Absolute Radiometric Calibration Method for Color Area Array CCD Camera Based on Bayer Array

Abstract

The present invention discloses an absolute radiometric calibration method for a color area array CCD camera based on a Bayer array. By using a highly stable LED light source combined with an integrating sphere to generate a stable and energy-adjustable uniform surface light source, and transferring the spectral radiance at the exit of the integrating sphere to the pixels of the area array CCD detector through a standard radiance meter, absolute radiation response calibration is achieved. In the present invention, by setting the magnitude of the LED light source current and setting the minimum value, typical value, and maximum value of the spectral radiance, an absolute radiation response calibration experiment of the color area array CCD camera is carried out under the condition of a single light source to determine the corresponding absolute radiation response coefficient, thereby eliminating the calibration uncertainty caused by detector response aliasing and effectively improving the accuracy of absolute radiometric calibration.

Description

Technical Field

[0001] The present invention relates to the field of optical camera calibration, and particularly to an absolute radiometric calibration method and device for a color area array CCD camera based on a Bayer array. Background Art

[0002] In the field of remote sensing imaging for earth observation, CCDs are widely used due to their ultra-high quantum efficiency, ultra-low readout noise, high resolution, and wide spectral response range. The Bayer array is one of the main technologies for realizing color image capture by CCD or CMOS sensors. It is a 4×4 array composed of 8 green, 4 blue, and 4 red pixels. As shown in Figure 4 When converting a grayscale pattern into a color picture, 9 operations are performed in a 2×2 matrix, and finally a color pattern is generated. The color area array CCD camera based on the Bayer array plays an important role in earth observation. After obtaining remote sensing information of ground objects on a satellite platform, radiometric calibration must be carried out for effective quantitative utilization. The significance of radiometric calibration is to correspond the data values obtained by the payload with the actual physical quantities, establish the numerical relationship between the radiation quantity and the output quantity of the detector, and thus establish the transfer link from the national radiation reference to the output quantity of the detector to ensure the stability and effectiveness of the payload calibration reference. Absolute radiometric calibration aims to determine the correspondence between the image gray value and the measured radiance, so as to finally obtain an accurate reflectivity calculation result.

[0003] Currently, the commonly used laboratory radiometric calibration methods at home and abroad include standard lamp-diffuse reflection plate radiometric calibration, integrating sphere radiometric calibration, blackbody point source-collimating system calibration, radiometric calibration based on standard detectors, etc. There is little research on the absolute radiometric calibration method for visible light color CCD cameras. Affected by the factors of the Bayer filter itself, there will be crosstalk in the spectral responses of different channels of the area array CCD detector within the required narrowband wavelength range after adding a pre-filter. The traditional calibration method does not consider the crosstalk problem, and the crosstalk accounts for 1% to 10% in the response channel. With the development of technology, the accuracy and precision of information are required to be higher and higher, and the traditional method can no longer meet the calibration accuracy requirements. Summary of the Invention

[0004] To solve the problems existing in the above-mentioned prior art, the present invention provides an absolute radiometric calibration method for a color area array CCD camera based on a Bayer array, in order to eliminate the calibration uncertainty caused by detector response aliasing, thereby improving the accuracy of absolute radiometric calibration.

[0005] The solution adopted by the present invention to solve its technical problems is:

[0006] The feature of an absolute radiometric calibration method for a color area array CCD camera based on a Bayer array lies in that it is applied to a device composed of an LED light source, an integrating sphere, a standard irradiance meter, a computer, and a color area array CCD camera to be measured based on a Bayer array; the LED light source includes a red LED, a blue LED, and a green LED; the color area array CCD camera based on a Bayer array includes: a lens, a filter, and a area array CCD detector based on a Bayer array; the absolute radiometric calibration method includes the following steps:

[0007] Step 1: Build an absolute radiometric calibration system for the color area array CCD camera;

[0008] Install the color area array CCD camera, the standard irradiance meter, and the integrating sphere on the optical platform, and adjust the angles of the color area array CCD camera and the standard irradiance meter so that they are respectively aligned with the center of the light output port of the integrating sphere, so that the radiation source output surface of the integrating sphere fills the entire field of view of the color area array CCD camera;

[0009] Step 2: Adjust the current magnitude of the LED light source and set the spectral irradiance;

