Spectral reflectivity sensor solar altitude correction method based on sky light distribution

Abstract

The invention discloses a spectral reflectance sensor solar altitude correction method based on sky light distribution, and belongs to the technical field of optical remote sensing. The method comprises the following steps: calculating the light energy transmittance of light rays refracted among different media based on a vegetation spectral reflectance sensor, calculating the light energy entry rate of sky light in an optical window of the sensor based on a CIE sky brightness distribution model, and finally calculating the average transmittance of the sky light emitted from the lower surface of a cosine corrector. And the solar altitude correction of the spectral reflectivity sensor is realized. The light energy transmission condition of each beam of sky light in the optical window of the sensor is calculated, and the average transmittance of the sky light is obtained through integration, so that the solar altitude correction of the spectral reflectivity sensor is realized, the influence of the solar altitude is reduced, and the accuracy of all-day measurement is improved; the daily effective working time of the sensor can be doubled, and the working efficiency is improved; the method is firm in technical theory and good in universality, and can be used for sensors of the same type.

Description

Technical Field

[0001] The invention belongs to the technical field of optical remote sensing, and in particular relates to a solar altitude correction method for a spectral reflectance sensor based on sky light distribution. Background Art

[0002] During the growth of vegetation, it is crucial to quickly and accurately capture its growth information for agricultural production regulation. Using canopy reflectance, vegetation growth information such as chlorophyll content, biomass, and leaf area index can be obtained. Vegetation spectral reflectance sensors using sunlight as the light source have been widely used in obtaining growth information. The sensor receives both sky light and vegetation reflected light. The vegetation canopy has a characteristic similar to that of a Lambertian surface, and its reflected light is not affected by the altitude of the sun; however, the direct light component in the sky light will change its incident angle at the sensor optical window due to changes in the altitude of the sun, thereby affecting the transmittance and ultimately reducing the accuracy of the reflectance measurement. This phenomenon is manifested as follows: as the solar altitude angle increases, the reflectance shows a downward trend, especially at noon, reaching the lowest point, and the overall trend is "U" shaped. Although the noon period (such as 10:00 to 14:00) is theoretically the best measurement time, in practice the time window available for measurement is very limited every day, and the high temperature environment around noon in summer is not suitable for field or outdoor operations. In order to minimize the impact of the sun's altitude on the measurement, the optical window of the sensor that receives sunlight is generally equipped with a cosine corrector. The materials used are usually opalescent glass, polytetrafluoroethylene, etc. However, actual applications have shown that the cosine corrector has limited effect in reducing the impact of the sun's altitude. A "U"-shaped change trend can still be observed in the measurement data of one day, which reduces the accuracy of the measurement. Summary of the invention

[0003] In view of the problems existing in the prior art mentioned in the background technology, the present invention provides a method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution.

[0004] Technical solution: In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0005] A solar altitude correction method for a spectral reflectance sensor based on sky light distribution, which calculates the light energy transmittance when light is refracted between different media based on a vegetation spectral reflectance sensor, calculates the light energy entry rate of sky light in the sensor optical window based on a CIE sky brightness distribution model, and finally calculates the average transmittance of sky light emitted from the lower surface of a cosine corrector to achieve solar altitude correction of the spectral reflectance sensor.

[0006] Preferably, the specific content of calculating the light energy transmittance when the light is refracted between different media based on the vegetation spectral reflectance sensor is as follows:

[0007] The reflection and refraction characteristics of light at the interface of the medium are closely related to the vibration direction of the electric vector; due to the transverse wave characteristics of plane light waves, the electric vector can vibrate in any direction in the plane perpendicular to the propagation direction, and be decomposed into a component vibrating perpendicular to the incident plane and a component vibrating parallel to the incident plane.

[0008] Preferably, the component vibrating perpendicular to the incident surface is called the s component, and the component vibrating parallel to the incident surface is called the p component. The amplitude reflection coefficient and transmission coefficient of the s component and the p component are calculated by the Fresnel formula. The transmission coefficient is related to the light energy received inside the sensor.

