There is no census on whether white light or red + blue light is better to maximize crop yield in indoor growing systems. s–1), although it decreased lettuce dry weight under high blue light (60 or 100 µmol.Our previous research ( bit.ly/greenblueLEDs) showed that the inclusion of 33% green light did not affect lettuce dry weight under low blue light (0 or 20 µmol Green light drives photosynthesis and controls plant shape. However, these do not consider the roles that green light plays in plant growth. Red and blue light are also perceived as the “optimal” wavebands for plant growth because: 1) in the lab, chlorophylls mainly absorb red and blue light, but not green light and 2) the McCree curve shows high quantum efficiency of photosynthesis (on an instantaneous basis) under red and blue light. Red + blue LEDs are common in horticulture primarily because of their higher efficacy than white LEDs. White LEDs create a visually pleasant environment for workers to inspect crops, unlike red + blue LEDs, which poorly render colors of objects. Some broad-spectrum LED fixtures include additional blue and/or red LEDs alongside white LEDs to create distinct broad spectra. White LEDs emit blue, green, red, and sometimes far-red light, thereby covering a broad range of photosynthetically active radiation. Warm-white LEDs emit a relatively small blue fraction (e.g., 7%), whereas cool-white and daylight LEDs emit larger blue fractions (e.g., 20% and 30%, respectively). The remaining blue fraction varies depending on the phosphor material. White LEDsĬommercial white LEDs are blue LEDs with phosphor coatings that distribute most of that light at longer wavelengths. These are the reasons behind a recurring debate within controlled environment agriculture on broad-spectrum (white) LEDs versus red + blue LEDs. To further complicate matters, fixture efficacy is related to its light spectrum. While it is straightforward to compare the fixture cost and efficacy, it is often unclear how different commercial light spectra compare in crop production. The selection depends on factors including the fixture cost, efficacy (the photon output per unit energy), the light spectrum, the form factor, and the light responses of crops. Today, growers can choose from a wide range of LED fixtures with different specifications. These improvements helped to drive the commercial adoption of light-emitting diode (LED) fixtures, especially in indoor vertical farms. In the last decade, lighting for controlled environment agriculture has evolved rapidly thanks to improved energy efficiency, spectral tuning and fixture design. Photos courtesy of Qingwu Meng and Erik Runkle.
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