Abstract
To overcome the dependence on sunlight of multi-spectral cameras, an active light source multi-spectral imaging system was designed and a preliminary experimental study was conducted at night without solar interference. The system includes an active light source and a multi-spectral camera. The active light source consists of four integrated LED (Light Emitting Diode) arrays and adjustable constant current power supplies. The red LED arrays and the near-infrared LED arrays are each driven by an independently adjustable constant current power supply. The center wavelengths of the light source are 668 nm and 840 nm, which are consistent with that of filter lens of the Rededge-M multi-spectral camera. This paper shows that the radiation intensity measured is proportional to the drive current and is inversely proportional to the radiation distance, which is in accordance with the inverse square law of light. Taking the inverse square law of light into account, a radiation attenuation model was established based on the principle of image system and spatial geometry theory. After a verification test of the radiation attenuation model, it can be concluded that the average error between the radiation intensity obtained using this model and the actual measured value using a spectrometer is less than 0.0003 w/m2. In addition, the fitting curve of the multi-spectral image grayscale digital number (DN) and reflected radiation intensity at the 668 nm (Red light) is y = −3484230x2 + 721083x + 5558, with a determination coefficient of R2 = 0.998. The fitting curve with the 840 nm (near-infrared light) is y = 491469.88x + 3204, with a determination coefficient of R2 = 0.995, so the reflected radiation intensity on the plant canopy can be calculated according to the grayscale DN. Finally, the reflectance of red light and near-infrared light can be calculated, as well as the Normalized Difference Vegetation Index (NDVI) index. Based on the above model, four plants were placed at 2.85 m away from the active light source multi-spectral imaging system for testing. Meanwhile, NDVI index of each plant was measured by a Greenseeker hand-held crop sensor. The results show that the data from the two systems were linearly related and correlated with a coefficient of 0.995, indicating that the system in this article can effectively detect the vegetation NDVI index. If we want to use this technology for remote sensing in UAV, the radiation intensity attenuation and working distance of the light source are issues that need to be considered carefully.
Highlights
The Normalized Difference Vegetation Index (NDVI) crop growth parameter is an important indicator of crop growth status [1]
In order to improve the efficiency of multi-spectral camera imaging technology, the combination of active non-parallel light source technology and multi-spectral imaging technology is proposed according to the respective advantages of active light source and spectral imaging technology
Sensor and small coverage area of single operation, we propose to combine multi-spectral imaging technology with active light source technology to collect the spectrum over a wide range
Summary
The Normalized Difference Vegetation Index (NDVI) crop growth parameter is an important indicator of crop growth status [1]. Non-destructive and accurate monitoring of the crop nitrogen status is important for diagnosing crop growth characteristics, improving the nitrogen management level and utilization efficiency, and reducing farmland environmental pollution brought by excessive nitrogen application [2,3,4]. Canopy spectral sensing technology based on remote sensing sensors [8] is an important means to obtain crop growth information indicators [9,10]. Passive sensors receive solar radiation reflected by the vegetation or their own thermal radiation energy [12]. Since the passive light source measuring instrument mainly regards sunlight as a radiation source, its greatest limitation is that it is affected by insufficient sunlight intensity and the solar zenith angle. It generally needs to be used when the weather is clear and using a small solar zenith angle [13,14,15]
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