reduction in speed generates heat, which causes the filament to glow. The hotter the filament gets, the brighter the light becomes. The key point: incandescent light sources have a filament that carries the electricity through the lamp and provides the visible light.

HID lamps—metal halide (MH) and high-pressure sodium (HPS)—on the other hand do not have a filament. Instead, they contain an arc tube made from either quartz or ceramic materials. This is where the electrical discharge (hence the term “high-intensity discharge”) happens. Electrical discharge is similar to the arcing of a lightning strike, only much lower in terms of power and much longer in time. Electricity flows into the lamp from the ballast and stops at an electrode on one side of the arc tube. The ballast then pushes the electricity across the gap (from one electrode to the other) inside the arc tube, creating a mini lightning bolt called an arc stream. The ballast then works hard to maintain this arc stream inside the arc tube for as long as the lamp is powered.

The HID lamp gives off a fair amount of light while this arc stream is happening, but not anywhere near the amount required for plant growth. However, the arc stream also heats up chemicals inside the arc tube that produces an incredible amount of light once the lamp comes to full power. These chemicals are unique in their composition and ratios, and each lamp has a specific chemical formula that produces the light spectrums advertised on its packaging. This unique formula is also what produces the energy that plants use to grow. Exactly how much growth is achieved is a result of a lamp’s intensity, spectrum and ability to put the light where it’s needed. Let’s look closer at these three key elements.

Lighting Intensity

Intensity is the most important aspect of lighting. High-intensity lighting is required for rapid plant growth. The indoor horticulture market still measures light intensity in terms of lumens. While one could argue that the lumen does not exactly correlate to plant growth, for all practical purposes, the more lumens a light source produces, the better plants will grow.

More recently, photosynthetically active radiation (PAR) has become a more meaningful way of quantifying light energy for plant growth. In very simple terms, PAR is the portion of a light’s energy that plants use to trigger photosynthesis. Some argue that ultraviolet light (UV) and infrared light (IR) are also critical for healthy plant growth, but these types of light are not included in the PAR scale.

However, no matter whether you describe light intensity in lumens or PAR, it remains true that your plants will grow better when you have more intensity. It is often asked, “Can a plant have too much light?” The answer is yes, but these levels are typically extremely high and are plant specific. For example, desert plants tolerate much higher light levels than flower species that grow under the canopy of the rain forest. Other environmental issues also come into play and limit how much light should be used in a grow area. In other words, the better a grow environment is controlled, the higher the light levels can be provided.

Light Spectrum

Spectrum is the second most significant criterion for plant lighting. As light intensity increases, light spectrum becomes more important—this cannot be stressed enough.

As stated earlier, solar radiation is perfect for plant growth. Sunlight is intense and has a broad and balanced spectrum. Indoor plants require this same high-intensity, broad-but-balanced spectrum.

The chemistry inside a HID lamp’s arc tube is what produces the light that provides those spectrums. When considering different light sources for indoor gardening, pay attention to the spectral distribution charts published for the lamps. These charts explain the quality of light produced by each lamp and offers the opportunity to compare the light sources and make the best choice for your garden.

Keep in mind too that there are only a few laboratories in the world that make chemistry for HID lamps. As a result, many companies use the same lamp chemistries, meaning that they produce similar light spectrums.

Light Focus

Focus is the final essential element for high-quality plant lighting. Delivering light where it is needed is the job of a reflector. Well-designed reflectors deliver with excellent uniformity a higher percentage of the available light to where it’s needed.

Choosing a good reflector is worth the additional up-front cost; high-quality reflectors use better materials that hold up to the heat, dust, dirt and moisture found in indoor gardens. They also use more effective designs. Using cheaper reflectors that more quickly degrade and lose their effectiveness could result in the light energy not being delivered to the plants.

High-quality indoor plant lighting is not a mystery. Good results can be achieved by utilizing high-quality light sources and lighting systems. Good lighting comes from well-engineered materials and designs that produce intense light with broad and balanced spectrums.

Focusing this light requires well-designed reflectors with high-quality materials that ensure a majority of the light reaches the plants in a uniform fashion over a long period of time.

While the incredible intensity, spectrum and focus of our sun’s solar radiation cannot be duplicated in indoor gardens, high-quality indoor lighting is in some ways superior to outdoor growing…there are no cloudy days in indoor gardening!