*Article* **Detection of Canopy Chlorophyll Content of Corn Based on Continuous Wavelet Transform Analysis**

**Junyi Zhang 1,2, Hong Sun 1, Dehua Gao 1, Lang Qiao 1, Ning Liu 1, Minzan Li 1,\* and Yao Zhang <sup>1</sup>**


Received: 23 July 2020; Accepted: 20 August 2020; Published: 24 August 2020

**Abstract:** The content of chlorophyll, an important substance for photosynthesis in plants, is an important index used to characterize the photosynthetic rate and nutrient grade of plants. The real-time rapid acquisition of crop chlorophyll content is of great significance for guiding fine management and differentiated fertilization in the field. This study used the method of continuous wavelet transform (CWT) to process the collected visible and near-infrared spectra of a corn canopy. This task was conducted to extract the valuable information in the spectral data and improve the sensitivity of chlorophyll content assessment. First, a Savitzky–Golay filter and standard normal variable processing were applied to the spectral data to eliminate the influence of random noise and limit drift on spectral reflectance. Second, CWT was performed on the spectral reflection curve with 10 frequency scales to obtain the wavelet energy coefficient of the spectral data. The characteristic bands related to chlorophyll content in the spectral data and the wavelet energy coefficients were screened using the maximum correlation coefficient and the local correlation coefficient extrema, respectively. A partial least-square regression model was established. Results showed that the characteristic bands selected via local correlation coefficient extrema in a wavelet energy coefficient created a detection model with optimal accuracy. The determination coefficient (*Rc* 2) of the calibration set was 0.7856, and the root-mean-square error (*RMSE*) of the calibration set (*RMSEC*) was 3.0408. The determination coefficient (*Rv* 2) of the validation set is was 0.7364, and the *RMSE* of the validation set (*RMSEV*) was 3.3032. Continuous wavelet transform is a process of data dimension enhancement which can effectively extract the sensitive variables from spectral datasets and improve the detection accuracy of models.

**Keywords:** canopy spectra; chlorophyll content; continuous wavelet transform (CWT); correlation coefficient; partial least square regression (PLSR)
