Real-time calibration method for the scattering channel of a spectroradiometer based on direct solar irradiance

Abstract

The present invention discloses a real-time calibration method for the scattering channel of a spectroradiometer based on direct solar irradiance, which includes: obtaining the calibration values of the entire wavelength band of the direct channel of the spectroradiometer through a hybrid calibration method that combines the Langley calibration method in the non-absorbing wavelength band and the theoretical calculation of the solar irradiance at the top of the atmosphere in the strong absorption wavelength band; accurately obtaining the spectral irradiance reaching the optical acquisition system of the radiometer and the atmospheric transmittance through the calibration of the direct channel; using the measured direct solar radiation passing through the atmosphere and the atmospheric transmittance in real time, combining the solar irradiance at the top of the atmosphere and the optical component parameters of the instrument to obtain the real-time calibration values of the entire wavelength band of the scattering channel. Without using other auxiliary materials, the present invention transfers the calibration of the direct channel of the spectroradiometer to the calibration of the scattering channel, and will be applied to devices that simultaneously measure direct solar light and atmospheric scattered light.

Description

Technical Field

[0001] The present invention relates to the field of calibration research of optical parameter measurement instruments, and particularly relates to a real-time calibration method for the scattering channel of a spectroradiometer based on direct solar irradiance. Background Art

[0002] Solar radiation is scattered in the atmosphere to form sky spectral brightness. By measuring the sky spectral brightness reaching the ground, a large amount of information on atmospheric components and states can be retrieved, thus having many applications in the field of atmospheric science. As a widely used instrument, a spectroradiometer can be used to measure atmospheric optical parameters such as atmospheric transmittance, total water vapor amount, and optical thickness. In the measurement of the scattering channel, it is necessary to convert the output signal of the instrument into the actual value of the all-sky radiance, which is the calibration process. Then, by integrating the spectral radiance of all wavelengths, the sky spectral brightness can be obtained. The calibration of the traditional solar radiometer scattering channel often adopts absolute radiometric calibration, which relies on a stable indoor light source with known spectral radiance, such as an integrating sphere. By establishing a quantitative relationship between the digital signal value output by the spectral radiometer and the actual radiance of the integrating sphere, the calibration value of the scattering channel is obtained. Also, because the radiometer has different integration times during measurement, there is a set of calibration values for different integration times. Additionally, the integrating sphere needs to be cooled when adjusting different brightness levels, which makes this calibration method time-consuming and inefficient. At the same time, the integrating sphere calibration is an instrument with relatively high maintenance and operation costs, which also greatly increases the calibration cost; and over time, the stability of the integrating sphere changes, and the calibration accuracy will also be affected.

[0003] Therefore, there is an urgent need to propose a real-time calibration method for the scattering channel with high accuracy to reduce costs and improve calibration efficiency and calibration accuracy. Summary of the Invention

[0004] To solve the problem of low calibration efficiency of the spectroradiometer scattering channel, the present invention provides a real-time calibration method for the scattering channel of a spectroradiometer based on direct solar irradiance, which does not rely on a stable light source with known external radiance, saves costs, and improves calibration efficiency at the same time.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A real-time calibration method for the scattering channel of a spectroradiometer based on direct solar irradiance, comprising the following steps:

[0007] Step 1) When the atmospheric condition is clean and stable, use the Langley method to calculate discrete calibration values in the non-absorbing band, and combine with the solar irradiance at the top of the atmosphere to calculate the discrete instrument response values:

[0008] In direct measurement mode, the voltage response of the instrument to the solar spectrum reaching the ground is:

[0009] V(λ)=I0(λ)K(λ)T(λ) (1)

[0010] Where: V(λ) is the instrument measurement value, I0(λ) is the solar irradiance at the top of the atmosphere, K(λ) is the instrument response function, T(λ) is the slant transmittance of solar radiation through the atmosphere, and λ is the wavelength measured by the spectroradiometer;

[0011] When the atmospheric quality factor is 0, the atmospheric slant transmittance T(λ) is 1. At this time, the value measured by the instrument is the calibration coefficient. The full-band calibration coefficient V0(λ) of the direct channel is expressed as:

[0012] V0(λ)=I0(λ)K(λ) (2)

[0013] The solar irradiance I0(λ) at the top of the atmosphere is expressed as:

[0014] I0(λ)=∫I0(λ')φ(λ-λ')dλ' (3)

[0015] Where: φ(λ-λ') is the instrument function, λ' is the wavelength of the solar irradiance at the top of the atmosphere;

[0016] The instrument response function K(λ) is assumed to vary linearly between wavelengths within a narrow band, that is:

[0017] K(λ)=a+bλ (4)

[0018] Among them, a and b are coefficients;

[0019] The instrument response function K(λ) with continuous full-band is obtained by interpolation fitting through the adjacent discrete instrument response values K.

[0020] After obtaining the instrument response function K(λ), the direct channel calibration value of the spectroradiometer is calculated based on the solar irradiance I0(λ) at the top of the atmosphere according to formula (2).

