A method and device for measuring the spectral reflectance of plants

Abstract

The present application discloses a method and device for measuring the spectral reflectance of plants, which relates to the field of spectral reflectance measurement. The method includes: obtaining an initial measurement signal of the plant to be measured; converting the initial measurement signal into the form of a voltage signal to obtain a first measurement signal; performing high-pass filtering on the first measurement signal to obtain a second measurement signal; obtaining an initial excitation signal having the same frequency as the modulated light, adding a 90° phase shift to the initial excitation signal to obtain a first excitation signal; performing low-pass filtering on the signal after multiplying the initial excitation signal and the second measurement signal to obtain an in-phase component; performing low-pass filtering on the signal after multiplying the first excitation signal and the second measurement signal to obtain a quadrature component; calculating a second amplitude based on the in-phase component, the quadrature component, and a first amplitude; and determining the spectral reflectance of the plant to be measured based on the second amplitude and a spectral reflectance calibration equation. The present application realizes the accurate measurement of the spectral reflectance of plants.

Description

Technical Field

[0001] This application relates to the technical field of spectral reflectance measurement, and particularly to a method and device for measuring plant spectral reflectance. Background Art

[0002] The measurement of plant spectral reflectance is an important part of modern agriculture and ecological research. It can provide key information about the nutritional status, water content, etc. of plants. By analyzing the reflection characteristics of plants at different wavelengths, the health status of plants can be detected. For example, within the visible light band, chlorophyll is one of the main factors affecting the spectral response of plants, and healthy green vegetation usually exhibits a relatively high reflectance in the near-infrared band. When plants are attacked by diseases or lack necessary nutrients, their spectral characteristics will change. Using vegetation spectral indices composed of reflectances in specific bands, such as the Normalized Difference Vegetation Index (NDVI), the growth and development status of vegetation can be judged. In addition, reflectance spectroscopy analysis can not only identify different vegetation types, but also evaluate vegetation coverage, and can be used to predict vegetation productivity and pest and disease conditions.

[0003] Currently, devices for non-destructively obtaining plant spectral reflectance are mainly divided into two categories. One is a passive device that uses sunlight as a light source, and the other is an active device with a specific band light source. Due to the limitation of sunlight, the working conditions of passive devices are greatly affected by the external environment. Changes in light intensity and angle under natural conditions will lead to unstable measurement results. Especially in outdoor environments, factors such as cloud cover and solar altitude angle will introduce additional errors, greatly limiting the application space of passive devices. Since active devices have their own light sources and rely little on the external environment, they have the advantages of good real-time performance, low external interference, low cost, large working range, etc., and have become one of the core devices for precision agriculture to obtain crop physiological information, with a very broad application prospect.

[0004] However, since the spectral range of sunlight is very wide and its intensity is very high, the spectral bands emitted by the active light sources used in active devices are basically within the solar spectrum. This means that the photodetector module can not only receive the modulated light emitted by the active light source, but also receive sunlight, which brings trouble to subsequent signal processing and analysis. Although existing methods also use a filter or filter for an active light source band to filter out most of the sunlight, this will cause attenuation of the effective signal or phase delay of the effective signal, resulting in inaccurate measurement of the plant spectral reflectance. Summary of the Invention

[0005] The purpose of this application is to provide a method and device for measuring the spectral reflectance of plants, which avoids the influence of sunlight and realizes the accurate measurement of the spectral reflectance of plants.

[0006] To achieve the above object, the present application provides the following solutions.

