CN113218916A - Method for estimating chlorophyll content by utilizing anisotropy coefficient - Google Patents

Method for estimating chlorophyll content by utilizing anisotropy coefficient Download PDF

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CN113218916A
CN113218916A CN202110522178.2A CN202110522178A CN113218916A CN 113218916 A CN113218916 A CN 113218916A CN 202110522178 A CN202110522178 A CN 202110522178A CN 113218916 A CN113218916 A CN 113218916A
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reflection
chlorophyll content
observation
coefficient
anif
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CN113218916B (en
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卢珊
孙仲秋
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Northeastern University China
Northeast Normal University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/55—Specular reflectivity
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N2021/1793—Remote sensing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/55—Specular reflectivity
    • G01N2021/551—Retroreflectance

Abstract

The invention discloses a method for estimating chlorophyll content by utilizing an anisotropic coefficient, belonging to the technical field of optical remote sensing detection. The method can estimate the chlorophyll by utilizing the anisotropy coefficient, can exert the advantage of multi-angle observation, and provides a basis for agricultural and forestry science remote sensing.

Description

Method for estimating chlorophyll content by utilizing anisotropy coefficient
Technical Field
The invention belongs to the technical field of optical remote sensing detection, and particularly relates to a method for estimating chlorophyll content by utilizing an anisotropy coefficient.
Background
Chlorophyll is one of main pigments of plant leaves, can reflect important indexes of plant physiological conditions, photosynthetic capacity, stress and aging, and the change of the content can be used for representing the physiological conditions of plants. Therefore, the estimation of the chlorophyll content of the plant from the physiological and botanical aspects has important research significance. The traditional field measurement adopts a solution extraction analysis method, so that the chlorophyll content can be estimated most accurately, but the method is time-consuming and is not suitable for continuously monitoring large-area vegetation.
Chlorophyll of leaves absorbs incident light of a specific waveband, so that the chlorophyll content of various plants can be detected by an optical method. The spectral reflectance measurement is quite suitable for measuring the chlorophyll content on the level of leaf or canopy because of the characteristics of no damage and rapidness. In the visible light band, chlorophyll strongly absorbs blue and red light, and as the content of chlorophyll changes, the absorption characteristics also change. This allows the researcher to use the spectral index to estimate chlorophyll content.
The spectral index allows estimation of chlorophyll content using reflections at different wavelengths, whereas most previous studies on estimation of chlorophyll content have been performed using a spectrometer with an integrating sphere. In an integrating sphere, the angular reflection is negligible because the light reflected from different angles is integrated. However, these studies neglected the effect of multi-angle reflection on estimation of chlorophyll content in green leaves, and researchers have also pointed out that reflection coefficients obtained from different observation directions affect the spectral index value due to the anisotropic nature of reflection from plant leaves, and thus affect the accuracy of spectral index estimation of chlorophyll content.
Therefore, how to provide a method for estimating chlorophyll content by using anisotropy coefficient is a problem that needs to be solved by those skilled in the art.
Disclosure of Invention
In view of the above, the invention provides a method for estimating chlorophyll content by using an anisotropy coefficient, which can estimate chlorophyll by using the anisotropy coefficient, can exert the advantage of multi-angle observation, and provides a basis for agricultural and forestry science remote sensing.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for estimating chlorophyll content by utilizing anisotropy coefficients is characterized in that near a specular reflection direction, reflection information of plant leaves at different detection zenith angles is measured, a relation between a spectral index of the anisotropy coefficients in leaf reflection light and the chlorophyll content of vegetation is established, and estimation of chlorophyll content of different types of plant leaves by utilizing the anisotropy coefficients is achieved.
