Development of a nondestructive measurement system to obtain exuberance index of foliage during greenhouse crop growth by low-cost near-infrared light sensors

A nondestructive measurement system for continuously counting the average number of leaf layers (ANL) of a crop canopy was developed to evaluate the vegetative growth instead of the leaf area index (LAI). A virtual near-infrared light sensor with a peak between 700 and 800 nm was developed by calculating the difference in electrical current outputs of two low-cost Si photodiodes for detecting visible light and visible plus near-infrared light. Two virtual sensors were placed on the top and bottom of the crop canopy each, and apparent ANL as the value of the light path length was calculated from the measured values using the Beer-Lambert Law. The far red light transmittance of the leaves of most greenhouse fruit vegetables is approximately 60%. The ANL may be measured in up to approximately six leaves when a near-infrared light sensor with a 13.3 dB dynamic range is used. To measure more accurately the amount of received light from leaves that spreads in various directions, the virtual sensor was covered by a diffusion hemisphere made from a half of a Ping-Pong ball. The sensing system was installed in a device based on a ubiquitous environment control system (UECS), a type of decentralized autonomous computer measurement and control system for crop production in greenhouses. The measurement system was located and tested in muskmelon, cucumber, and tomato production greenhouses in Japan. The calculated ANL from measured daily cumulative optical sensing data for muskmelons was highly correlated (R ²=0.90) with LAI obtained manually. The correlation between ANL and LAI of the other tests was relatively high.