[0007] In a first aspect, the present application provides a method for measuring the spectral reflectance of plants. The method for measuring the spectral reflectance of plants includes: obtaining an initial measurement signal of a plant to be measured; the initial measurement signal is a current signal corresponding to the reflected light generated after the plant to be measured is irradiated by modulated light and ambient light; converting the initial measurement signal into a voltage signal form to obtain a first measurement signal; performing high-pass filtering on the first measurement signal to obtain a second measurement signal; obtaining an initial excitation signal having the same frequency as the modulated light, adding a 90° phase shift to the initial excitation signal to obtain a first excitation signal; performing low-pass filtering on the signal after multiplying the initial excitation signal and the second measurement signal to obtain an in-phase component; performing low-pass filtering on the signal after multiplying the first excitation signal and the second measurement signal to obtain a quadrature component; calculating a second amplitude based on the in-phase component, the quadrature component, and a first amplitude; the first amplitude is the amplitude of the initial excitation signal and the first excitation signal; the second amplitude is the amplitude of the second measurement signal; the amplitude of the second measurement signal is the amplitude of the reflected light; determining the spectral reflectance of the plant to be measured based on the second amplitude and a spectral reflectance calibration equation.

[0008] In a second aspect, the present application further provides a device for measuring the spectral reflectance of plants. The device for measuring the spectral reflectance of plants includes: an active light source module for emitting modulated light with a specific frequency to the plant to be measured; a spectral reflectance measurement module connected to the active light source module for performing the method for measuring the spectral reflectance of plants described in the first aspect.

[0009] According to the specific embodiments provided by the present application, the following technical effects are disclosed.

[0010] In order to separate the reflection signals of sunlight and active light in this application, after converting the initial measurement signal into voltage, high-pass filtering is performed on it, thereby filtering out most of the DC components. At this time, the measurement signal mainly consists of the reflection signal of active light and a small amount of noise signals. To solve the problem of phase delay of the measurement signal caused by filters, circuits, and noise interference, this application multiplies the high-pass filtered measurement signal by the initial excitation signal of the same frequency, and at the same time multiplies the high-pass filtered measurement signal by the first excitation signal with a 90° phase shift increase, respectively extracts the in-phase component and the quadrature component of the measurement signal, and accurately obtains the actual amplitude of the measurement signal, avoiding the interference of phase delay. Finally, based on the actual amplitude of the measurement signal, this application accurately determines the spectral reflectance of the plant to be measured using the spectral reflectance calibration equation, and at the same time the entire measurement process is not affected by sunlight. Description of the Drawings

[0011] In order to more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of this application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0012] Figure 1 It is a flowchart of the method for measuring the spectral reflectance of plants provided by the embodiment of this application.

[0013] Figure 2 It is an internal structure diagram of the spectral reflectance measurement module provided by the embodiment of this application.

[0014] Figure 3 It is a module structure diagram of the plant spectral reflectance measurement device provided by the embodiment of this application.

[0015] Figure 4 It is a front view of the plant spectral reflectance measurement device provided by the embodiment of this application.

[0016] Figure 5 It is a bottom view of the plant spectral reflectance measurement device provided by the embodiment of this application.

[0017] Symbol description: Active light source module - 1, Spectral reflectance measurement module - 2, Photoelectric detector module - 21, I / U conversion module - 22, High-pass filter - 23, Lock-in amplifier module - 24, Microprocessor - 25, Button module - 3, Display module - 4, Wireless communication module - 5, Battery module - 6, Battery charge and discharge module - 7, Voltage stabilization power supply module - 8, Connection hole - 9. Detailed Embodiments

[0018] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts belong to the scope of protection of the present application.