Preferably, reflection measurement is carried out on the plant leaf, and the optical characteristics of the leaf need to be expressed through a bidirectional reflection coefficient BRF; the BRF is the ratio of the radiant flux measured on the surface of the blade to the radiant flux of a standard white board under the same illumination condition and the same observation condition, and the formula is as follows:
Figure BDA0003064409620000021
wherein λ represents a wavelength, θsAt the angle of incidence zenith, θvIn order to observe the zenith angle,
Figure BDA0003064409620000022
as a result of the azimuth angle of incidence,
Figure BDA0003064409620000023
for observation of azimuth angle, ρλThe reflectivity of a standard whiteboard in the hemispherical direction is reasonable.
Preferably, the anisotropy coefficient ANIF normalizes all reflection data to the standard reflection signature of the respective target, by which the anisotropy effect is separated from the underlying target reflection signature, the anisotropy coefficient ANIF being expressed as:
Figure BDA0003064409620000024
wherein R is a bidirectional reflection coefficient in a specific view field direction, R0The reflection coefficient of the observation in the vertical detection direction is shown, lambda represents the wavelength, theta represents the zenith angle, phi represents the azimuth angle, s represents the incident direction, and v represents the observation direction.
Preferably, when the surface roughness and the refractive index of the blade of different species are changed, the reflection peak value of the blade is changed along with the change, and the reflection peak value appears in an observation direction larger than the specular reflection direction; in the vertical and backscattering directions, the reflection of the blade is almost isotropic and is approximately considered to be the diffuse reflection generated inside the blade; vertical direction reflection R in the definition of ANIF0Approximately equal to the diffuse reflection portion Rdiff,λR for different detection angles is expressed as the sum R of the diffuse reflection portion and the specular reflection portionspec,λ+Rdiff,λ(ii) a Thus, ANIF is represented as:
Figure BDA0003064409620000031
since specular reflection hardly changes with wavelength, the difference ratio spectral index consisting of three bands will weaken the specular reflection affecting the estimation of chlorophyll content, i.e. in the form of the difference ratio:
Figure BDA0003064409620000032
wherein, ANIFλ1Value of anisotropy coefficient for the first wavelength band, ANIFλ2Value of anisotropy coefficient of the second band, ANIFλ3The value of the anisotropy coefficient for the third wavelength band; rspec，λ1Is the diffusely reflecting portion of the first wavelength band, Rspec，λ2Is the diffusely reflecting portion of the second wavelength band, Rspec，λ3Is a diffuse reflection part of the third wavelength band, Rdiff，λ1Is a specularly reflective part of the first wavelength band, Rdiff，λ2Is a specularly reflective part of the second wavelength band, Rdiff，λ3Is the specular reflection part of the third wavelength band.
Preferably, the anisotropy coefficients of the blade under different observation angles are a bidirectional reflection coefficient R measured through each observation angle and a bidirectional reflection coefficient R observed vertically0Calculating to obtain; in practical measurements, since an ideal diffuse reflection plate is almost isotropic, the radiances dL in each observation direction are considered to be consistent, and assuming that the reflected radiances of the whiteboard are the same at different observation angles, equation (4) is further derived as:
Figure BDA0003064409620000041
wherein dLSampleThe radiation flux measured at the blade surface.
Preferably, the relationship between the spectrum index of the anisotropy coefficient in the leaf reflected light and the chlorophyll content of the vegetation is represented by establishing a regression estimation model through the chlorophyll content and the vegetation index value calculated at each observation angle:
y＝ax+b (6)
wherein a is slope, b is intercept, x is vegetation index value, and y is chlorophyll content.
The invention has the beneficial effects that:
according to the method, the anisotropy coefficient of the plant leaf is utilized, and higher chlorophyll estimation results can be obtained in multiple observation directions. Since the anisotropy coefficient utilizes the ratio relation of the reflection coefficients, the calibration of a standard diffuse reflection plate is not needed in the actual measurement process, and only the radiation fluxes at different observation angles and in the vertical observation direction are needed, so that the great convenience condition is provided for the actual operation. The reflection coefficient can be obtained without measuring the reflection radiance of the surface of the blade for the first time like measuring the reflection coefficient of the blade and then carrying out normalization processing on the reflection radiance of the blade and the reflection radiance of the standard white board. But the anisotropic coefficient is used for representing the optical characteristics of the surface of the blade, so that the normalization processing can be performed without an additional standard white board, and the method has the advantage of convenience in reflection information acquisition.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
FIG. 1 is a diagram showing the distribution of anisotropy coefficients with wavelength bands calculated at different observation angles.
