Comparison of SunScan and HemiView plant canopy analysis systems in LAI assay applications
Foreword
Measurement of the leaf area index of LAI plants usually requires a non-destructive method. The SunScan Plant Canopy Analyzer and the HemiView Digital Plant Canopy Analyzer are two typical instruments that use different models to calculate the leaf area index. This article will compare the models, measurement parameters, specific applications, etc., and provide guidance and reference for how to choose SunScan and HemiView. In addition, this article will also be the first to cover the application case of HemiView and BF5.
background
Campbell (1986) proposed the elliptic angle distribution function of the Direct solar beam through the canopy in 1986. This function can describe many different types of canopies by a single parameter, the Elliptical Leaf Angle Distribution Parameter (ELADP). In this model, the elliptical leaf angular distribution coefficient of the plant canopy is obtained by dividing the plane radius of the ellipsoid by the vertical radius.
Based on the calculation of leaf area index and elliptical leaf angle distribution coefficient of plant canopy, the extinction coefficient of incident light passing through a plant canopy at a specific zenith angle can be calculated by the following formula:
At the same time, the canopy porosity is also in accordance with Beer's law, which can be calculated by:
After obtaining the canopy porosity index, it can be transformed into plant leaf area index by hemispherical image analysis and incident light cut-off measurement. Especially when obtaining canopy porosity of multiple zenith angles, the evaluation of the angular distribution of the elliptical leaves can also be converted into plant leaf area index. This is the theoretical background shared by the HemiView plant canopy analyzer and the SunScan plant canopy analyzer. And because of the independence of the model scale, the practical application does not need to consider the actual thickness of the plant canopy, only the leaf area index of the entire canopy is evaluated.
LAI model in SunScan plant canopy analysis system
In the SunScan plant canopy analysis system, the BF5 optical radiation meter was used to monitor the condition of direct and scattered light radiation above the canopy, and the total transmitted light below the canopy was measured by the SS1 probe. We assume that solar radiation is a normal state and the attenuation of direct light is homogenized in order to apply the Campbell model for calculation. To do this, we also need to know the angle of incidence of the incident light and the distribution of the angle of the canopy blade for input into the SunScan system. Then the third type of parameter that is needed in the SunScan model is to evaluate the absorbance of the leaf surface, because the transmitted light measured below the canopy, in addition to the portion of the canopy that passes through the canopy, is certainly a small part due to the blade not being completely reflected or Light radiation that is absorbed and penetrated.
LAI model in HemiView plant canopy analysis system
From a certain perspective, HemiView's theory of computation is relatively simple. First, we assume that when the sky hemisphere image is acquired, it is in the absence of the sun and the illumination conditions are uniform. Secondly, the image obtained by HemiView is thresholded, mainly analyzing the transmission of incident light passing through the canopy in all directions above the canopy. Light and interception rate. Therefore, there is no need to consider light reflection in this process, and the scattered light does not need attention. This simplifies the calculation process because it does not need to consider light intensity and light reflection, and only pays attention to transmitted light and light absorption. And the transmitted light measured by HemiView takes into account various possible angles, from which the leaf angle distribution and the LAI index can be evaluated. Therefore, in HemiView, the canopy leaf angle distribution is an output value, not a parameter like SunScan that needs to be evaluated in advance.
Optical Radiation Calculation in HemiView Plant Canopy Analysis System
HemiView does not use light intensity in calculating the LAI index, but the intensity of the light intensity above and below the canopy of the plant needs to be evaluated. The calculation of optical radiation is mainly based on the preset canopy structure and correlation coefficient in the hemisphere image, so that we can calculate the illumination conditions under the canopy regardless of the illumination conditions above the canopy. Therefore, through a picture, we can even calculate the lighting conditions under the canopy within a period of one year below the canopy or a period of time when a certain canopy structure is basically unchanged. HemiView also contains optical models for evaluating direct and scattered light (under clean sky conditions), but HemiView does not consider blade reflections in optical radiation calculations, and SunScan does better in this regard.
Application of actual forest vegetation canopy
Really, especially the forest vegetation canopy of mixed tree species, due to the high degree of heterogeneity and uncertainty of its canopy structure, will make the assumptions and calculation results of the LAI model less than a thousand miles. Suppose you think that the LAI calculated by HemiView is effective, that is, the LAI index of a uniform plant canopy predicts uniform light interception in the canopy, but this is not exactly the same as the LAI under actual conditions. The growth of plant canopies is closely related to light interception. Therefore, if the canopy visible light actually measured in HemiView is used to evaluate its effect on plant canopy, it will reflect the difference of plant canopy more than the conclusions from various hypothetical inferences. Qualitative. If BF5, an illuminating radiation sensor that measures the actual total radiation and scattered radiation above the canopy, is introduced into the HemiView system, the reliability of HemiView's computational theory and results can be better optimized.