[0021] Step 2) In actual measurement, the direct channel of the spectroradiometer obtains the direct atmospheric irradiance, and the real-time atmospheric transmittance is calculated using the direct channel calibration value. The real-time calibration value of the scattered channel is calculated again in combination with the solar irradiance at the top of the atmosphere and the optical parameters of the instrument. The calculation formula is as follows:

[0022]

[0023] Where C a (λ) is the full-band calibration coefficient of the scattering channel, D is the transmittance of the attenuation plate, Ω νis the field of view of the spectroradiometer, I0(λ) is the solar irradiance at the top of the atmosphere, T(λ) is the atmospheric transmittance, V(λ) is the direct solar radiation measured in real time by the spectroradiometer, and λ is the wavelength measured by the spectroradiometer.

[0024] The beneficial effects of the present invention are:

[0025] (1) The present invention can perform real-time calibration of the spectroradiometer without relying on auxiliary materials and calculate the total radiance of the sky spectrum brightness. Compared with integrating sphere calibration, the present invention does not require an external stable light source with known radiance, saving costs and improving calibration efficiency.

[0026] (2) The present invention can improve the calibration accuracy. The performance of the integrating sphere gradually becomes unstable over time, while the sun is a stable light source. On the basis of the high calibration accuracy of the direct channel, the calibration accuracy of the present invention can be guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart of the real-time calibration method of the scattering channel of the spectroradiometer based on direct solar irradiance;

[0028] Figure 2a , Figure 2b This calibration method is used to calculate the calibration value of the scattering channel; where, Figure 2a For visible light tube, Figure 2b It is a near-infrared light tube;

[0029] Figure 3 It is the full sky radiance distribution map calculated using the calibration value;

[0030] Figure 4 The spectral radiance diagram at a zenith angle of 0° is calculated using the calibration values calculated using the integrating sphere calibration and the real-time calibration. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more precise definition of the protection scope of the present invention.

[0032] like Figure 1 As shown, the real-time calibration method of the scattering channel of the spectroradiometer based on direct solar irradiance according to an embodiment of the present invention includes the following steps:

[0033] Step 1: When the atmospheric conditions are clean and stable, calculate the discrete calibration values using the Langley method in the non-absorbing band. Combine the solar irradiance at the top of the atmosphere to calculate the discrete instrument response values. Obtain the instrument response function through linear interpolation, and then combine it with the solar irradiance at the top of the atmosphere to calculate the calibration values of the direct channel of the spectroradiometer, specifically including:

[0034] The spectroradiometer has two measurement channels: direct and scattered. To retrieve the atmospheric optical parameters, it is necessary to calibrate these two channels, that is, convert the measured values of the instrument into the actual spectral irradiance. The direct and scattered channels of the spectroradiometer share the same optical path. The difference in the working mode during measurement is only that an attenuation filter to prevent optical sensor saturation is required for the direct channel measurement. Additionally, according to the optical path design of the spectroradiometer, the field of view size is known. Relying on the sun itself being a stable light source, based on the direct solar irradiance measured in real time by the direct channel, the direct calibration coefficient can be transferred to the calibration process of the scattered channel.

[0035] In the calibration process of the direct channel, the hybrid calibration method is used. That is, when the atmospheric conditions meet the requirements, enough discrete calibration values can be directly calculated using the Langley method in the non-absorbing band. Divide these discrete calibration values by the solar irradiance at the top of the atmosphere in the corresponding band to obtain the discrete instrument response values. Since the response of the instrument is a slowly changing process, the instrument response function can be obtained through linear interpolation. Obtaining the instrument response function and the solar irradiance at the top of the atmosphere, and then combining the following formula can obtain the calibration coefficient of the entire band of the direct channel:

[0036] V0(λ) = I0(λ)K(λ) (2)

[0037] In the formula, V0(λ) is the calibration coefficient of the entire band of the direct channel, I0(λ) is the solar irradiance at the top of the atmosphere, K(λ) is the instrument response function, and λ is the wavelength.

[0038] The solar irradiance at the top of the atmosphere I0(λ) can be expressed as:

[0039] I0(λ) = ∫I0(λ')φ(λ - λ')dλ' (3)

[0040] In the formula: φ(λ - λ') is the instrument function, and λ' is the wavelength of the solar irradiance at the top of the atmosphere.

[0041] The instrument response function K(λ) is a slowly changing function with wavelength. In a narrow band range, it can be simply assumed that it changes linearly between wavelengths, that is:

[0042] K(λ) = a + bλ (4)

[0043] Where a and b are coefficients.

[0044] Therefore, the continuous K(λ) across all bands can be obtained by interpolation fitting based on the discretely spaced K values at adjacent intervals.

[0045] Step 2: During the actual measurement process, use the calibration value of the direct channel to calculate the real-time atmospheric transmittance. At the same time, the direct channel of the spectroradiometer can measure the real-time direct solar radiation. According to the optical path design of the spectroradiometer, the field of view angle of the instrument and the transmittance of the attenuation filter to prevent direct saturation can be known. According to the definitions of the field of view angle, irradiance, and radiance, when aiming at the same light source, the ratio of the output signals of the scattered channel and the direct channel is equal to the gain ratio of the instrument itself, that is, the reciprocal of the transmittance D of the attenuation filter. Combining the above, the following formula can be obtained:

[0046]

[0047] In the formula, C a (λ) is the calibration coefficient across all bands of the scattered channel, D is the transmittance of the attenuation filter, Ω v is the solid angle of the instrument, I0(λ) is the solar irradiance at the top of the atmosphere, and V0(λ) is the calibration coefficient of the direct channel of the spectroradiometer.