FIG. 2 is a graph of the change of anisotropy coefficient calculated at an observation angle of 60 degrees for samples with different chlorophyll contents of a Reptilia chinensis Benth sample.
FIG. 3 is a graph of a regression estimation model established for chlorophyll content (LCC) and vegetation index values (industries Value) of a sample in a modeling dataset according to the present invention.
FIG. 4 is a schematic diagram of the reflection spectrum measurement of the plant leaf according to the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention provides a method for estimating chlorophyll content by utilizing an anisotropic coefficient, which is characterized in that the relationship between the spectral index of the anisotropic coefficient in leaf reflected light and the chlorophyll content of vegetation is established by measuring the reflection information of different detection zenith plant leaves near the specular reflection direction, so that the estimation of the chlorophyll content of different types of plant leaves by utilizing the anisotropic coefficient spectral index is realized.
Performing reflection measurement on plant leaves, and expressing the optical characteristics of the leaves by a bidirectional reflection coefficient BRF; BRF is the radiation flux (dL) measured at the blade surface under the same illumination condition and the same observation conditionSample) Ratio of radiant flux (dL) to a standard white board (ideal diffuse reflector, and same as the sample blade viewing area). The formula is as follows:
Figure BDA0003064409620000061
wherein λ represents a wavelength, θsAt the angle of incidence zenith, θvIn order to observe the zenith angle,
Figure BDA0003064409620000062
as a result of the azimuth angle of incidence,
Figure BDA0003064409620000063
for observation of azimuth angle, ρλThe reflectivity of a standard whiteboard in the hemispherical direction is reasonable.
The anisotropy coefficient (ANIF) normalizes all reflection data to the standard reflection signature of the respective target. In this way the anisotropic effects are separated from the underlying target reflection features. In many cases, it is more intuitive to use a vertically observed reflectance as the standard reflectance feature. The anisotropy coefficient (ANIF) can be expressed as:
Figure BDA0003064409620000064
wherein R is a bidirectional reflection coefficient in a specific view field direction, R0The reflection coefficient of the observation in the vertical detection direction is shown, lambda represents the wavelength, theta represents the zenith angle, phi represents the azimuth angle, s represents the incident direction, and v represents the observation direction.
In the measurement of the reflection of the plant leaf, the leaf reflection can be divided into two parts of diffuse reflection and specular reflection. The diffuse reflection contains information on the biochemical parameters inside the leaf. Since specular reflection is caused by reflection from the leaf surface and its reflected radiation component does not enter the interior of the leaf, specular reflection is independent of leaf pigment content and is therefore an important contributor to chlorophyll content estimation. The anisotropic character of the plant leaves has very distinct angular distribution characteristics. In the forward observation direction, especially the specular reflection direction (the incident direction is at the same angle as the observation direction), the anisotropic characteristic is very obvious. When the surface roughness and the refractive index of the blade of different species are changed, the reflection peak value of the blade is changed, and the reflection peak value may appear in a larger observation direction than the mirror reflection direction. Whereas in the perpendicular and back-scattering directions the reflection of the blade is almost isotropic and can be approximated as a diffuse reflection generated inside the blade. Thus, the vertical direction reflection R in the ANIF definition0Can be approximately equal to the diffuse reflection part (R)diff,λ) R for different detection angles is expressed as the sum of the diffuse reflection portion and the specular reflection portionAnd (R)spec,λ+Rdiff,λ) (ii) a Thus, ANIF is represented as:
Figure BDA0003064409620000071
since specular reflection hardly changes with wavelength, the difference ratio spectral index consisting of three bands will weaken the specular reflection affecting the estimation of chlorophyll content, i.e. in the form of the difference ratio:
Figure BDA0003064409620000072
wherein, ANIFλ1Value of anisotropy coefficient for the first wavelength band, ANIFλ2Value of anisotropy coefficient of the second band, ANIFλ3The value of the anisotropy coefficient for the third wavelength band; rspec，λ1Is the diffusely reflecting portion of the first wavelength band, Rspec，λ2Is the diffusely reflecting portion of the second wavelength band, Rspec，λ3Is a diffuse reflection part of the third wavelength band, Rdiff，λ1Is a specularly reflective part of the first wavelength band, Rdiff，λ2Is a specularly reflective part of the second wavelength band, Rdiff，λ3Is the specular reflection part of the third wavelength band.