[0019] With the development of the times, the agricultural field has experienced a transformation from traditional chemical analysis to modern spectroscopic non-destructive testing methods. The traditional acquisition of crop growth information is mainly through field sampling and chemical analysis. This method has defects such as damaging plants, high cost, and inability to obtain crop physiological information in large-scale and real-time, and this method is not suitable for precision agriculture online operations. At present, non-destructive testing has been widely used in the industrial field. It refers to a method of using the changes in magnetic, electrical, acoustic, optical, thermal and other reactions caused by abnormalities or defects existing in the internal structure of the object to be detected, using advanced technologies and instruments, to detect and distinguish the types, properties, quantities, shapes, positions, sizes, distributions and their changes of the abnormalities and defects inside and outside the object to be detected, without damaging or affecting the use performance of the object to be detected and without harming the internal tissues of the object to be detected. In the agricultural field, the non-destructive acquisition of crop growth information has broad application prospects. The early research on the spectral characteristics of plants mainly relied on integrating sphere measurements under laboratory conditions. This method can accurately measure the directional-hemispherical reflectance, but the operation is complex and not suitable for field work. With the progress of portable devices, such as the emergence of handheld ground spectrometers, it has become feasible to quickly obtain vegetation reflectance spectra on-site. Such instruments can perform continuous wavelength scanning in the visible to short-wave infrared range (usually 350 - 2500nm), and ensure data accuracy through standard whiteboard calibration. However, the data is redundant, very troublesome to process, and calibration must be carried out before each measurement, with cumbersome operation, large equipment, and great inconvenience for field operations.

[0020] The purpose of the present application is to provide a method and device for measuring plant spectral reflectance, which avoids the influence of sunlight and realizes the accurate measurement of plant spectral reflectance.

[0021] To make the above objects, features, and advantages of the present application more obvious and understandable, the present application will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0022] Embodiment 1

[0023] As Figure 1 shown, this embodiment provides a method for measuring plant spectral reflectance, and the method for measuring plant spectral reflectance is specifically as follows.

[0024] Step S1: Obtain the initial measurement signal of the plant to be measured.

[0025] In this embodiment, the initial measurement signal is a current signal corresponding to the reflected light generated after the plant to be measured is irradiated by modulated light and ambient light (mainly sunlight). In the subsequent process, sunlight will be filtered out, and finally only the measurement signal related to the modulated light will be retained.

[0026] Step S2: Convert the initial measurement signal into a voltage signal form to obtain a first measurement signal.

[0027] In this embodiment, for the convenience of subsequent calculations, it is necessary to first convert the initial measurement signal in current form into a first measurement signal in voltage form.

[0028] Step S3: Perform high-pass filtering on the first measurement signal to obtain a second measurement signal.

[0029] In this embodiment, high-pass filtering will filter out most of the measurement signals related to sunlight, and only retain the measurement signals related to the modulated light. At the same time, the phase delay caused by high-pass filtering or signal interference will also be eliminated in the subsequent steps.

[0030] The expression of the second measurement signal is: ; is the second measurement signal, is the second amplitude, is the frequency, is the time, is the phase shift.

[0031] Step S4: Obtain an initial excitation signal with the same frequency as the modulated light, and add a 90° phase shift to the initial excitation signal to obtain a first excitation signal.

[0032] In this embodiment, the modulated light, the initial excitation signal, the first excitation signal, and the second measurement signal have the same frequency; the initial excitation signal and the first excitation signal have the same amplitude.

[0033] The expression of the initial excitation signal is: ; is the initial excitation signal, is the first amplitude.

[0034] The expression of the first excitation signal is: ; is the first excitation signal.

[0035] Step S5: Perform low-pass filtering on the signal after multiplying the initial excitation signal and the second measurement signal to obtain an in-phase component.

[0036] In this embodiment, the in-phase component is , and its specific derivation process is as follows.

[0037]

[0038] Since , the following relationship exists.

[0039] .

[0040] The DC component obtained after low-pass filtering it is , and this value is the measured in-phase component I .

[0041] Step S6: Perform low-pass filtering on the signal after multiplying the first excitation signal and the second measurement signal to obtain the quadrature component.

[0042] In this embodiment, the quadrature component is , and its specific derivation process is as follows.

[0043] .

[0044] Since , the above formula is as follows.

[0045]

[0046] Since , then:

[0047] .

[0048] The DC component obtained after low-pass filtering it is , and this value is the measured quadrature component .

[0049] Step S7: Calculate the second amplitude based on the in-phase component, the quadrature component, and the first amplitude.