[0010] Adjust the current magnitudes of the red LED, blue LED, and green LED in the LED light source in sequence, and only keep one color LED on during the adjustment process, and the rest of the LEDs are in the off state;

[0011] Determine the minimum value, typical value, and maximum value of the spectral irradiance according to the spectral irradiance of the incident light at the required spectral band, adjust the current of one color LED in the LED light source so that the spectral irradiance reaches the minimum value, typical value, and maximum value respectively. After the LED light source is stable, record the corresponding current values of the red LED, blue LED, and green LED in the LED light source;

[0012] Denote the minimum value, typical value, and maximum value of the spectral irradiance of the red LED at the spectral band r as RL min 、RL typ 、RL max ;

[0013] Denote the minimum value, typical value, and maximum value of the spectral irradiance of the blue LED at the spectral band b as BL min 、BL typ 、BL max ;

[0014] Denote the minimum value, typical value, and maximum value of the spectral irradiance of the green LED at the spectral band g as GL min 、GL typ 、GL max ;

[0015] Step 3: Computer-controlled measurement;

[0016] Step 3.1: Under the control of the computer, turn on the color area array CCD camera and the standard irradiance meter, and set the integration time t of the color area array CCD camera so that the minimum value, typical value, and maximum value of all spectral irradiances of the camera are within the normal working range, and preheat to a stable state;

[0017] Step 3.2: Under the control of the computer, adjust the current of the red LED in the LED light source, and only keep the red LED on during the adjustment process, and the other LEDs are in the off state; let the red LED be the currently turned-on color LED;

[0018] Step 3.3: After the currently turned-on color LED is in a stable state, make the irradiance at the radiation source outlet of the integrating sphere be the minimum value of the currently turned-on color LED;

[0019] Step 3.4: The standard irradiance meter continuously collects M data of the radiation source of the integrating sphere, and at the same time, the color area array CCD camera collects N images of the radiation source of the integrating sphere, and then collects N background images of the color area array CCD camera, where N≥10;

[0020] Step 3.5: Under the control of the computer, make the irradiance at the radiation source outlet of the integrating sphere be the typical value and maximum value of the currently turned-on color LED in turn, and return to Step 3.4 respectively, so as to obtain data in three different situations under the currently turned-on color LED;

[0021] Step 3.6: Let the blue LED and the green LED be the currently turned-on color LEDs in turn, and process according to the process of Step 3.3 - Step 3.5, so as to obtain data in nine different situations;

[0022] Step 3.7: The computer calculates the response mean value of the central pixel in the p-th row and q-th column of the channel c of the color area array CCD camera at the spectral band k and the integration time t according to Equation (1)

[0023]

[0024] In Equation (1): n is the current acquisition order of the color area array CCD camera; represents the response DN value of the pixel in the i-th row and j-th column of the radiation source image collected for the n-th time by the channel c of the color area array CCD camera at the spectral band k and the integration time t; It represents the background DN value of the pixel at the i-th row and j-th column of the background image collected for the n-th time by the channel c of the color area array CCD camera in the spectral band k and integration time t.

[0025] Step 3.8: The computer calculates the in-band average radiance value of the integrating sphere corresponding to the measured spectral band k according to Equation (2).

[0026]

[0027] In Equation (2): They respectively represent the in-band average radiances of the integrating sphere corresponding to the measured spectral band k received by the channels R, B, and G of the color area array CCD camera, and there are:

[0028]

[0029] In Equation (3): λ1 and λ2 respectively represent the upper wavelength and lower wavelength of the measured spectral band k; m is the current acquisition order of the standard radiance meter; Lc m (λ) represents the spectral radiance of the integrating sphere radiation source collected by the standard radiance meter for the m-th time, and it is a function of the wavelength λ, r c (λ) represents the relative spectral response function of the channel c of the color area array CCD camera. It represents the in-band average radiance of the integrating sphere corresponding to the measured spectral band k received by the channel c of the color area array CCD camera.

[0030] Step 3.9: The computer performs a least-squares linear fitting on the center pixel response means of the channel c of the color area array CCD camera corresponding to the minimum, typical, and maximum spectral radiances under the currently turned-on color LED in turn, in the spectral band k and integration time t, so as to obtain the absolute radiation response coefficient matrix of the R, B, and G channels in the three spectral bands r, b, g, and integration time t. And output, where It represents the absolute radiation response coefficient of the channel c in the spectral band k and integration time t, that is, the fitting slope.