Therefore, according to the formulas (2), (3) and (4), it can be found that the spectral index based on the ANIF actually utilizes the characteristic that the specular reflection does not change along with the change of the wavelength, and the specular reflection is eliminated in the form of a ratio, so that the spectral index based on the ANIF fully utilizes the diffuse reflection information of the leaves, the influence of the specular reflection on the estimation of the chlorophyll content is reduced, and the estimation of the chlorophyll content with high precision is realized.
According to the method, the reflection information of the plant leaves at different detection zenith angles is measured, the anisotropy coefficients of different observation angles are calculated based on the formula (2), and the estimation of the chlorophyll content of the leaves by using the anisotropy coefficients is finally realized based on the relation between the spectral index of the anisotropy coefficients and the chlorophyll content of the vegetation in the formula (4).
The anisotropy coefficients of the blade under different observation angles are a bidirectional reflection coefficient R measured through each observation angle and a bidirectional reflection coefficient R vertically observed0And (4) calculating. In practical measurements, since an ideal diffuse reflector is almost isotropic, it can be considered that its radiance dL in each observation direction is uniform. We can therefore assume that the brightness of the reflected radiation of the whiteboard is the same at different angles of observation, so equation (4) can be further derived:
Figure BDA0003064409620000081
wherein dLSampleThe radiation flux measured at the blade surface.
From equation (5), the spectral index obtained by the ratio of the differences between the ANIFs at the two wavelengths may not require measurement of the radiant flux of a standard diffuse reflector. The results obtained according to the formula (4) and the formula (5) are approximately equal, so that the estimation accuracy using the vegetation index based on the bidirectional reflection coefficient and based on the radiance is approximate. Therefore, the method does not need to measure the reflection coefficient of the blade, and the reflection radiance of the surface of the blade needs to be measured for the first time, and then the reflection coefficient can be obtained by normalizing the reflection radiance of the blade and the reflection radiance of the standard white board. Therefore, the anisotropic coefficient is used for representing the optical characteristics of the blade surface, normalization processing can be carried out without an additional standard white board, and the method has the advantage of convenience in reflection information acquisition.
Establishing a regression estimation model for the relationship between the spectrum index of the anisotropy coefficient in the leaf reflected light and the chlorophyll content of the vegetation through the chlorophyll content and the vegetation index value obtained by calculation at each observation angle, wherein the regression estimation model is represented as follows:
y＝ax+b (6)
wherein a is slope, b is intercept, x is vegetation index value, and y is chlorophyll content.