[0009] The amplitude transmission coefficient t of the s-component and p-component s and t p The calculation formula is as follows:

[0010]

[0011] Where n is the relative refractive index on both sides of the medium, θ i is the angle of incidence.

[0012] As a preference, the energy transmittances T of the s component and the p component are s ′ and T p The calculation formula of ′ is as follows:

[0013]

[0014] For the case of natural light, the incident angle is θ i When the light energy transmittance The specific calculation is as follows:

[0015]

[0016] Preferably, the specific content of calculating the light energy entry rate of sky light into the sensor optical window based on the CIE sky brightness distribution model is:

[0017] The CIE standard defines the general concept of the sky and the angle between the sun and the sky element; Z S is the angular distance between the sun and the zenith, Z is the angular distance between the sky element and the zenith, γ is the sky element altitude angle, θ is the sun altitude angle, χ is the angular distance between the sky element and the sun, α is the azimuth angle of the sky element, α S is the azimuth of the sun;

[0018] The solar azimuth angle α defined by CIE S With due north as 0°, the azimuth gradually increases clockwise, with a value range of 0-360°; χ is calculated by formula (6);

[0019] χ=arccos[sinθ·sinγ+cosθ·cosγ·cos(aa S )] (6)

[0020] Among them, arccos() represents the inverse cosine function;

[0021] Morning solar azimuth α S The calculation formula is:

[0022]

[0023] Afternoon solar azimuth α S The calculation formula is:

[0024]

[0025] Among them, λ represents the latitude of the calculation location; σ represents the solar declination;

[0026] The brightness L of any sky element γ and the zenith brightness L Z The ratio of is calculated according to formula (9);

[0027]

[0028] in, is the brightness standard absorption index function, and f() is the scattering index function;

[0029]

[0030] Among them, a, b, c, d, e are standard sky parameters, and exp() represents the exponential function.

[0031] As a preferred method, normalize the brightness of any sky element to facilitate data processing, and obtain the relative sky brightness L R , L R The calculation formula is as follows:

[0032]

[0033] Among them, L R(γ,α) is the relative brightness of the sky element when its altitude angle and azimuth angle are γ and α respectively; L γ,sun is the brightness of the sky element in the direction of the sun, L γ is the brightness of the sky element with altitude and azimuth angles γ and α respectively;

[0034] Assume that the altitude angle and azimuth angle of a sky element are γ and α respectively, and the ratio of the brightness of the sky element to the brightness of the entire sky is R γ,α The light energy entering the cosine corrector is T γ,α ;

[0035] R γ,α The calculation formula is as follows:

[0036]

[0037] The light energy entry rate of the sky element light into the sensor optical window is calculated according to formula (5);

[0038] T γ,α The calculation formula is as follows:

[0039] T γ,α =R γ,α ·T′ 90°-γ (16)

[0040] Among them, T′ 90°-γ is the light energy transmittance of the light in the sensor optical window when the sky element height is γ calculated by formula (5).

[0041] Preferably, the specific content of calculating the average transmittance of skylight emitted from the lower surface of the cosine corrector is:

[0042] Assume that the light intensity of the sky element with altitude angle γ and azimuth angle α entering the cosine corrector is I0, and the refraction angle is θ R , the thickness of the cosine corrector is h, the refractive index is n, the extinction coefficient is k, and the change in light intensity when the length of the light transmission path in the cosine corrector is infinitesimal dl is dI;

[0043] According to the law of refraction, 1·sin(90°-γ)=n·sinθ R , the path length of the light entering from the upper surface of the cosine corrector and emitting from the lower surface in the cosine corrector is:

[0044]

[0045] Where k′ is the light path length amplification factor caused by the scattering of the cosine corrector; the following relationship exists when the light is transmitted in the cosine corrector:

[0046] dI=-kIdl(17)

[0047] Integrating equation (17) yields the output light intensity I, which is:

