Development of a Multi-Spectral Detector Determining Leaf Chlorophyll Content of the Cucumber in a Greenhouse

Abstract

A multi-spectral detector was developed to determine leaf chlorophyll content of the cucumber in a greenhouse. The detector consists of two major units: light collecting and data processing. The light-collecting unit includes an optical fiber and two light measurement devices. A light measurement device is composed of an inlet, two gaskets, an interferential filter, and a photoelectric cell. It can form a closed space in the center to measure reflecting light without external noise. The fiber is designed as Y type to divide the reflecting light into two beams in order to obtain two preset wavebands of light, 527 and 765 nm, under the ambient light illumination. The data processing unit is composed of several operational amplifiers, an A/D converter, a ROM, and a MCU. The function of the data processing unit is to amplify signals, display data, and calculate the leaf chlorophyll content of cucumbers in a greenhouse from measured reflectance by an embedded estimation model. Performance tests show that the effect of the distance from the fiber to target on sensor response is quite small and linear. Thus, this effect can be eliminated by keeping the fiber in a constant height from the leaf surface, or by using a reference panel. The effect of the observation angle of the fiber is very small and random, and it can be ignored in measuring reflectance in greenhouses. Calibration tests show that the detector can meet the demands of practical use in a greenhouse. The spectral information measured by the detector has high correlation with leaf chlorophyll content of the cucumber plants and can be used as a tool for farmers to diagnose the nutrient status of cucumber plants in greenhouses.