According to the method, the anisotropy coefficient of the plant leaf is utilized, and higher chlorophyll estimation results can be obtained in multiple observation directions. Using reflection coefficient due to anisotropy coefficientThe ratio relation of (A) and (B) can be corrected without a standard diffuse reflection plate in the actual measurement process, and only the radiation fluxes at different observation angles and in the vertical observation direction need to be acquired, so that great convenience is provided for actual operation. The reflection coefficient can be obtained without measuring the reflection radiance of the surface of the blade for the first time like measuring the reflection coefficient of the blade and then carrying out normalization processing on the reflection radiance of the blade and the reflection radiance of the standard white board. But the anisotropic coefficient is used for representing the optical characteristics of the surface of the blade, so that the normalization processing can be performed without an additional standard white board, and the method has the advantage of convenience in reflection information acquisition. The invention can establish anisotropy index ((ANIF) in blade reflected light near the specular reflection direction750-ANIF705)/(ANIF750+ANIF705-2*ANIF445) Relation between chlorophyll content (coefficient of determination R)2Equal to 0.9), the estimation of chlorophyll content of different plant leaves by using the anisotropy coefficient spectral index is realized, and the root mean square error RMSE is 8.15 mu g/cm2. The method for estimating chlorophyll has the advantages of convenience, stability and universality, and the chlorophyll content can be monitored nondestructively and continuously in the fields of forestry and agriculture.
According to the method provided by the invention, multi-angle reflection spectrum measurement is carried out on the leaves of plants of multiple species. By measuring the reflection coefficient of the blade in the 400-nm and 1000-nm wave band range, the anisotropy coefficient of each observation angle is calculated based on the formula (3). The experimental measurement sample set was approximated as 3: 2 is divided into: modeling (sample size 166) and testing (sample size 102); the results of chlorophyll content (LCC) distribution are shown in Table 1, wherein Table 1 shows the species, number of samples and distribution of chlorophyll content of the measured leaf modeling dataset and the validation dataset. The modeling dataset and the validation dataset each comprise a plurality of species having different surface structures, such as scindapsus aureus, populus, apricot trees with smooth leaf surfaces, or raspberries and maples with rough leaf surfaces containing fine fuzz. The chlorophyll content of the sample leaf is also widely distributed from a lower value to a higher value, and can represent the distribution condition of most of the chlorophyll content in the nature.
Figure BDA0003064409620000091
Figure BDA0003064409620000101
TABLE 1
Taking a daphne blade as an example, the change condition of the anisotropy coefficients measured at different observation angles along with the wave band is calculated, and the obtained result is shown in fig. 1.
The results of the anisotropy coefficients calculated at an observation angle of 60 ° with the chlorophyll content are shown in fig. 2, taking 6 samples of the daphne leaf as an example.
Then, the anisotropy coefficient obtained by calculating each observation angle of the sample in the modeling data set, and the determining coefficient R obtained by various forms of vegetation indexes and chlorophyll content based on the anisotropy2As shown in table 2. It can be seen from equation (4) and table 2 that the difference ratio between the three bands can well remove the influence of specular reflection at each observation angle, so that each observation angle has a high correlation, for example (ANIF)750-ANIF705)/(ANIF750+ANIF705-2*ANIF445)。
Figure BDA0003064409620000102
TABLE 2
Taking the vegetation index with the best coefficient performance (ANIF750-ANIF705)/(ANIF750+ ANIF705-2 ANIF445) "as an example, a regression estimation model was established using the chlorophyll content and the vegetation index Value (industries Value) calculated from each observation angle, as shown in fig. 3. As can be seen from fig. 3, there is a strong linear relationship between the vegetation index and chlorophyll content.
Then, based on the 5 vegetation indices (R) that performed well2>0.8) on the basis of the verification datasetThe precision of estimation of chlorophyll content was obtained from the vegetation index values and expressed in RMSE (root mean square error). The estimation accuracy is shown in table 3. Using the estimation model between chlorophyll content and vegetation index value in table 3 (y ═ ax + b, where a is slope, b is intercept, x is vegetation index value, and y is chlorophyll content), the chlorophyll content precision (RMSE) estimated by the validation set is obtained. As can be seen from Table 3, the 5-difference ratio type vegetation indexes all have better estimation accuracy (R) of chlorophyll content2≥0.85,RMSE<12μg/cm2) And wherein the vegetation index "(ANIF 750-ANIF705)/(ANIF750+ ANIF 705-2. ANIF 445)" has the highest estimation accuracy and RMSE of 8.15. mu.g/cm2。
Figure BDA0003064409620000111
According to the technical effect obtained by the invention, the higher chlorophyll estimation results can be obtained in a plurality of observation directions by utilizing the anisotropy coefficient of the plant leaves. Since the anisotropy coefficient utilizes the ratio relation of the reflection coefficients, the calibration of a standard diffuse reflection plate is not needed in the actual measurement process, and only the radiation fluxes at different observation angles and in the vertical observation direction are needed, so that the great convenience condition is provided for the actual operation. This also brings scientific support for the design of remote sensing sensors, and can have wider selection. In addition, the experimental sample selected by the invention comprises a plurality of species, the surface structure of the leaf is complex and various, and the chlorophyll content is related from small to large. The technical effect obtained by the invention has higher estimation precision in a larger chlorophyll content range. Has enough precision guarantee for different species structures and a wide chlorophyll content range.