[0048]

[0049] Among them, C is the coefficient to be determined;

[0050] From formula (18), we can see that when l = 0,

[0051]

[0052] Then C = I0, and formula (18) can be transformed into:

[0053]

[0054] Then, when the sky element height is γ, the light entering the cosine corrector exits from the lower surface of the cosine corrector at an emission rate T R,γ for:

[0055]

[0056] When the sky element height is 90°, the light is incident vertically on the cosine corrector, and then formula (21) yields:

[0057]

[0058] Substituting formula (22) into formula (21), we can obtain:

[0059]

[0060] Combining equations (16) and (23), we can obtain the average transmittance T of the whole process of skylight entering the cosine corrector and emitting from the lower surface: θ , the specific calculation formula is as follows:

[0061]

[0062] Where θ is the solar altitude angle; T 90°-γ is the light energy transmittance of the light in the sensor optical window when the sky element height is γ calculated by formula (5);

[0063] Reflectivity ρ after solar altitude correction θ The calculation formula is as follows:

[0064]

[0065] Where ρ is the original reflectivity output by the sensor.

[0066] As a preference, if there is no diffuse scattering optical device in the path of sky light reaching the photodetector, then equation (16) is integrated over the entire sky light, and the average transmittance T of sky light reaching the photodetector surface is θ It can be calculated by formula (26);

[0067]

[0068] Where θ is the solar altitude angle.

[0069] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0070] The present invention calculates the light energy transmittance of each beam of sky light in the optical window of the sensor, integrates it to obtain the average transmittance of the sky light, and then realizes the solar altitude correction of the spectral reflectance sensor, reduces the influence of the solar altitude, and improves the accuracy of all-day measurement; the present invention can double the effective daily working time of the sensor and improve work efficiency; the technical theory of the present invention is solid, and the universality is good, and the same type of sensors can be used. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 It is a graph showing the variation of the average light energy transmittance of sky light on the sensor at different sun altitudes of the present invention;

[0072] Figure 2 It is a daily variation diagram of the reflectivity of the 710nm band before and after correction of the present invention;

[0073] Figure 3 It is a daily variation diagram of the reflectivity before and after correction in the 870nm band of the present invention. DETAILED DESCRIPTION

[0074] The present invention is further illustrated below in conjunction with specific examples. The examples are implemented based on the technical solutions of the present invention. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0075] Example 1

[0076] The VCR vegetation reflectance sensor is used. The sensor has two bands, 710nm and 870nm. The cosine corrector of the sensor is polytetrafluoroethylene with a refractive index of n = 1.38. The light energy transmittance under direct illumination is T R,90° =0.49, taking the standard reflectivity grayscale plate as the measurement object, the reflectivity of the grayscale plate at 710nm is 24.3%, and the reflectivity at 870nm is 23.0%.

[0077] The measurement location is Huaiyin Normal University, the measurement date is October 2, 2024, the measurement time range is 8:43-16:30, and measurement is conducted once every minute. The solar altitude is recorded synchronously with the measurement.

[0078] The present embodiment provides a method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution, comprising the following steps:

[0079] S1: Calculate the light energy transmittance when the light is refracted between different media;

[0080] Sky light shines on the surface of the sensor, refracts and reflects at the optical window, and the refracted light enters the sensor, eventually reaching the photodetector and being converted into an electrical signal. The output voltage of the sensor reflects the amount of light energy entering the sensor.

[0081] The reflection and refraction characteristics of light at the interface of the medium are closely related to the vibration direction of the electric vector. Due to the transverse wave characteristics of plane light waves, the electric vector can vibrate in any direction in the plane perpendicular to the propagation direction, and it can always be decomposed into a component vibrating perpendicular to the incident surface and a component vibrating parallel to the incident surface. The component vibrating perpendicular to the incident surface is usually called the s component, and the component vibrating parallel to the incident surface is called the p component. The amplitude reflection coefficient and transmission coefficient of the s component and the p component can be calculated by the Fresnel formula. The transmission coefficient is related to the light energy received inside the sensor.