[0031] Step 3.10: The computer uses Equation (3) to establish the relationships between the response DN values of the pixels of the R, B, and G channels of the color area array CCD camera based on the Bayer array after background subtraction and the in-band average radiance values in the three spectral bands r, b, g at the integration time T:

[0032]

[0033] In Equation (4): The subscripts R, B, and G respectively represent the R, B, and G channels of the Bayer array; r, b, and g respectively represent the corresponding spectral bands; DN R,T , DN B,T , DN G,T respectively represent the response DN values of the R, B, and G channel pixels of the color area array CCD camera based on the Bayer array under the integration time T; respectively represent the average in-band radiance values of the measured spectral bands r, b, and g; respectively represent the absolute radiation response coefficients of the R channel of the Bayer array to the spectral bands r, b, and g under the integration time t, respectively represent the absolute radiation response coefficients of the B channel of the Bayer array to the spectral bands r, b, and g under the integration time t, respectively represent the absolute radiation response coefficients of the G channel of the Bayer array to the spectral bands r, b, and g under the integration time t.

[0034] The feature of the absolute radiometric calibration method of the color area array CCD camera based on the Bayer array according to the present invention also lies in that: the relative spectral response function r c (λ) is obtained by the following steps:

[0035] Step 3.8.1: A relative spectral response function test system is composed of the computer, the color area array CCD camera, a monochromator, a light source, a beam splitter, and a standard detector. The monochromator is connected to the light source and preheated to a stable state;

[0036] The beam splitter is arranged at the light exit position of the monochromator, and the outgoing light energy of the monochromator is separated into two outgoing lights with the same light intensity and spectral composition;

[0037] Step 3.8.2: The color area array CCD camera is installed on another optical platform and its relative position to the beam splitter is adjusted so that the outgoing light of the monochromator is located at the central pixel of the color area array CCD camera;

[0038] Step 3.8.3: According to the working spectral band and bandwidth of the color area array CCD camera, set the lower limit of the wavelength working range of the monochromator to λ min , the upper limit of the wavelength working range to λ max and the stepped interval wavelength to λ inter ;

[0039] Step 3.8.4: Turn on the standard detector for preheating, set the integration time of the color area array CCD camera so that the color area array CCD camera is within the optimal response range, and set the wavelength λ of the outgoing light of the monochromator to λ min ;

[0040] Step 3.8.5: The standard detector continuously collects U times of data on the outgoing light of the monochromator at wavelength λ. Meanwhile, the color area array CCD camera continuously collects V images of the outgoing light of the monochromator, and then collects V background images of the color area array CCD camera, where V ≥ 10;

[0041] Step 3.8.6: The computer performs data preprocessing on the DN value measured by the central pixel of the color area array CCD camera according to Equation (5) to obtain the response mean value of the central pixel of channel c of the color area array CCD camera at wavelength λ

[0042]

[0043] In Equation (5): v is the current acquisition order of the color area array CCD camera; represents the response DN value of the pixel at the i-th row and j-th column of the radiation source image collected for the v-th time by channel c of the color area array CCD camera at wavelength λ; represents the background DN value of the pixel at the i-th row and j-th column of the background image collected for the v-th time by channel c of the color area array CCD camera at wavelength λ;

[0044] Step 3.8.7: The computer performs data preprocessing on the data measured by the standard detector according to Equation (6) to obtain the output value of the standard detector at wavelength λ

[0045]

[0046] In Equation (6): u is the current acquisition order of the standard detector; P ref,u,λ represents the output value collected for the u-th time by the standard detector at wavelength λ;

[0047] Step 3.8.8: The computer calculates the spectral responsivity of channel c at wavelength λ using Equation (7)

[0048]

[0049] Step 3.8.9: After assigning λ min + λ inter to λ, return to Step 3.8.5 and execute sequentially until λ ≥ λ max is reached, so as to obtain spectral responsivity data under different wavelengths. The computer performs normalization processing on the spectral responsivity data under different wavelengths and then performs interpolation processing to obtain the relative spectral response function r of channel c of the color area array CCD camera c(λ).