[0050] In this embodiment, the first amplitude is the amplitude of the initial excitation signal and the first excitation signal, and the second amplitude is the amplitude of the second measurement signal. The amplitude of the second measurement signal is the amplitude of the reflected light. Since amplitude = , and , , are all known quantities, so the following relationship is satisfied.

[0051] .

[0052] Derive the above formula to obtain the calculation formula for the second amplitude.

[0053] .

[0054] Step S8: Determine the spectral reflectance of the plant to be measured based on the second amplitude and the spectral reflectance calibration equation.

[0055] In this embodiment, it is also necessary to perform analog-to-digital conversion on the second amplitude obtained in step S7, and then substitute it into the spectral reflectance calibration equation with the parameters already determined for calculation. The final calculation result is the spectral reflectance of the plant to be measured. Among them, the determination process of the spectral reflectance calibration equation is as follows:

[0056] The first step is to obtain a spectral reflectance calibration plate.

[0057] The second step is to irradiate the spectral reflectance calibration plate with modulated light of a set frequency and read multiple groups of calibration data; the calibration data includes the amplitude of the reflected light and the corresponding spectral reflectance.

[0058] The third step is to use the least squares method to fit the spectral reflectance calibration equation based on multiple groups of calibration data.

[0059] In addition, since the modulated light signal and the initial excitation signal (or the first excitation signal with a 90° phase shift added) in the second measurement signal of this embodiment have the same frequency, the final result is a DC signal and a signal twice the original frequency. Multiplying will only shift the signal of the modulation frequency back to DC, and all other frequency components in the second measurement signal (mainly the sunlight signal) will shift to other non-zero frequencies, thereby realizing the separation of the modulated light and the sunlight.

[0060] Embodiment 2

[0061] This embodiment provides a device for measuring the spectral reflectance of plants, and the device for measuring the spectral reflectance of plants is as follows.

[0062] The active light source module 1 is used to emit modulated light of a specific frequency to the plant to be measured. Among them, the inside of the active light source module 1 is a light-emitting diode or a laser diode.

[0063] The spectral reflectance measurement module 2 is connected to the active light source module 1 and is used to execute the method for measuring the spectral reflectance of plants in Embodiment 1.

[0064] As Figure 2 shown, the spectral reflectance measurement module 2 is as follows.

[0065] The photodetector module 21 is used to obtain the initial measurement signal of the plant to be measured; the initial measurement signal is the current signal corresponding to the reflected light generated after the plant to be measured is irradiated with modulated light and ambient light. Among them, the inside of the photodetector module 21 is a photodiode or a photovoltaic cell.

[0066] The I / U conversion module 22, connected to the photodetector module 21, is used to convert the initial measurement signal into the form of a voltage signal to obtain the first measurement signal.

[0067] The high-pass filter 23, connected to the I / U conversion module 22, is used to perform high-pass filtering on the first measurement signal to obtain the second measurement signal.

[0068] The lock-in amplifier module 24, connected to the high-pass filter 23, is used to obtain the initial excitation signal with the same frequency as the modulated light, add a 90° phase shift to the initial excitation signal to obtain the first excitation signal; perform low-pass filtering on the signal after multiplying the initial excitation signal and the second measurement signal to obtain the in-phase component; perform low-pass filtering on the signal after multiplying the first excitation signal and the second measurement signal to obtain the quadrature component; calculate the second amplitude based on the in-phase component, quadrature component, and the first amplitude.

[0069] The microprocessor 25, connected to the active light source module 1 and the lock-in amplifier module 24, is used to send a PWM signal to drive the active light source module 1 to emit modulated light with a specific frequency; determine the spectral reflectance of the plant to be measured based on the second amplitude and the spectral reflectance calibration equation. Among them, the lock-in amplifier module 24 is connected to the ADC interface of the microprocessor 25.

[0070] As Figure 3 shown, the plant spectral reflectance measurement device further includes: a key module 3, connected to the I / O interface of the microprocessor 25, used to trigger the microprocessor 25 to send a PWM signal to the active light source module 1.