[0082] The amplitude transmission coefficient t of the s-component and p-component s and t p The calculation method of is shown in formula (1) and formula (2).

[0083]

[0084]

[0085] Where n is the relative refractive index on both sides of the medium, θ i is the angle of incidence, that is, the angle between the incident light and the normal direction of the incident surface.

[0086] Energy transmittance T of s component and p component s ′ and T p ' can be expressed as formula (3) and formula (4).

[0087]

[0088] For the case of natural light, the incident angle is θ i Light transmittance T θi ′ can be expressed as formula (5).

[0089]

[0090] S2: Calculate the light energy entry rate of sky light into the sensor optical window based on the CIE sky brightness distribution model;

[0091] The CIE (International Commission on Illumination) standard defines the general concept of the sky and the angle between the sun and the sky element. S is the angular distance between the sun and the zenith, Z is the angular distance between the sky element and the zenith, γ is the sky element altitude angle, θ is the sun altitude angle, χ is the angular distance between the sky element and the sun, α is the azimuth angle of the sky element, α S is the azimuth of the sun. The solar azimuth angle α defined by CIE SWith due north as 0°, the azimuth increases gradually clockwise, with a value range of 0-360°. χ can be calculated using formula (6).

[0092] χ=arccos[sinθ·sinγ+cosθ·cosγ·cos(aa S )] (6)

[0093] Among them, arccos() represents the inverse cosine function.

[0094] Morning solar azimuth α S The calculation formula is:

[0095]

[0096] Afternoon solar azimuth α S The calculation formula is:

[0097]

[0098] Among them, λ represents the latitude of the calculation location; σ represents the solar declination.

[0099] The brightness L of any sky element γ and the zenith brightness L Z The ratio can be calculated according to formula (9).

[0100]

[0101] in, is the brightness standard absorption index function, and f() is the scattering index function.

[0102]

[0103] In equations (10) to (13), a, b, c, d, and e are standard sky parameters, and exp() represents an exponential function.

[0104] CIE divides the sky into 15 types. Spectral reflectance sensors using the sun as the light source almost all work on sunny days. The standard sky parameters on sunny days are shown in Table 1.

[0105] Table 1 CIE standard clear sky parameters

[0106] Sky Type a b c d e Sky Light Description 12 -1.0 -0.32 10 -3.0 0.45 CIE standard clear sky, clear atmosphere 13 -1.0 -0.32 16 -3.0 0.30 CIE standard clear sky, turbid atmosphere 14 -1.0 -0.15 16 -3.0 0.30 Clear cloudy sky with a broad corona 15 -1.0 -0.15 24 -2.8 0.15 Blue and white cloudy sky with a broad solar corona

[0107] From equations (9) to (13), we can see that when χ = 0, equation (9) reaches its maximum value, that is, the sky brightness in the direction of the sun is the largest. Normalize the brightness of any sky element to facilitate data processing, and get the relative sky brightness L R , L R The calculation method of is shown in formula (14), and the value range is 0-1.

[0108]

[0109] Among them, L R(γ,α) is the relative brightness of the sky element when its altitude angle and azimuth angle are γ and α respectively. γ,sun is the brightness of the sky element in the direction of the sun, L γ is the brightness of the sky element with altitude and azimuth angles γ and α respectively.

[0110] Assume that the altitude angle and azimuth angle of a sky element are γ and α respectively, and the ratio of the brightness of the sky element to the brightness of the entire sky is R γ,α The light energy entering the cosine corrector is T γ,α . R γ,α The calculation method of is shown in formula (15).

[0111]

[0112] Among them, L R(γ,α) The calculation method of is shown in formula (14).

[0113] The light energy entry rate of the sky element light into the sensor optical window can be calculated according to formula (5), T γ,α The calculation method of is shown in formula (16).

