Are white LEDs efficient for growing plants?
What about these 10-watt white LEDs?
First, it is important to know what "white" light really is. White is not a spectral color, but rather a combination of different light colors. Human eyes only have 3 kinds of color-sensitive cells called cones-- red, green and blue-- and any light which stimulates all three of these at similar levels will appear white. There are many different ways that humans perceive white light from the combination of different component colors. Equal amounts of red, green and blue light, even without any other colors, will appear white to the human eye. Yellow light stimulates both the red and green cones, so blue and yellow light combined will also appear white. This is just one example; there are many, many other combinations which appear white to the human eye, even though the light is not a complete spectrum.
LEDs' electroluminescence technology (how LEDs make light) is not capable of producing white light directly from the diodes; individual LEDs can only produce one color of light. The first "white" LEDs were actually red, green and blue (RGB) LEDs combined, and indeed the light appears white to the human eye. However, if you view something that only reflects orange light under the RGB light, it will appear black, as there is no true orange from the light source that the object can reflect back. This means that RGB LEDs have a poor Color Rendition Index (CRI).
Almost all "white" LED grow lights on the market today are actually just a blue LED with a phosphor coating which converts much of the blue light into different colors. The most commonly available "white" LEDs use a phosphor called Yttrium Aluminium Garnet (YAG) which predominantly creates yellow light; the combination looks white to the human eye and has a much better Color Rendition Index (CRI) than RGB LEDs due to the wider spectrum created by the phosphor. However, 20%-40% of the light produced by the blue LED is lost in this process, so these "white" LEDs cannot be as efficient at creating light as a pure-color LED (and LED's cannot be made to produce a "pure-color" white). White LED grow lights are good if you're looking to illuminate your home or office as this efficiency loss is easily justified by being able to see comfortably, but for growing plants they are wasteful.
Plants preferentially absorb red and blue light. Much of the light produced by "white" LEDs are in spectra (colors) that plants do not use. This unused light is just converted to heat within the leaves, requiring lower environmental temperatures to maintain optimal leaf surface temperatures. When combined with the 20%-40% efficiency loss, "full spectrum" white LEDs are less than half as efficient for growing plants than the correct mix of pure-color LEDs-- white LED grow lights force you to cool your growing environment more, just like HPS and MH, losing a lot of the other advantages LEDs offer.
White LEDs are designed and manufactured for humans- they are ideal for our eyes' color needs, but not for plants!The Phyto-Genesis Spectrum (purple) versus "full spectrum" white LEDs (gray):
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