Design and Testing of an Active Light Source Apparatus for Crop Growth Monitoring and Diagnosis

Portable spectrometers can extract crop growth information in a non-destructive, rapid, and efficient way. However, most existing instruments can only be used to collect spectral reflectance and calculate vegetation indices. They are costly and lack crop spectral monitoring models and diagnosis models; therefore, crop growth information cannot be directly output from these instruments. In this paper, an active light source apparatus for crop growth monitoring and diagnosis (CGMD) was developed based on the canopy shape characteristics and spectral monitoring mechanisms of row-cultivated crops. A modulated light source was used in this apparatus to effectively eliminate the interference of ambient light. At the same time, a high-pass filter was used in the apparatus to extract the canopy reflectance spectral information, which improves the signa-noise ratio. Crop canopy vegetation indexes-the normalized difference vegetation index (NDVI) and the ratio vegetation index (RVI)-and crop growth parameters-leaf area index (LAI), leaf dry weight (LDW), leaf nitrogen accumulation (LNA), and leaf nitrogen content (LNC)-were obtained in real time. The ambient light influence test and the comparison test of different test heights of the CGMD apparatus show that it has high test stability and can effectively overcome ambient light interference within the standard test height range. The rice and wheat experimental results demonstrated that the test results of the CGMD apparatus and the commercial Analytical Spectral Devices field spectrometer (Malvern Panalytical, Malvern, Worcs, UK) were consistent. Comparing the vegetation indexes and agronomic parameters obtained from the chemical analysis, the CGMD apparatus has a good predictive ability for LAI, LDW, LNC, and LNA.