[0114] T γ,α =R γ,α ·T′ 90°-γ (16)

[0115] Among them, T′ 90°-γ is the light energy transmittance of the light in the sensor optical window when the sky element height is γ calculated according to formula (5), because the angle in formula (5) is the incident angle, which is complementary to the solar altitude angle.

[0116] S3: Finally, the average transmittance of skylight emitted from the lower surface of the cosine corrector is calculated.

[0117] Assume that the light intensity of the sky element with altitude angle γ and azimuth angle α entering the cosine corrector is I0, and the refraction angle is θ R The thickness of the cosine corrector is h, the refractive index is n, the extinction coefficient is k, and the change in light intensity when the length of the light transmission path in the cosine corrector is infinitesimal dl is dI.

[0118] According to the law of refraction, 1·sin(90°-γ)=n·sinθ R , the path length of the light entering from the upper surface of the cosine corrector and exiting from the lower surface inside the cosine corrector:

[0119]

[0120] Where k′ is the light path length amplification factor caused by the scattering of the cosine corrector, which is related to the material of the cosine corrector. When light is transmitted in the cosine corrector, the following relationship exists:

[0121] dI=-kIdl(17)

[0122] Integrating equation (17) yields the output light intensity I, which is:

[0123]

[0124] Where C is the coefficient to be determined. From formula (18), we can see that when l = 0,

[0125]

[0126] Then we can get C = I0, and formula (18) can be transformed into:

[0127]

[0128] It can be obtained that when the sky element height is γ, the light entering the cosine corrector has an emission rate T from the lower surface of the cosine corrector. R,γ for:

[0129]

[0130] When the sky element height is 90°, the light is incident vertically on the cosine corrector (when the sensor is working, the cosine corrector of the optical window is in a horizontal state), then formula (21) can be obtained:

[0131]

[0132] Substituting formula (22) into formula (21), we can obtain:

[0133]

[0134] Therefore, in practical applications, it is only necessary to determine the light energy transmittance T of the vertical incident light of the cosine corrector in advance. R,90 °You can use formula (23).

[0135] Combining equations (16) and (23), we can obtain the average transmittance T of the whole process of skylight entering the cosine corrector and emitting from the lower surface: θ , the specific calculation formula is as follows:

[0136]

[0137] Where θ is the solar altitude angle; T′ 90°-γis the light energy transmittance of the light in the sensor optical window when the sky element height is γ, calculated according to formula (5); because the angle in formula (5) is the incident angle, it is complementary to the solar altitude angle.

[0138] Reflectivity ρ after solar altitude correction θ It can be calculated by formula (25).

[0139]

[0140] Where ρ is the original reflectivity output by the sensor.

[0141] In this embodiment, if there is no diffuse scattering optical device (such as a cosine corrector) in the path of sky light reaching the photodetector, equation (16) can be integrated for all sky light, and the average transmittance T of sky light reaching the photodetector surface is θ It can be calculated by formula (26), where θ is the solar altitude angle.

[0142]

[0143] After the reflectivity is corrected for the sun altitude using the method of the present invention, the sensor needs to be recalibrated before it can be shipped out of the factory for use.

[0144] In this embodiment, the numerical integration method can be used to calculate formula (24) to reduce the amount of calculation in practical applications and improve the applicability of the model.

[0145] The numerical integration method is used to calculate equation (24). The sky type is 15 in Table 1. The solar altitude range is 5-90° with a step size of 5°. The sky element altitude range is 0-90° with an integration step size of 0.05°. The sky element azimuth range is 0-360° with an integration step size of 0.05°. The integration results are shown in Figure 1 .

[0146] like Figure 2 As shown, it is the daily variation of reflectance before and after correction in the 710nm band; Figure 3 Shown is the daily variation of reflectance in the 870nm band before and after correction.

[0147] Example 2

[0148] An electronic device comprises a processor and a memory storing a computer program. When the processor executes the computer program, the steps of the above-mentioned method for correcting the sun altitude of a spectral reflectance sensor based on sky light distribution are implemented.