[0071] The display module 4, connected to the SPI interface of the microprocessor 25, is used to display the spectral reflectance of the plant to be measured determined by the plant spectral reflectance measurement device.

[0072] The wireless communication module 5, connected to the UART interface of the microprocessor 25, is used to realize the transmission and positioning of signals between the plant spectral reflectance measurement device and the outside world. Among them, the wireless communication module 5 is 4G + GNSS.

[0073] In addition, the plant spectral reflectance measurement device further includes: a battery module 6, a battery charge and discharge module 7, and a regulated power supply module 8.

[0074] The battery module 6 is connected to the battery charge and discharge module 7; the battery charge and discharge module 7 is connected to the regulated power supply module 8; the regulated power supply module 8 is connected to the microprocessor 25. During actual use, it is necessary to first charge the battery module 6 through the charging function of the battery charge and discharge module 7. After the battery module 6 is fully charged, further discharge the regulated power supply module 8 through the discharge function of the battery charge and discharge module 7.

[0075] As a preferred embodiment, the above-mentioned plant spectral reflectance measuring device is Figure 4 the pistol-like structure shown, which includes a gun body part and a handle part. As Figure 5 shown, a main light source module 1, a photodetector module 21 and a connection hole 9 are sequentially arranged on the lower surface of the gun body part. The handle part is connected to the gun body part by inserting into the connection hole 9. A display module 4 is also arranged on the upper surface of the gun body part. The display module 4 is an OLED display screen. The handle part is also provided with an outward protruding button corresponding to the internal key module 3.

[0076] In summary, the present application measures the plant spectral reflectance based on the lock-in amplification technology, which can not only separate sunlight from the modulated light, but also ensure that the effective signal does not attenuate. At the same time, the plant spectral reflectance measuring device of the present application is not affected by sunlight and can accurately measure the plant spectral reflectance under cloudy, overcast or dark conditions, expanding the use environment for measuring the plant spectral reflectance.

[0077] All actions of obtaining signals, information or data in this application are carried out on the premise of complying with the corresponding data protection regulations and policies of the country where it is located and with the authorization given by the owner of the corresponding device.

[0078] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. The same or similar parts among the embodiments can be referred to each other.

[0079] Specific examples are used in this article to elaborate on the principle and implementation manner of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application; at the same time, for those of ordinary skill in the art, according to the idea of the present application, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present application.

Claims

1. A method for measuring plant spectral reflectance, characterized in that: The plant spectral reflectance determination method comprises: Acquire an initial measurement signal of the plant to be measured; the initial measurement signal is a current signal corresponding to the reflected light generated by the plant to be measured after being irradiated by modulated light and ambient light; Converting the initial measurement signal into a voltage signal to obtain a first measurement signal; Performing high-pass filtering on the first measurement signal to obtain a second measurement signal; Acquire an initial excitation signal having the same frequency as the modulated light, and add a 90° phase shift to the initial excitation signal to obtain a first excitation signal; Performing low-pass filtering on a signal obtained by multiplying the initial excitation signal and the second measurement signal to obtain an in-phase component; Performing low-pass filtering on a signal obtained by multiplying the first excitation signal and the second measurement signal to obtain an orthogonal component; Calculate a second amplitude based on the in-phase component, the quadrature component and the first amplitude; the first amplitude is the amplitude of the initial excitation signal and the first excitation signal; the second amplitude is the amplitude of the second measurement signal; the amplitude of the second measurement signal is the amplitude of the reflected light; Based on the second amplitude and the spectral reflectance calibration equation, the spectral reflectance of the plant to be measured is determined.

2. The method for measuring plant spectral reflectance according to claim 1, characterized in that: The frequencies of the modulated light, the initial excitation signal, the first excitation signal and the second measurement signal are the same; and the amplitudes of the initial excitation signal and the first excitation signal are the same.

