Implications of open circuit voltage of light-emitting diodes installed for plant cultivation

This study investigated a light sensor function of light-emitting diodes (LEDs), the use of which is expanding in plant cultivations. Unimodal spectral LEDs of four types with respective emission peak wavelengths of 464, 501, 634, and 849 nm, and a white LED with a bimodal spectrum with 455-nm primary and 574-nm secondary peaks were used for this study. Open circuit voltage (VOC) of up to 1-2 V was induced in the LEDs when they were irradiated with sunlight. The VOC value of the 634 nm LED saturated with a low photon flux density can be used for binary discrimination between daylight and night. Each LED had a VOC inducible threshold wavelength of incident light. By virtue of the wavelength threshold feature, existence of a plant leaf between a light source and the LEDs is detectable by comparing the VOC values of blue-green range LEDs (464 nm, 501 nm) and near-infrared (NIR) LEDs (849 nm). Under leaf shadow, the NIR LED VOC exceeded those of the blue-green range LEDs. Under natural incident sunlight, the VOCs of the blue-green range LEDs were greater than that of the NIR LED. Another function of LEDs in agricultural use has been demonstrated.