[0149] The processor may be a CPU (Central Processing Unit), a general-purpose processor, a DSP (Digital Signal Processor), an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array) or other programmable logic devices, transistor logic devices, hardware components or any combination thereof. It may implement or execute various exemplary logic blocks, modules and circuits described in conjunction with the disclosure of this application. The processor may also be a combination that implements computing functions, such as a combination of one or more microprocessors, a combination of a DSP and a microprocessor, etc.

[0150] The memory can be a ROM (Read Only Memory) or other types of static storage devices that can store static information and instructions, a RAM (Random Access Memory) or other types of dynamic storage devices that can store information and instructions, or an EEPROM (Electrically Erasable Programmable Read Only Memory), a CD-ROM (Compact Disc Read Only Memory) or other optical disk storage, optical disk storage (including compressed optical disk, laser disc, optical disk, digital versatile disc, Blu-ray disc, etc.), a magnetic disk storage medium or other magnetic storage device, or any other medium that can be used to carry or store the desired computer program in the form of instructions or data structures and can be accessed by a computer, but is not limited to these.

[0151] Example 3

[0152] A computer-readable storage medium stores a computer program or instruction, which, when executed by a processor, implements the steps of the above-mentioned method for correcting the sun's altitude of a spectral reflectance sensor based on sky light distribution.

[0153] The computer-readable storage media provided herein include, but are not limited to, any type of disk (including floppy disk, hard disk, optical disk, CD-ROM, and magneto-optical disk), ROM, RAM, EPROM (Erasable Programmable Read-Only Memory), EEPROM, flash memory, magnetic card or optical card. In other words, the readable medium includes any medium that can store or transmit information in a readable form by a device (e.g., a computer).

[0154] In this embodiment, a VCR vegetation reflectance sensor is used, and the MCU (microcontroller unit) of the sensor is MSP430I2041, which has built-in storage.

[0155] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution, characterized in that: The light energy transmittance when light is refracted between different media is calculated based on the vegetation spectral reflectance sensor. The light energy entry rate of sky light into the sensor optical window is calculated based on the CIE sky brightness distribution model. Finally, the average transmittance of sky light emitted from the lower surface of the cosine corrector is calculated to realize the sun altitude correction of the spectral reflectance sensor.

2. The method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution according to claim 1, characterized in that: The specific content of calculating the light energy transmittance when light is refracted between different media based on the vegetation spectral reflectance sensor is as follows: The reflection and refraction characteristics of light at the interface of the medium are closely related to the vibration direction of the electric vector; due to the transverse wave characteristics of plane light waves, the electric vector can vibrate in any direction in the plane perpendicular to the propagation direction, and be decomposed into a component vibrating perpendicular to the incident plane and a component vibrating parallel to the incident plane.

3. The method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution according to claim 2, characterized in that: The component vibrating perpendicular to the incident plane is called the s component, and the component vibrating parallel to the incident plane is called the p component. The amplitude reflection coefficient and transmission coefficient of the s component and the p component are calculated by the Fresnel formula. The transmission coefficient is related to the light energy received inside the sensor. The amplitude transmission coefficient t of the s-component and p-component s and t p The calculation formula is as follows: Where n is the relative refractive index on both sides of the medium, θ i is the angle of incidence.

4. The method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution according to claim 3, characterized in that: Energy transmittance T of s component and p component s ′ and T p The calculation formula of ′ is as follows: For the case of natural light, the incident angle is θ i When the light energy transmittance The specific calculation is as follows:

