CN114965306B - Calibration method of optical sensor for measuring reflectivity - Google Patents
Calibration method of optical sensor for measuring reflectivity Download PDFInfo
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- CN114965306B CN114965306B CN202210596026.1A CN202210596026A CN114965306B CN 114965306 B CN114965306 B CN 114965306B CN 202210596026 A CN202210596026 A CN 202210596026A CN 114965306 B CN114965306 B CN 114965306B
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- 238000002310 reflectometry Methods 0.000 title claims abstract description 40
- 230000003287 optical effect Effects 0.000 title claims abstract description 39
- 238000000034 method Methods 0.000 title claims abstract description 17
- 238000005259 measurement Methods 0.000 claims abstract description 15
- 238000002834 transmittance Methods 0.000 claims abstract description 7
- 238000006243 chemical reaction Methods 0.000 claims description 10
- 230000003321 amplification Effects 0.000 claims description 5
- 238000002474 experimental method Methods 0.000 claims description 5
- 239000011521 glass Substances 0.000 claims description 5
- 238000003199 nucleic acid amplification method Methods 0.000 claims description 5
- 239000011022 opal Substances 0.000 claims description 5
- 230000003595 spectral effect Effects 0.000 claims description 5
- 239000000463 material Substances 0.000 claims description 4
- -1 polytetrafluoroethylene Polymers 0.000 claims description 4
- 229920001343 polytetrafluoroethylene Polymers 0.000 claims description 4
- 239000004810 polytetrafluoroethylene Substances 0.000 claims description 4
- 238000012937 correction Methods 0.000 claims description 2
- 230000005855 radiation Effects 0.000 description 8
- 230000002354 daily effect Effects 0.000 description 4
- 238000005286 illumination Methods 0.000 description 4
- 238000013461 design Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000001228 spectrum Methods 0.000 description 2
- 238000012360 testing method Methods 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 238000012935 Averaging Methods 0.000 description 1
- 239000002028 Biomass Substances 0.000 description 1
- 241000195493 Cryptophyta Species 0.000 description 1
- 230000032683 aging Effects 0.000 description 1
- 238000012271 agricultural production Methods 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 230000003203 everyday effect Effects 0.000 description 1
- 230000005251 gamma ray Effects 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 230000026267 regulation of growth Effects 0.000 description 1
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Classifications
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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
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/27—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands using photo-electric detection ; circuits for computing concentration
- G01N21/274—Calibration, base line adjustment, drift correction
Abstract
The invention discloses a calibration method of an optical sensor for measuring reflectivity, which is used for measuring the reflectivity of ground objectsThe influence of factors such as dark current of the sensor, light transmittance of the cosine corrector, half field angle of the sensor and the like is comprehensively considered, so that the sensor has good adaptability to different light intensity environments after calibration, and measurement accuracy is improved.
Description
Technical Field
The invention relates to the field of optical measurement, in particular to a calibration method of an optical sensor for measuring reflectivity.
Background
The spectral reflectivity of the ground object can be measured by using the sensor, so that the ground object can be identified, and specific information of the ground object, such as biomass of algae in water environment, growth vigor of crops and the like, can be obtained. The obtained spectral reflectivity can provide data support for the protection and treatment of water environment and decision basis for the crop growth regulation in agricultural production. The accuracy of reflectivity measurement directly influences actual application effect, so that the sensor needs to be calibrated after production to ensure the accuracy of measurement, and the accuracy of the sensor in daily use is reduced due to factors such as aging, so that the sensor also needs to be calibrated regularly. The conventional method for calibrating the sensor is to take gray plates with different standard reflectances as measurement objects, calculate the original reflectances of the sensor according to measured data, establish a regression equation of the original reflectances and the standard reflectances of the gray plates, and realize the calibration of the sensor. The sensor calibrated by the calibration method can only accurately measure when the solar altitude is large in one day, generally works before and after noon, the accuracy of other periods is seriously reduced, and the working is required to be a sunny day. The analysis is that the sensor has dark current, no calibration equation is considered in the current sensor reflectivity construction, the influence of the dark current is minimum when the sky light is strongest, and the larger the influence of the sky light is weakened, the larger the deviation of measurement data is, so that the requirements of a test time period and clear weather are limited. Meanwhile, in practical application, the solar altitude angle is changed before and after the noon every day along with the change of seasons, so that the sensor needs to be calibrated frequently, otherwise, the accuracy of measurement is seriously reduced.
