Fluorescent lights work by passing electricity between two electrodes in a glass tube filled with a low-pressure mixture of gasses including xenon and mercury. When electricity excites a mercury atom, it gives off a photon of ultraviolet (UV) light, which is not visible to humans. To convert this UV light into visible light, the inside of the glass tube is covered with chemicals called phosphors. When a UV photon hits one of these phosphor molecules, some of its energy is converted into heat and the rest is re-emitted as a different color of light. Carefully selecting and mixing the phosphor chemicals creates several different colors of light, which the human eye perceives as white. Varying the proportions of the phosphors can create redder, “warm white” light, or bluer, “cool white” light.
Intensity of Fluorescent vs. LED Grow Lights
Fluorescent lights don’t produce very intense light, so you have to keep them close to your plants, and they don’t penetrate plant canopies very well so they don’t work well for growing taller plants. High-intensity LED grow lights can work much better for taller plants.
Spectrum of Fluorescent vs. LED Grow Lights
The balance of red and blue light in fluorescent grow lights can keep plants happy, but they produce a lot of green light that plants don’t use well. Only certain phosphors are available for fluorescent lights, so it isn’t possible to cover the light spectrum as evenly as is possible with LEDs. The phosphors allow some tuning of the light spectrum, but again not as completely as is possible with LED grow lights.
Lifespan of Fluorescent vs. LED Grow Lights
Each time a fluorescent light is turned on, a small amount of metal from the electrodes vaporizes as the electric arc gets started. This metal deposits itself on the glass tube, blocking light from getting out. This is why the ends of fluorescent tubes become black over time.
Essentially, every time you turn on the fluorescent light, its light output decreases and the lifespan is shortened! Typically, fluorescent tubes degrade quickly enough that you need to replace them every 6 months of use; they may still light up but they aren’t as bright as they need to be for growing plants.
LEDs don’t degrade every time they are turned on, and typically remain bright enough to grow plants with for 50,000 hours or more- over 5 years of continuous use, or 11 years at 12 hours per day!
Since all fluorescent lights contain mercury, they contaminate the environment when you throw them out at the end of their life, unlike LEDs.
Efficiency of Fluorescent vs. LED Grow Lights
LEDs are more efficient at creating light than fluorescent tubes. Per watt used, LEDs can be over twice as efficient as fluorescent lights at creating photons.