HemiView plant canopy analysis system combined with BF5 solar radiation sensor
The canopy hemisphere image is a geometric projection of the canopy that provides many canopy structural parameters. If other measures are used to introduce direct and scattered light measurements (such as small weather stations, total radiation models, or direct measurements of BF5 solar radiation sensors), it will be more convincing to assess the underlying conditions of the canopy.
How is the HemiView plant canopy analysis system combined with the BF5 solar radiation sensor?
It depends on what you are going to do:
A: Match the HemiView 's solar model with the local climate conditions, which are generally used for long-term monitoring. The HemiView 's solar model requires input of the transmission coefficient of the atmosphere and the ratio of scattered light in the total incident light.
- Place BF5 above the canopy for long-term operation to obtain monitoring data
- According to this, the monthly total direct radiation and scattered radiation of the monitoring point are obtained.
- Calculate the correlation value using HemiView
- Correct the transmission coefficient of the atmosphere and the ratio of the scattered light in the total incident light
Such a correction will result in more convincing calculations using HemiView and its solar model.
B. BF5 is used to measure the actual optical radiation conditions above the plant canopy so that HemiView calculates the canopy porosity.
B.1 Long-term monitoring
1. Analyze the monitoring data of BF5 to give the total monthly scattered radiation, as well as the total monthly value of direct radiation and the time-sharing value of each day.
2. These data are multiplied by the site factor (calculated scattering radiation) and canopy porosity (calculated direct radiation)
3. Add the data to obtain the transmitted light radiation under the canopy
- = upper scattered radiation (measured by BF5) xISF (from HemiView)
- = above direct radiation (measured by BF5) x canopy porosity (fromHemiView)
B.2 Spot or timetable analysis
Correcting the timetable in HemiView with measured data from BF5
Radiation below the canopy = solar trajectory in the top direct radiation x schedule
B.3 Time equation correction
When HemiView has embedded the sun, that is, when the sun is in the south, it is defined as 12 noon. However, due to the orbital deviation of the earth around the sun, the sun and the clock may have a time difference of up to 20 minutes, which must be often Corrected.
How to choose the SunScan and HemiView plant canopy analysis system
HemiView evaluates the geometric effects of the canopy in detail with respect to SunScan
BF5 combined with HemiView application provides fast and direct measurement of solar radiation
The SunScan plant canopy analysis system measures the PAR illumination levels above and below the canopy in real time. Combined with the theoretical model of the canopy structure, the plant canopy LAI index is calculated, which is easy to repeat and easy to carry. The HemiView plant canopy analysis system calculates the LAI index by analyzing the geometric features of the canopy hemisphere image, and calculates the upper and lower illumination levels of the canopy.
The crops are relatively low-rise and the canopy is relatively uniform, matching the theoretical assumptions and models of the SunScan plant canopy analysis system, so the SunScan plant canopy analysis system is ideal for low-crop crop and vegetation canopy analysis due to tall trees or The illumination above the canopy of forest plants is difficult to measure, so the SunScan plant canopy analysis system is powerless.
The HemiView plant canopy analysis system is ideal for evaluating canopy structures and is less affected by canopy height, making it more suitable for the analysis of tall canopy canopies, especially in combination with BF5 solar radiation sensors. Conversely, the hemispherical image of the canopy of the low crop is difficult to obtain, and the rapid change of the canopy structure with rapid growth makes it difficult to apply HemiView to evaluate its LAI.
So, in general, both can be analyzed for multiple canopy types. SunScan plant canopy analysis is more suitable for low-profile, regular growth, canopy uniform analysis applications. HemiView plant canopy analysis system (better with BF5 solar radiation sensor) is more suitable for analysis of tall and canopy structures. application.
Comparison of SunScan and HemiView plant canopy analysis systems
HemiView plant canopy analysis system | SunScan Plant Canopy Analysis System | |
Technical principle | Obtaining canopy characteristics by acquiring canopy hemisphere images and then analyzing the images | Calculate plant canopy parameters by measuring direct and scattered light |
System composition | The hardware includes camera, fisheye lens, self-balancing bracket and necessary accessories. The software is HemiView analysis software. | SunScan probe sensor, SunData software, BF5 optical radiation sensor and handheld data terminal |
Applicable canopy type | Ideal for tall, irregular canopy structure types such as forests | Ideal for low-profile, regular canopy structure types such as crops that reflect well canopy PAR rejection |
Light condition measurement | Need uniform lighting conditions (dawn, dusk, and cloudy or cloudy) | Can be used under any lighting conditions (preferably measured during the day) |
Sun radiation | Calculate solar radiation based on geometric features using canopy hemisphere images | Direct measurement of solar radiation using sensors: total incident and transmitted light, and direct and scattered light above the canopy |
LAI leaf area index | Calculated by image analysis and embedded model | Calculated by sensor measurements and embedded models |
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