[0048] Since V(λ) = V0(λ)·T(λ), and the direct channel of the spectroradiometer can measure the atmospheric transmittance T(λ) at different times. Therefore, during actual measurement, the near real-time calibration coefficient of the scattered channel can be obtained using the following formula:

[0049]

[0050] In the formula, V(λ) is the direct solar radiation measured in real time by the spectroradiometer. Limited by the fact that the instrument can only measure direct data or scattered data at one time, but according to the uniqueness of the calibration coefficient of the scattered channel, the calibration values calculated at any time should be consistent.

[0051] Combined with Figure 2, since the spectral resolution of the spectroradiometer is less than 1 nm and the integration times of the spectrometers used internally are different, when calculating the calibration coefficient, it is necessary to smooth the solar irradiance at the top of the atmosphere and the output value of the instrument using a unified instrument function width, and at the same time normalize the integration time. Finally, a unique calibration coefficient for the scattered channel will be obtained. The spectral range of the visible light tube of the spectroradiometer is 400 - 700 nm, and the spectral range of the near-infrared light tube is 700 - 1100 nm, with a spectral resolution of 1 nm.

[0052] Combined with Figure 3, the calibration coefficient of the scattering channel is applied to the calculation of the all-sky background radiation. To obtain a complete all-sky background map, it is necessary to calculate the integrated radiance at all measured azimuth and zenith angles. The data in the 400 - 700 nm band of the visible light tube measurement data and the data in the 700 - 1100 nm band of the near-infrared light tube are multiplied by the calibration coefficient and integrated to obtain the actual all-sky background radiation.

[0053] Combined with Figure 4 , the calibration coefficient of the scattering channel is applied to the calculation of the spectral radiance and compared with the calibration result of the integrating sphere. It can be found from the figure that the spectral radiance distributions calculated by the two calibration methods are consistent, the radiance decreases with the decrease of the wavelength, and shows a concave state in the strong absorption band. This shows that the real-time calibration has high accuracy.

[0054] The above are only the embodiments of the present invention, and do not limit the protection scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied to other related technical fields, shall be equally included in the protection scope of the present invention.

Claims

1. A real-time calibration method for the scattering channel of a spectroradiometer based on direct solar irradiance, characterized in that, It includes the following steps: Step 1) When the atmospheric conditions are clean and stable, calculate the discrete calibration values using the Langley method in the non-absorption band, and combine with the solar irradiance at the top of the atmosphere to calculate the discrete instrument response values: In the direct measurement mode, the voltage response of the instrument to the solar spectrum reaching the ground is: V(λ) = I0(λ)K(λ)T(λ) (1) Where: V(λ) is the instrument measurement value, I0(λ) is the solar irradiance at the top of the atmosphere, K(λ) is the instrument response function, T(λ) is the slant path transmittance of solar radiation through the atmosphere, and λ is the wavelength measured by the spectroradiometer; When the air mass factor is 0, the atmospheric slant path transmittance T(λ) is 1. At this time, the value measured by the instrument is the calibration coefficient. Then, the calibration coefficient V0(λ) of the direct channel over the entire band is expressed as: V0(λ) = I0(λ)K(λ) (2) The solar irradiance I0(λ) at the top of the atmosphere is expressed as: I0(λ) = ∫I0(λ')φ(λ - λ')dλ' (3) Where: φ(λ - λ') is the instrument function, and λ' is the wavelength of the solar irradiance at the top of the atmosphere; The instrument response function K(λ) is assumed to vary linearly between wavelengths within a narrow band range, that is: K(λ) = a + bλ (4) Where a and b are coefficients; Through interpolation and fitting of the discrete instrument response values K at adjacent intervals, the continuous instrument response function K(λ) over the entire band is obtained; After obtaining the instrument response function K(λ), combine it with the solar irradiance I0(λ) at the top of the atmosphere again. According to formula (2), calculate the calibration value of the direct channel of the spectroradiometer; Step 2) In actual measurement, the direct channel of the spectroradiometer obtains the direct atmospheric irradiance, calculates the real-time atmospheric transmittance using the calibration value of the direct channel, and then combines it with the solar irradiance at the top of the atmosphere and the optical parameters of the instrument to calculate the real-time calibration value of the scattering channel. The calculation formula is as follows: where C a (λ) is the calibration coefficient of the scattering channel over the full wavelength range, D is the transmittance of the attenuation filter, Ω ν is the field of view of the spectroradiometer, I0(λ) is the solar irradiance at the top of the atmosphere, T(λ) is the atmospheric transmittance, V(λ) is the direct solar radiation measured in real time by the spectroradiometer, and λ is the wavelength measured by the spectroradiometer.

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