Photoreceptors in plant leaves transform light into sugars and starches for food. Sunlight supplies the full color spectrum, but plants use primarily the red and blue wavelengths. Besides the original incandescent light bulb, light manufacturers make grow lights in different intensities and colors. Choosing which depends on how big your garden is. Greenhouses, botanical gardens, commercial settings and home gardens use different plant lights based on cost, intensity and lifespan.
Sunlight produces at least 100,000 watts. While free, sunlight is undependable, difficult to direct indoors and limited in winter months. Artificial lights enabled plant growers to operate year round. Early light bulbs couldn't rival the intensity of the sun until halogen bulbs were developed in 1958. Plants lights are now made to focus color wavelengths to stimulate growth.
Incandescent lights are made from a tungsten filament placed in a near vacuum inside a glass bulb. Argon or krypton gas inside the bulb glows when electrical current flows through the filament. Incandescent bulbs are hot and inefficient but inexpensive and provide the full light spectrum.
Tungsten evaporates and coats the inside of the bulb, dimming its brightness. Halogen added to the bulb attracts tungsten and returns it to the filament, keeping halogen bulbs bright longer. They must be 200 degrees Celsius for this to work, so are made in small capsules placed inside a larger bulb that is cooler.
Fluorescent lights have an anode and a cathode at opposite ends of a glass tube. Mercury vapor in the tube emits ultraviolet radiation when energized. The inside of the tube is coated with powdered phosphor, which fluoresces under ultraviolet radiation to produce visible light. Grow lights focus on the blue/red spectrum by changing the chemistry of the phosphor. Fluorescent plant lights are cheap to run, expensive to install and must be replaced in six to 12 months.
High Intensity Discharge
HIDs are made with a small capsule containing vapor placed inside a larger bulb. An arc sent through the capsule produces ultraviolet light that is filtered by the outer bulb to produce visible light. HIDs get very hot and need expensive ballasts, but last 10,000 hours.
Mercury vapor HIDs produce bright blue/white light and lack the spectrum plants need.
Sodium vapor HIDs emit pure yellow light and must be combined with sunlight or metal halide lights for plants. They are inexpensive, efficient and last 24,000 hours.
Metal halide plant lights provide a wide spectrum like sunlight and are best for large growers that need a lot of light.
LED bulbs are made with a transparent plastic case. Terminal pins form a current into and out of the case through the diode platform. This contains semiconductor material made up of positive and negative electrons. When energized in the diode, the electrons release light energy in the form of photons in a process called electroluminescence.
LEDs are cheaper to operate and provide the optimum light frequency for blooming and plant growth but are more costly to buy.