The specific implementation process comprises the following steps:
the spectral measurement of plant leaves requires a collimated incident light source, a spectrometer, an angle measuring device and a detection lens. The collimated light source is a halogen lamp, has a continuous spectrum as the sun, can freely rotate on a quarter circular arc with the rotation precision of 0.25 degrees, and simulates different incident zenith angles. The detection direction is controlled by the multi-angle measuring device, the optical fiber can freely rotate with the precision of 0.25 degrees in the detection direction of plus or minus 90 degrees of the zenith, and the optical fiber of the spectrometer is installed on a rotating arm of the multi-angle measuring device, as shown in figure 4.
The selection of the leaves follows the principle of multi-species and extensive chlorophyll distribution, and the leaves are selected from the upper layer, the middle layer and the lower layer of the vegetation canopy. And selecting the leaves which are flat, have no obvious spots and are uniform in color. The chlorophyll is uniformly selected from low to high chlorophyll value, and the chlorophyll content range of the sample is enlarged. During measurement, the field angle is 8 degrees, and the distance between the detector and the water surface is 0.2 meter. The measurement process is carried out for 10 minutes as far as possible, and the fact that the chlorophyll content of the leaves is not changed in the measurement process is guaranteed.
The detection lens is mounted at the front end of the optical fiber of the spectrometer, as shown in fig. 4. The spectrometer used by the invention is an ASD (automatic absorption spectroscopy) surface feature spectrometer, and the effective waveband is 400-1000 nm. During measurement, the incident zenith angle theta is ensured by the angle measuring deviceiEqual to the detected zenith angle thetasIn the range of 20-60 degrees, the measurement is carried out once every 10 degrees, and finally the measurement is carried out in the vertical observation direction, and the measurement is carried out in 6 directions. And measuring the spectral data of the blade and the ideal reflector data for 5 times in each direction, and calculating the bidirectional reflectance and the anisotropic coefficient of the blade at each observation angle according to a formula (1) and a formula (2).
After obtaining the anisotropy coefficient, based on a formula (4) and a nonlinear least square fitting method in Matlab, based on the anisotropy coefficient values of 5 different angles, an estimation model is established on vegetation indexes of various forms and the chlorophyll content. And verifying by using an estimation model obtained by a plurality of species sample data sets on the vegetation index based on ANIF to obtain better estimation accuracy. The estimation model obtained on the vegetation index based on the ANIF has certain universality, and can be directly used for estimating the chlorophyll content of the vegetation under the condition of accurately obtaining the reflection information of the vegetation. In addition, the invention can carry out standardized correction without a standard white board, can simplify the measurement process and provides convenient conditions for actual measurement.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. The device disclosed by the embodiment corresponds to the method disclosed by the embodiment, so that the description is simple, and the relevant points can be referred to the method part for description.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (6)

1. A method for estimating chlorophyll content by utilizing an anisotropy coefficient is characterized in that near a specular reflection direction, reflection information of plant leaves at different detection zenith angles is measured, a relation between a spectral index of the anisotropy coefficient in leaf reflection light and the chlorophyll content of vegetation is established, and estimation of chlorophyll content of different types of plant leaves by utilizing the spectral index of the anisotropy coefficient is realized.