Disclosure of Invention
Aiming at the defects existing in the prior art, the invention aims to provide a calibration method of an optical sensor for measuring reflectivity, which solves the problems, and realizes accurate measurement of the reflectivity of ground objects in environments with different seasons, different light intensities and rapid change light intensity.
In order to achieve the above purpose, the present invention provides the following technical solutions:
the calibration method of the optical sensor for measuring the reflectivity comprises the steps of receiving sky light upwards and receiving ground object reflected light downwards, wherein the calibration equation of the spectral reflectivity of the sky light received by the sensor and the ground object reflected light received by the sensor is as follows:ρ is the calibrated reflectivity, V U For receiving the voltage of the optical sensor output of sky light upwards, +.>Voltage of dark current of optical sensor for receiving sky light upward after IV conversion, V D For receiving the voltage of the optical sensor output of the ground object reflected light downwards, +.>Dark current IV conversion for an optical sensor receiving reflected light from a surface featureThe voltage after the calibration is alpha is the transmittance of the opal glass or polytetrafluoroethylene material of the cosine corrector on the upper surface of the sensor, omega is the half field angle of the sensor, and gamma is the correction coefficient;
(1) The sensor is placed under the condition of complete darkness, the dark current is converted through IV to obtain voltage, and the voltage output by the sensor is measured to obtain
(2) Taking gray plates with different standard reflectivities as measurement objects, recording the output voltage of a sensor, and respectively calculating gamma corresponding to the gray plates with different standard reflectivities of the optical channel according to experimental results R For gamma R The average value is taken to obtain gamma,
wherein ρ is R Is the reflectance value of the standard reflectance gray scale.
According to the further improvement scheme, the sensor is multiband, the optical channels for receiving sky light and ground object reflected light in each waveband are a group of optical channels, and gamma is required to be determined through experiments in each group of optical channels of the sensor.
In a further improvement scheme, the amplification factor of the sensor current-voltage conversion circuit is adjustable, so that the voltage after dark current IV conversion under different amplification factors is measured.
In a still further improvement scheme, the number of the gray plates with different standard reflectivities is more than or equal to 2, preferably 3-10.
The specific scheme deducing process comprises the following steps:
the sensor is provided with one or more groups of upper and lower optical channels, each group is a wave band, each wave band has a certain spectral range, an upward optical channel in the sensor receives sun and sky light, and a downward optical channel receives the reflection spectrum of a measured object. Reflectivity is the ratio of the reflected light energy to the incident light energy,
ρ is the reflectivity, E I Is the incident light energy received by the measured object in unit area and unit time E R Is the reflected light energy of the measured object per unit area per unit time. The detector on the upper surface of the sensor receives incident light irradiated to the measuring object, and irradiance H of the detector sky Can replace E in formula (1) I The detector on the lower surface of the sensor receives the reflected light of the measuring object, and irradiance H of the detector object Radiation exitance M convertible into canopy object ，M object Can replace E in formula (1) R Therefore, the formula (1) can be converted into the formula (2).
For a diffuse reflecting surface, when the incident irradiance is constant, the reflecting surface is observed from any angle, the reflecting brightness is constant, the reflecting surface is called a lambertian surface, and the reflection spectrum information of the measured object received by the sensor corresponds to the irradiance generated by a lambertian light source on the detection surface of the sensor. Irradiance H generated by the point light source is shown as (3),
wherein I is the radiation intensity of the point source, r is the distance between the point source and the illuminated surface element, and θ is the angle between the normal line of the illuminated surface element and r.
