(LED) fixtures have improved dramatically since they were introduced a few decades ago, but you still need to know how to grow in innovative ways to get the most for your investment. Replacing conventional high-intensity discharge (HID) lamps like high-pressure sodium and metal halide with an LED fixture requires an LED with proper spectrum and spacing between leaves and lights, and even distribution over the grow space.
Spectrum and Fixture Form Have Come a Long Way
Plant growth is driven by the light spectrum available. The shape of the light fixture (fixture form) has a huge impact on how light is delivered to the canopy. Higher amounts of blue light make plants shorter, darker in pigment, and richer in cannabinoids and terpenes. Red light is a powerful driver for photosynthesis, whereas green, orange, and yellow light provide only a fraction of energy to a plant. Some LEDs emit specific wavelengths as an attempt to conserve electricity by providing isolated colors. However, this neglects the capacity your plant has to absorb a wide range of spectra.
It is also important to understand that you cannot take down an HID fixture and place an LED in the same spot and expect the same results. An LED fixture can deliver a huge amount of light, but the light intensity drops quickly when moving away from the fixture. This is the reason many LED fixtures disperse light evenly over a large area, as opposed to one small crowded center of light as with an HID. The best approach is to use a fixture with a large surface area and a complete spectrum (“narrow band” wavelengths are more likely to light-burn leaves), and place it close to the canopy.
Light Penetration is About Spectrum
Light does one of two things when it is intercepted by a leaf. It is absorbed as energy for growth or it is perceived as a signal (inducing flowering, producing protective compounds, etc.). Red and blue light are absorbed rapidly by the top leaves, with little passing into the lower canopy. Green, orange, and yellow light drive growth, but are weakly absorbed and pass through leaves easily. Since most of the red and blue light is absorbed by the upper canopy which can only provide so much energy for the plant, it is critical to provide these “middle” wavelengths that allow the entire canopy to photosynthesize, as opposed to relying entirely on the top leaves to support growth.
Heat from LEDs is Different from HIDs
Since LEDs give off almost no radiant heat, you can grow close to the fixture without heat stress. Some of the best fixtures allow you to get as close as six inches. With a balanced spectrum and even light distribution, you can safely move your lights close to the canopy, drastically increasing your light intensity.
There is a common misconception that radiant heat is simply the warmth from a light fixture. Radiant heat is light in the form of infrared radiation (IR), an electromagnetic wave just like red, blue, and green. When this IR is absorbed, energy is released as heat by the object. LEDs hardly put off any IR, but HIDs emit a huge amount of IR that increases the leaf and room temperature.
Wattage Does Not Indicate Amount of Photons
Wattage is the amount of electricity a fixture consumes, not the amount of energy supplied to plants. In HIDs, much of this electricity is converted into IR, which does not contribute to growth (although it can be used as an inefficient heater). Many light manufacturers use the value photosynthetic photon flux density (PPFD) to indicate how much light emitted from the fixture is used by plants.
When selecting a fixture, look for this value coupled with the footprint of the light to determine how much energy you will supply to your plant. This will help you make a fair shopping comparison. When designing your grow, consider the possibilities of properly used LEDs. HIDs put off a huge amount of light, but they also create excess heat, the spectrum is limited, and you need more space to grow.