[0050] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0051] 1. The absolute radiometric calibration method of the color area array CCD camera based on the Bayer array of the present invention takes into account the crosstalk problem caused by the light received by the detector of the color area array CCD camera based on the Bayer array in different spectral bands compared with the current traditional absolute radiometric calibration method. The higher-level primary standard is transferred to the camera detector through a standard radiance meter, thereby eliminating the systematic error caused by the traditional calibration model.

[0052] 2. The absolute radiometric calibration method of the color area array CCD camera based on the Bayer array of the present invention analyzes the imaging process of the color area array CCD camera based on the Bayer array and finds that the radiation response characteristics of its R, B, and G channels overlap within the spectrum. Based on this, a calibration model for the radiation response of the multi-channel CCD detector is established, providing a support basis for the absolute radiometric calibration of the color area array CCD camera.

[0053] 3. In steps 3.2 to 3.6 of the present invention, by using red LED, blue LED, and green LED as the light source of the integrating sphere, setting the minimum value, typical value, and maximum value of the spectral radiance, the absolute radiation response calibration experiment of the color area array CCD camera is carried out under the condition of a single light source, and the corresponding absolute radiation response coefficient is determined by linear fitting, thereby improving the accuracy of the absolute radiation response coefficient and reducing the error.

[0054] 4. In step 3.4 of the present invention, the computer is used to control the standard radiance meter and the color area array CCD camera to collect data simultaneously, avoiding the error caused by different acquisition times and improving the accuracy of calibration.

[0055] 5. In steps 3.7 and 3.8 of the present invention, the computer takes the average value of multiple groups of data, reducing the measurement error in the multiple measurement process and being conducive to improving the credibility of the data. Description of the Drawings

[0056] Figure 1 is a schematic diagram of the absolute radiometric calibration standard transfer method of the present invention;

[0057] Figure 2 is a component diagram of the optical system of the color area array CCD camera of the present invention;

[0058] Figure 3 is a schematic diagram of the test calibration method of the spectral response function of the color area array CCD camera of the present invention at each wavelength;

[0059] Figure 4 is a schematic diagram of the Bayer array filter template in the prior art;

[0060] Figure 5 It is the quantum efficiency curve graph of different channels of the CCD detector in the embodiment of the present invention;

[0061] Figure 6 It is the schematic diagram of the transmittance of the filter in the embodiment of the present invention;

[0062] Figure 7 It is the spectral response curve of the R channel of the color area array CCD camera described in the embodiment of the present invention;

[0063] Figure 8 It is the spectral response curve of the B channel of the color area array CCD camera described in the embodiment of the present invention. Specific implementation mode

[0064] In this embodiment, as Figure 1 shown, an absolute radiometric calibration method for a color area array CCD camera based on a Bayer array is applied to a device composed of an LED light source, an integrating sphere, a standard irradiance meter, a computer, and a color area array CCD camera to be measured based on a Bayer array; wherein, the LED light source includes a red LED, a blue LED, and a green LED;

[0065] As Figure 2 shown, the color area array CCD camera based on a Bayer array includes: a lens, a filter, and a area array CCD detector based on a Bayer array;

[0066] In specific implementation, the color area array CCD camera detector can select the KAI-08050 of ON Semiconductor. This CCD detector is an interline transfer type CCD, and there are primary color filters arranged according to the Bayer array on the pixel surface for primary color separation, Figure 5 which is the quantum efficiency curve graph of different channels of this CCD detector;

[0067] In specific implementation, a narrowband filter is designed for band selection. The filter of the present invention can select a filter with a transmittance as Figure 6 shown. The dual-channel transmittances of the filter are 36% and 90% respectively; as Figure 7 and Figure 8 shown, there will be crosstalk in the R channel and B channel of the color area array CCD camera after selecting this filter for the dual-band spectral response within the narrowband. Among them, the response of the B channel to band r accounts for about 9% of its total response, and the response of the R channel to band b accounts for about 1.5% of its total response;

[0068] In specific implementation, this absolute radiometric calibration method includes the following steps:

[0069] The first step: Build an absolute radiometric calibration system for the color area array CCD camera;