The area of the facet source is delta A s The radiation brightness is L, the area of the irradiated surface is delta A, delta A s Distance from DeltaA is r, deltaA s The angle between the normal line of (2) and r is theta s The angle between the normal of delta A and r is theta. Due to delta A s Is small, so the radiation intensity I of the facet source can be expressed as formula (4),
I＝L cosθ s ·ΔA s (4)
the irradiance H generated by the facet source at delta A can be obtained by combining the formula (3) and the formula (4), and is shown as the formula (5),
the irradiance H generated at the detector surface by the lambertian extended source can be further deduced from equation (5),
H＝M sin 2 ω (6）
m is the radiation exitance of the radiation source, the invention is the radiation exitance of the measured object, and omega is the half field angle of the sensor. M can thus be represented as formula (7).
The photoelectric detector of the sensor generates photo-generated current under illumination, the current is converted and amplified by the current-voltage circuit to output voltage, and the output voltage represents the intensity of the illumination. One of the inherent characteristics of the photodetector is that it has a dark current, i.e., it has a current in a completely dark condition without illumination, so that the output voltage of the photodetector includes a photo-generated current and a dark current, the voltage generated by illumination can be calculated by equation (8),
V UL voltage generated by sensor for sunlight and sky light irradiation, V U For the output voltage of the sensor,a voltage generated for a dark current.
The upper surface of the sensor is provided with a cosine corrector which is made of opal glass or polytetrafluoroethylene material, and irradiance generated by sunlight and skylight on the upper surface of the sensor is H sky (W·m -2 ) The conversion relation between irradiance and voltage is shown in formula (9),
V UL ＝α·β·H sky ·A U (9)
alpha (%) is the light transmittance of opal glass or polytetrafluoroethylene material, and beta (V.W) -1 ) Representing the ability of the photodetector's optical signal to convert to an electrical signal, A U (m 2 ) Is the photosensitive area of the photodetector. H can be obtained by transforming (9) sky ，
Voltage V output by diffuse reflection light irradiation sensor of measuring object DL Calculated by equation (11).
Wherein V is D For the output voltage of the sensor,a voltage generated for a dark current.
Voltage V DL Irradiance H generated on the lower surface of the sensor by diffuse reflected light from the measuring object object (W·m -2 ) There is a relationship of formula (12).
V DL ＝β·H object ·A D (12)
A D (m 2 ) Is the photosensitive area of the photodetector. Converting (12) to obtain H object ，
From the formulae (7) and (13), the radiation emittance M of the measurement object can be obtained object (W·m -2 )。
Omega is the half field angle of the sensor, typically A for the same sensor U And A is a D Equal, the reflectivity ρ of the sensor can be obtained from the equation (2), the equation (10), and the equation (14), as shown in the equation (15).
However, in reality, the optical element and the circuit element cannot be completely consistent, so that the output of the sensor is affected, and small changes in the mounting position of the optical element during assembly can also affect the optical signal, so that the coefficient gamma pair is necessary to be usedFine tuning is performed, and the value of γ is close to 1, so the reflectance of the sensor output should be formula (16).
Dark current is amplified simultaneously with the current-voltage conversion circuit, thusAnd->Can be determined experimentally. The measuring method comprises the following steps: the sensor is placed under the dark condition, the voltage output by the IV conversion circuit under different amplification factors of the sensor is measured in sequence, and the +.>And->a and ω are intrinsic parameters of the sensor, a is obtained by measuring the transmittance of the cosine corrector, and ω is obtained from the angle of view of the sensor design. When the sensor is produced, gamma becomes the sensorThe intrinsic parameters of the sensor, gamma, can be determined through experiments, and the specific method is as follows: gamma corresponding to gray plates with different standard reflectivities are calculated respectively according to experimental results by taking gray plates with different standard reflectivities as measuring objects R For gamma R And taking average to obtain gamma, wherein each group of optical channels of the sensor is required to determine gamma through experiments. Gamma ray R Calculated from equation (17).
Wherein gamma is R Gamma, ρ corresponding to standard reflectivity plate experiment R Is the reflectance value of the standard reflectance gray scale.
