Study On The Bidirectional Reflectance Characteristics Of Wheat Leaves

In recent years,with the application of hyperspectral and multi-angle remote sensing technology,the study of crop canopy optical properties has become a hotspot in quantitative vegetation remote sensing.The directional change of solar radiation determines the directionality of absorption and reflection of solar spectrum information in field crops.The model established by the study of the absorption and reflection spectral directivity has a very high application value,which be used to simulate and invert the geometric parameters and biochemical content.The relative concentration of leaf pigment has a significant effect on the physiology and spectroscopic characteristics of plant leaves.The leaf pigment content affects the process of photosynthesis,light utilization efficiency,quality and energy exchange and stress response in plant growth process,the study of the comprehensive relationship between the pigments not only can be used to study the visualization characteristics of the leaves,and can also be used in remote sensing to quantify the biochemical components and structural characteristics of the leaves.In this paper,wheat and maize leaves were used as the object of study.Based on the laboratory designed and the improved optical orientation characteristic measurement system,the optical orientation and bi-directional reflection distribution of crop leaves were studied.The main contents of the study are as follows:(1)Improve the laboratory's original optical direction reflection acquisition system,shorten the acquisition time and reduce the experimental error.The improved optical direction characteristic measuring apparatus can realize the function of measuring the distribution of the reflected light at a plurality of receiving zenith angles at a receiving azimuth angle.At the same time,the optical multiplexer and the bearing seat can be rotated to ensure that the relative position of the receiving sensors in the measurement process was fixed and the effect of the optical fiber disturbance on the experiment was reduced.Therefore,the measuring device measurement process was short,efficient,lower experimental error and higher measurement accuracy.(2)Studyed the bidirectional reflection distribution and influencing factors of wheat leaves in the near infrared band.When the light source was incident at a fixed angle,the reflected light distribution was concentrated in the micro-space with symmetrical symmetry of the incident light.And also,The larger the zenith angle of the light source,the greater the anisotropy index of the blade,the reason is that the mirror effect and the forward scattering effect together affect this change.At the same time,the higher the chlorophyll concentration,the lower the leaf anisotropy index,the reason may be that the increase of chlorophyll concentration will increase the photosynthesis rate and the light absorption rate increase,resulting in the relative decrease of the leaf reflectivity,which leads to the decrease of leaf anisotropy index.By analyzing the scanning electron microscopy of the two leaves,it is concluded that the microstructure difference(leaf spacing)or trichomes distribution of the leaves was one of the factors that affect the bidirectional reflection distribution of the leaves.(3)Analyzed the stability of chlorophyll spectrum model of wheat leaves at 30°Incident conditions.In this paper,four different pretreatment methods combined with PLS for full-band modeling analysis were used in this paper,selected the MSC-PLS model as modeling program.The spectral data at 144 receiving angles were processed to obtain the optimal receiving angle of the chlorophyll spectrum model,we got that when it was 30° incident condition,the optimal receiving angle was(50 °,15 °).The optimal model established under the 30° incident condition was BW-MLR model,and Rpre of the model was 0.968,RPD was 3.848,the model works better.Finally,the error analysis was carried out at different angles at five different receiving angles of chlorophyll spectroscopy.The results showed that the stability of the chlorophyll spectrum model of wheat leaves was affected by the zenith angle and azimuth angle,and the stability of the model decreased with the increase of the zenith angle.But the stability of the model was better at the receiving azimuth angle of 90°-270 °,which was less affected by the receiving zenith angle.At the same time,the factors that contribute to the stability of the chlorophyll spectrum model were analyzed,that is,the stability of the model was less affected by the receiving direction when the position of the light source is on the same side as the receiving azimuth.