When shopping for lights for your indoor garden, it’s important to know what you’re shopping for. Not only are there hundreds of brands to choose from, but there are dozens of different types of bulbs for each brand. Understanding what each type of light bulb does is the first step in the right growing direction.
High-Intensity Discharge (HID) grow lights provide a cost-efficient light source for indoor gardening. Using very intense light, the watt to lumen ratio for HID lighting is among the highest for all types of lighting. HID lights also pump out a lot of heat and may need to be cooled. HID bulbs must be replaced often to ensure optimal spectrums are maintained, typically bulbs last 9 months to 1 year. HID lighting comes in many different wattages to accommodate different garden needs. The total wattage of a HID lighting system determines its intensity and the number and quality of the plants that it can grow. The more watts, the more intense the light.
Two types of HID grow lights can be used for indoor gardening – High-Pressure Sodium (HPS) and Metal Halide (MH). The primary visual difference between them is that metal halide light is white and the light emitted from a High-Pressure Sodium bulb is amber orange. These bulbs cannot be interchanged without changing their ballast.
With High-Pressure Sodium bulbs the orange/red sector of the spectrum, simulating outside autumn light, enhancing a plants’ reproductive processes, and plants respond by producing larger harvests of high-quality fruits, vegetables, and flowers. High-Pressure Sodium, then, is used by indoor gardeners during the flowering and fruiting stage of plant life. When used alone throughout the plants’ life cycle, however, HPS causes plants to stretch and become leggy, with larger spacing between branches. Nevertheless, plants thrive under HPS lighting. High-pressure sodium lights are more efficient at converting electricity to light energy than metal halide lights are.
There are two types of High-Pressure Sodium lights: Single-Ended and Double Ended.
Double-ended (DE) HPS bulbs connect to the fixture on each end much like fluorescent tube bulbs. In this case, the inner, light-generating tube is supported by two small wires on each end of it and lacks a frame wire that crosses in front of the arc tube. Without any of the metallic structure that SE bulbs need to support the arc tube, DE bulbs are significantly thinner than SE bulbs. These bulbs also degrade slower than traditional, single-ended HPS bulbs. In fact, after 10,000 hours, double-ended lamps will still output approximately 90% of their original intensity.
With the exposed arc tube, the bulb, without any metal framing in the way, increases light delivery to the canopy of your plants. The symmetry of DE bulbs allows you to create an even spread of light much easier than is possible with SE bulbs. Not only does the lack of wireframe improve on its light delivery and reflectance, the thinner bulb also improves its optical properties.
DE bulbs, however, are not perfect. While SE bulbs are under vacuum, DE bulbs are full of nitrogen gas. The nitrogen atmosphere allows the bulb to operate at a higher temperature, which increases its efficiency. The downside of a gas-filled bulb is that the gas conducts heat from the outside of the bulb to the arc tube. Any air blowing directly on the bulb induces a temperature change in the arc tube, causing a loss in efficiency. Therefore, DE bulbs do not tolerate air blowing directly on them like in active cooling setups (aka “cool tubes” in which the SE bulb sits inside of a tube with a fan housed in the luminaire or lamp hood).
Metal halide lights give off an intense, bluish-white light that plants respond to with healthy, vigorous growth. Plants will be stocky and strong, with dense green foliage. Metal Halide is the light of choice for indoor plant gardeners. The downside to Metal Halide lights is that the light is poor in the orange/red part of the spectrum. Ceramic metal halide grow lights are made with an arc tube constructed of a ceramic composite instead of quartz or PCA. This allows the tube to reach a higher temperature. The higher operating temperature of the ceramic tube allows for an ideal mixture of gases. This creates a fuller spectrum of light that increases the growth, overall health and yield of your plants. A special ballast is needed for ceramic metal halides. Not just any digital ballast will do. CMH lamps require a low frequency, which makes high-frequency digital ballasts incompatible.
CMH also have a more natural CRI (Color Rendering Index). CRI describes how the color of a light source changes how an object appears to the human eyes and how well subtle variations in color shades are revealed. The higher the CRI, the more realistic things look. CMH grow lights are generally rated between 80 to 96 CRI, by comparison, the sun is rated at 100 CRI. This means that CMH grow lights will let you see the true color of your plants without making your grow room look yellow, blue or purple.
But when it comes to flowering crops, HPS lights are still a lot more efficient. This is because HPS grow lights output more light in the red spectrum than CMH lights.