Advantageous effects of the invention
The calibration method for constructing the reflectivity based on the working principle of the sensor has the advantages that the sensor has good adaptability to the environment after calibration, the influence of dark current of the sensor is removed, the sensor can work when the light is strong and weak all the year round, the sensor can still work normally when the light changes in cloudy weather in one day, namely, the environmental adaptability of the sensor is improved, meanwhile, the influence of the light transmittance a of the cosine corrector and the field angle omega of the sensor design is considered, and the measurement accuracy is improved, so that the use efficiency of the sensor is greatly improved.
Drawings
FIG. 1 is a graph of dark current test at different magnification of an embodiment sensor;
FIG. 2 is a graph showing daily changes in reflectance for a 60% standard reflectance gray scale as a monitor on a sunny day;
fig. 3 is a graph showing daily changes in reflectance of a 20% standard reflectance gray scale as a monitoring object in cloudy weather.
Detailed Description
Dark currents at different magnifications of the sensor were measured in completely dark conditions, see fig. 1. The self-developed sensor is calibrated by using 40%, 20% and 99% standard reflectivity gray plates as measuring objects, the reflectivity of the sensor is measured at 710nm, the half field angle of the sensor is 15 degrees, and the light transmittance of opal glass of the sensor is 47.5%.
Calculating gamma according to formula (17) R Averaging to obtain γ= 0.97787The calibration equation of the sensor is shown in formula (18).
The accuracy of the sensor measurement is detected by taking 60% and 20% standard reflectivity gray plates as measurement objects, and the results are shown in fig. 2 and 3. As can be seen from fig. 2, on sunny days, at 9: 00-15: within 00, the measurement is more accurate and no U-shaped change exists. As can be seen from fig. 3, in the weather condition that the change of the intensity of the cloudy light is faster, the daily change of the measured reflectivity is relatively flat and accurate, which indicates that the calibration method of the invention is effective.
Claims (5)
1. A calibration method of an optical sensor for measuring reflectivity is characterized in that: the optical sensor for measuring the reflectivity comprises an optical sensor for receiving the sky light upwards and an optical sensor for receiving the ground object reflected light downwards, and a calibration equation of the spectral reflectivity of the sky light received by the sensor and the ground object reflected light received by the sensor is as follows:ρ is the calibrated reflectivity, V U For receiving the voltage of the optical sensor output of sky light upwards, +.>Voltage of dark current of optical sensor for receiving sky light upward after IV conversion, V D For receiving the voltage of the optical sensor output of the ground object reflected light downwards, +.>The dark current of the optical sensor for receiving the reflected light of the ground object downwards is converted into voltage by IV, and alpha isThe light transmittance of the cosine corrector on the upper surface of the sensor is opal glass or polytetrafluoroethylene material, ω is the half field angle of the sensor, and γ is a correction coefficient;
(1) The sensor is placed under the condition of complete darkness, the dark current is converted through IV to obtain voltage, and the voltage output by the sensor is measured to obtain
(2) Taking gray plates with different standard reflectivities as measurement objects, recording the output voltage of a sensor, and respectively calculating gamma corresponding to the gray plates with different standard reflectivities of the optical channel according to experimental results R For gamma R The average value is taken to obtain gamma,
wherein ρ is R Is the reflectance value of the standard reflectance gray scale.
2. The method for calibrating an optical sensor for measuring reflectivity according to claim 1, wherein: the sensor is multiband, and each band of optical channels for receiving sky light and ground object reflected light is a group of optical channels, and each group of optical channels of the sensor determines gamma through experiments.
3. The method for calibrating an optical sensor for measuring reflectivity according to claim 1, wherein: and the amplification factor of the sensor IV conversion circuit is adjustable, so that the voltage after dark current IV conversion under different amplification factors is measured.
4. The method for calibrating an optical sensor for measuring reflectivity according to claim 1, wherein: the number of the different standard reflectivity gray plates is more than or equal to 2.
5. The method for calibrating an optical sensor for measuring reflectivity as defined in claim 4, wherein: the number of the different standard reflectivity gray plates is 3-10.
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