2. The method for estimating chlorophyll content by using anisotropy coefficient as claimed in claim 1, wherein the reflection measurement is performed on plant leaves, and the optical characteristics of the leaves need to be expressed by Bidirectional Reflectance (BRF); the BRF is the ratio of the radiant flux measured on the surface of the blade to the radiant flux of a standard white board under the same illumination condition and the same observation condition, and the formula is as follows:
Figure FDA0003064409610000011
wherein λ represents a wavelength, θsAt the angle of incidence zenith, θvTo observe the dayThe top angle is a vertical angle,
Figure FDA0003064409610000012
as a result of the azimuth angle of incidence,
Figure FDA0003064409610000013
for observation of azimuth angle, ρλThe reflectivity of a standard whiteboard in the hemispherical direction is reasonable.
3. The method according to claim 2, wherein the anisotropy coefficient ANIF normalizes all reflection data to a standard reflection feature of a respective target, and the anisotropy effect is separated from the potential target reflection feature by the method, and the anisotropy coefficient ANIF is expressed as:
Figure FDA0003064409610000014
wherein R is a bidirectional reflection coefficient in a specific view field direction, R0The reflection coefficient of the observation in the vertical detection direction is shown, lambda represents the wavelength, theta represents the zenith angle, phi represents the azimuth angle, s represents the incident direction, and v represents the observation direction.
4. The method according to claim 3, wherein when the surface roughness and refractive index of the leaves of different species are changed, the reflection peak of the leaf is changed and appears in a larger observation direction than the specular reflection direction; in the vertical and backscattering directions, the reflection of the blade is almost isotropic and is approximately considered to be the diffuse reflection generated inside the blade; vertical direction reflection R in the definition of ANIF0Approximately equal to the diffuse reflection portion Rdiff,λR for different detection angles is expressed as the sum R of the diffuse reflection portion and the specular reflection portionspec,λ+Rdiff,λ(ii) a Thus, ANIF is represented as:
Figure FDA0003064409610000021
since specular reflection hardly changes with wavelength, the difference ratio spectral index consisting of three bands will weaken the specular reflection affecting the estimation of chlorophyll content, i.e. in the form of the difference ratio:
Figure FDA0003064409610000022
wherein, ANIFλ1Value of anisotropy coefficient for the first wavelength band, ANIFλ2Value of anisotropy coefficient of the second band, ANIFλ3The value of the anisotropy coefficient for the third wavelength band; rspec，λ1Is the diffusely reflecting portion of the first wavelength band, Rspec，λ2Is the diffusely reflecting portion of the second wavelength band, Rspec，λ3Is a diffuse reflection part of the third wavelength band, Rdiff，λ1Is a specularly reflective part of the first wavelength band, Rdiff，λ2Is a specularly reflective part of the second wavelength band, Rdiff，λ3Is the specular reflection part of the third wavelength band.
5. The method for estimating chlorophyll content according to claim 4, wherein the anisotropy coefficients of the leaves at different observation angles are R, measured at each observation angle, and R, measured at right angles0Calculating to obtain; in practical measurements, since an ideal diffuse reflection plate is almost isotropic, the radiances dL in each observation direction are considered to be consistent, and assuming that the reflected radiances of the whiteboard are the same at different observation angles, equation (4) is further derived as:
Figure FDA0003064409610000031
wherein dLSampleThe radiation flux measured at the blade surface.
6. The method for estimating chlorophyll content by using anisotropy coefficient as claimed in claim 5, wherein the relationship between the spectrum index of anisotropy coefficient in leaf reflected light and the chlorophyll content of vegetation is represented by establishing a regression estimation model by using the chlorophyll content and the vegetation index value calculated from each observation angle:
y＝ax+b (6)
wherein a is slope, b is intercept, x is vegetation index value, and y is chlorophyll content.
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