A question that comes up quite often is “Why are LED Horticultural Lights Purple?” The answer is that the lights are not actually purple, although they appear to be. Horticultural LED grow lights are actually a mix combing red and blue LEDs. The reason for this specific colour combination lies in the way plants use light for photosynthesis and growth. Here’s a detailed explanation:
LED Grow Lights Often Appear Purple. Why?
Light Spectrum and Photosynthesis
Philip Ronan, CC BY-SA 3.0
Plants use specific ranges of the visible light spectrum to power the chemical engine of growth we call photosynthesis. Visible light comes in a range of colours which have different wavelengths. These are measured in nanometres (nm). Generally these range from red with a wavelength between 625 and 750nm to violet with a range between 380 and 450nm. See this article on Wikipedia for a more comprehensive explanation of the visible light spectrum
Photosynthetic Active Radiation (PAR)
The light that plants use to power photosynthesis is known as Photosynthetic Active Radiation. Plants primarily use light within the 400-700 nanometre (nm) range for photosynthesis. This is basically the same as the visible spectrum. However, within this range, blue light (400-500 nm) and red light (600-700 nm) are the most effective for driving photosynthesis. These relatively narrow bands are, therefore, the most important for growth. Accordingly, some LED grow lights are tuned to to output in these bands.
Chlorophyll Absorption
Chlorophyll is a natural compound present in green plants that gives them their colour. Chlorophyll’s purpose in a plant is to absorb light—usually sunlight but also artificial light. The energy absorbed from light is transferred to energy-storing molecules. Then the plant. through photosynthesis, uses the stored energy to convert carbon dioxide (absorbed from the air) and water into glucose, a type of sugar. This carbohydrate (glucose is a carbohydrate) powers other chemical reactions within the plant. Basically it is the fuel that powers the plant.
Chlorophyll absorbs light most efficiently in the blue and red wavelengths. Generally blue light promotes vegetative growth, making plants bushier. Red light, especially in the 640-680 nm range, is crucial for flowering and fruiting stages.
Combining Red and Blue LEDs
Efficiency and Effectiveness
Red and blue LEDs are combined in grow lights to provide a balanced spectrum that maximizes photosynthetic efficiency. This combination results in the characteristic purple hue of LED grow lights in commercial growing operations because human eyes perceive the mixture of red and blue light as purple.
Energy Efficiency
LEDs are energy-efficient, and by using specific wavelengths that plants need, LED grow lights can be more efficient than traditional grow lights like fluorescent or incandescent bulbs. They emit a broader spectrum, including wavelengths that plants do not use as effectively. They also use energy to produce heat. Often far more energy is used generating heat than light.
This heat can actually be a benefit when using grow lighting with a propagator, reducing the heat the propagator needs to produce to reach temperature. However in hot weather and indoor farms etc., this excess heat becomes a problem requiring additional energy to remove.
Is the Purple Colour of LED Grow Lights Because They Contain UV Light?
Often people mistakenly think the purple colour of LED grow lights is because they are UV lights. Ultraviolet – UV – lights often appear to be a violet or purple colour but LED grow lights get their purple colour, as stated above, because they are a mixture of red and blue lights.
Do Plants Need Ultraviolet Light?
Ultraviolet (UV) light, specifically UV-A (320-400 nm) and UV-B (280-320 nm), can have some beneficial effects on plants, such as enhancing colour, flavour, and certain protective mechanisms. However, UV light is not typically considered part of the main spectrum needed for photosynthesis and is not the primary reason for the purple colour of LED grow lights.
Full Spectrum vs. Targeted Spectrum
Some growers prefer to use LED and other grow lights offering a full spectrum that includes UV and infrared (IR) light, mimicking natural sunlight. These lights may appear less purple and more white. The purple colour is specific to those grow lights that use a targeted spectrum focused on the primary wavelengths (red and blue) that drive photosynthesis most efficiently.
Summary
LED grow lights are purple because they are designed to provide the optimal light spectrum for photosynthesis by combining red and blue LEDs. This targeted approach ensures energy efficiency and effectiveness in promoting plant growth and development. While UV light can have additional benefits for plants, it is not the reason for the purple colour in most LED grow lights.