5. The method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution according to claim 1, characterized in that: The specific content of calculating the light energy entry rate of sky light into the sensor optical window based on the CIE sky brightness distribution model is: The CIE standard defines the general concept of the sky and the angle between the sun and the sky element; Z S is the angular distance between the sun and the zenith, Z is the angular distance between the sky element and the zenith, γ is the sky element altitude angle, θ is the sun altitude angle, χ is the angular distance between the sky element and the sun, α is the azimuth angle of the sky element, α S is the azimuth of the sun; The solar azimuth angle α defined by CIE S With due north as 0°, the azimuth gradually increases clockwise, with a value range of 0-360°; χ is calculated by formula (6); x=arccos[sinθ·sinγ+cosθ·cosγ·cos(aa S )] (6) Among them, arccos() represents the inverse cosine function; Morning solar azimuth α S The calculation formula is: Afternoon solar azimuth α S The calculation formula is: Among them, λ represents the latitude of the calculation location; σ represents the solar declination; The brightness L of any sky element γ and the zenith brightness L Z The ratio of is calculated according to formula (9); in, is the brightness standard absorption index function, and f() is the scattering index function; Among them, a, b, c, d, e are standard sky parameters, and exp() represents the exponential function.

6. The method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution according to claim 5, characterized in that: Normalize the brightness of any sky element to facilitate data processing, and get the relative sky brightness L R , L R The calculation formula is as follows: Among them, L R(γ,α) is the relative brightness of the sky element when its altitude angle and azimuth angle are γ and α respectively; L γ,sun is the brightness of the sky element in the direction of the sun, L γ is the brightness of the sky element with altitude and azimuth angles γ and α respectively; Assume that the altitude angle and azimuth angle of a sky element are γ and α respectively, and the ratio of the brightness of the sky element to the brightness of the entire sky is R γ,α The light energy entering the cosine corrector is T γ,α ; R γ,α The calculation formula is as follows: The light energy entry rate of the sky element light into the sensor optical window is calculated according to formula (5); T γ,α The calculation formula is as follows: T γ,α =R γ,α ·T′ 90°-γ (16) Among them, T′ 90°-γ is the light energy transmittance of the light in the sensor optical window when the sky element height is γ calculated by formula (5).

7. The method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution according to claim 1, characterized in that: The specific contents of calculating the average transmittance of skylight emitted from the lower surface of the cosine corrector are: Assume that the light intensity of the sky element with altitude angle γ and azimuth angle α entering the cosine corrector is I0, and the refraction angle is θ R , the thickness of the cosine corrector is h, the refractive index is n, the extinction coefficient is k, and the change in light intensity when the length of the light transmission path in the cosine corrector is infinitesimal dl is dI; According to the law of refraction, 1·sin(90°-γ)=n·sinθ R , the path length of the light entering from the upper surface of the cosine corrector and emitting from the lower surface in the cosine corrector is: Where k′ is the light path length amplification factor caused by the scattering of the cosine corrector; the following relationship exists when the light is transmitted in the cosine corrector: dI=-kIdl(17) Integrating equation (17) yields the output light intensity I, which is: Among them, C is the coefficient to be determined; From formula (18), we can see that when l = 0, Then C = I0, and formula (18) can be transformed into: Then, when the sky element height is γ, the light entering the cosine corrector exits from the lower surface of the cosine corrector at an emission rate T R,γ for: When the sky element height is 90°, the light is incident vertically on the cosine corrector, and then formula (21) yields: Substituting formula (22) into formula (21), we can obtain: Combining equations (16) and (23), we can obtain the average transmittance T of the whole process of skylight entering the cosine corrector and emitting from the lower surface: θ , the specific calculation formula is as follows: Where θ is the solar altitude angle; T′ 90°-γ is the light energy transmittance of the light in the sensor optical window when the sky element height is γ calculated by formula (5); Reflectivity ρ after solar altitude correction θ The calculation formula is as follows: Where ρ is the original reflectivity output by the sensor.

8. The method for calibrating the sun altitude of a spectral reflectance sensor based on sky light distribution according to claim 7, characterized in that: If there is no diffuse scattering optical device in the path of sky light reaching the photodetector, then equation (16) is integrated over the entire sky light, and the average transmittance T of sky light reaching the photodetector surface is θ It can be calculated by formula (26); Where θ is the solar altitude angle.

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