3. The method for measuring plant spectral reflectance according to claim 2, characterized in that: The expression of the initial excitation signal is: ； In the formula, is the initial excitation signal, is the first amplitude, is the frequency, For time; The expression of the first excitation signal is: ； In the formula, is the first excitation signal; The expression of the second measurement signal is: ； In the formula, is the second measurement signal, is the second amplitude, For phase shift.

4. The method for measuring plant spectral reflectance according to claim 1, characterized in that: The in-phase component is , the orthogonal components are ; In the formula, is the first amplitude, is the second amplitude, For phase shift.

5. The method for measuring plant spectral reflectance according to claim 1, characterized in that: The calculation formula of the second amplitude is: ； In the formula, is the second amplitude, is the first amplitude, is the same direction component, are orthogonal components.

6. The method for measuring plant spectral reflectance according to claim 1, characterized in that: The determination process of the spectral reflectance calibration equation is: Obtain a spectral reflectance calibration plate; irradiating the spectral reflectance calibration plate with modulated light of a set frequency, and reading multiple sets of calibration data; the calibration data includes the amplitude of the reflected light and the corresponding spectral reflectance; Based on multiple groups of calibration data, the spectral reflectance calibration equation is obtained by using the least squares method.

7. A plant spectral reflectance measuring device, characterized in that: The plant spectral reflectance measuring device comprises: An active light source module, used for emitting modulated light of a specific frequency to the plant to be measured; A spectral reflectance measurement module is connected to the active light source module and is used to execute the plant spectral reflectance measurement method according to any one of claims 1 to 6.

8. The plant spectral reflectance measuring device according to claim 7, characterized in that: The spectral reflectance measurement module specifically includes: A photoelectric detector module is used to obtain an initial measurement signal of the plant to be measured; the initial measurement signal is a current signal corresponding to the reflected light generated by the plant to be measured after being irradiated by modulated light and ambient light; An I / U conversion module, connected to the photoelectric detector module, for converting the initial measurement signal into a voltage signal to obtain a first measurement signal; a high-pass filter, connected to the I / U conversion module, and configured to perform high-pass filtering on the first measurement signal to obtain a second measurement signal; A lock-in amplifier module, connected to the high-pass filter, used to obtain an initial excitation signal with the same frequency as the modulated light, add a 90° phase shift to the initial excitation signal to obtain a first excitation signal; perform low-pass filtering on the signal after multiplying the initial excitation signal and the second measurement signal to obtain an in-phase component; perform low-pass filtering on the signal after multiplying the first excitation signal and the second measurement signal to obtain an orthogonal component; calculate a second amplitude based on the in-phase component, the orthogonal component and the first amplitude; the first amplitude is the amplitude of the initial excitation signal and the first excitation signal; the second amplitude is the amplitude of the second measurement signal; the amplitude of the second measurement signal is the amplitude of the reflected light; A microprocessor is connected to the active light source module and the lock-in amplifier module, and is used to send a PWM signal to drive the active light source module to emit modulated light of a specific frequency; based on the second amplitude and the spectral reflectance calibration equation, the spectral reflectance of the plant to be measured is determined.

9. The plant spectral reflectance measuring device according to claim 8, characterized in that: The plant spectral reflectance measuring device also includes: A button module, connected to the I / O interface of the microprocessor, and used to trigger the microprocessor to send a PWM signal to the active light source module; A display module, connected to the SPI interface of the microprocessor, for displaying the spectral reflectance of the plant to be measured determined by the plant spectral reflectance measuring device; The wireless communication module is connected to the UART interface of the microprocessor and is used to realize the transmission and positioning of the plant spectral reflectance measuring device and external signals.

10. The plant spectral reflectance measuring device according to claim 8, characterized in that: The plant spectral reflectance measuring device also includes: a battery module, a battery charging and discharging module and a voltage-stabilizing power supply module; The battery module is connected to the battery charging and discharging module; the battery charging and discharging module is connected to the voltage-stabilizing power supply module; and the voltage-stabilizing power supply module is connected to the microprocessor.

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