[0070] Mount a color area array CCD camera, a standard irradiance meter, and an integrating sphere on an optical platform. Adjust the angles of the color area array CCD camera and the standard irradiance meter so that they are respectively aligned with the center of the light outlet of the integrating sphere, thereby filling the entire field of view of the color area array CCD camera with the radiation source exit surface of the integrating sphere;

[0071] Step 2: Adjust the current magnitude of the LED light source and set the spectral irradiance;

[0072] Adjust the current magnitudes of the red LED, blue LED, and green LED in the LED light source in sequence, and keep only one color of LED on during the adjustment process, with the remaining LEDs off;

[0073] Determine the minimum value, typical value, and maximum value of the spectral irradiance according to the spectral irradiance of the incident light at the required spectral band. Adjust the current of one color of LED in the LED light source so that the spectral irradiance reaches the minimum value, typical value, and maximum value respectively. After the LED light source stabilizes, record the current values of the red LED, blue LED, and green LED in the corresponding LED light source;

[0074] Denote the minimum value, typical value, and maximum value of the spectral irradiance of the red LED at the spectral band r as RL min 、RL typ 、RL max ;

[0075] Denote the minimum value, typical value, and maximum value of the spectral irradiance of the blue LED at the spectral band b as BL min 、BL typ 、BL max ;

[0076] Denote the minimum value, typical value, and maximum value of the spectral irradiance of the green LED at the spectral band g as GL min 、GL typ 、GL max ;

[0077] Step 3: Computer-controlled measurement;

[0078] Step 3.1: Under the control of the computer, turn on the color area array CCD camera and the standard irradiance meter, and set the integration time t of the color area array CCD camera so that the camera is within the normal working range for all minimum values, typical values, and maximum values of the spectral irradiance, and preheat to a stable state;

[0079] Step 3.2: Under the control of the computer, adjust the current magnitude of the red LED in the LED light source, and keep only the red LED on during the adjustment process, with the remaining LEDs off; Let the red LED be the currently turned-on color LED;

[0080] Step 3.3: After the currently turned-on color LED reaches a stable state, make the radiance at the exit of the radiation source of the integrating sphere be the minimum value of the currently turned-on color LED.

[0081] Step 3.4: The standard radiance meter continuously collects M data of the radiation source of the integrating sphere. At the same time, the color area array CCD camera collects N images of the radiation source of the integrating sphere, and then collects N background images of the color area array CCD camera, where N ≥ 10.

[0082] Step 3.5: Under the control of the computer, make the radiance at the exit of the radiation source of the integrating sphere be the typical value and the maximum value of the currently turned-on color LED in sequence, and respectively return to Step 3.4, so as to obtain data under three different conditions for the currently turned-on color LED.

[0083] Step 3.6: Make the blue LED and the green LED be the currently turned-on color LEDs in sequence, and process according to the process of Step 3.3 - Step 3.5, so as to obtain data under nine different conditions; in specific implementation, if only the R channel and the B channel are calculated, the green LED can be not turned on, so as to only obtain data under six different conditions.

[0084] Step 3.7: The computer calculates the response mean value of the central pixel at the p-th row and q-th column of the channel c of the color area array CCD camera at the spectral band k and the integration time t according to Equation (1).

[0085]

[0086] In Equation (1): n is the current acquisition order of the color area array CCD camera. represents the response DN value of the pixel at the i-th row and j-th column of the radiation source image collected for the n-th time by the channel c of the color area array CCD camera at the spectral band k and the integration time t. represents the background DN value of the pixel at the i-th row and j-th column of the background image collected for the n-th time by the channel c of the color area array CCD camera at the spectral band k and the integration time t.

[0087] Step 3.8: The computer calculates the in-band average radiance value of the integrating sphere corresponding to the measured spectral band k according to Equation (2).

[0088]

[0089] In Equation (2): respectively represent the in-band average radiances received by the channels R, B, and G of the color area array CCD camera corresponding to the measured spectral band k of the integrating sphere, and there is:

[0090]

[0091] In Equation (3): λ1 and λ2 respectively represent the upper wavelength and the lower wavelength of the measured spectral band k; m is the current acquisition order of the standard radiance meter; Lc m (λ) represents the spectral radiance of the integrating sphere radiation source collected by the standard radiance meter for the m-th time, and is a function of the wavelength λ, r c (λ) represents the relative spectral response function of channel c of the color area array CCD camera, represents the average irradiance within the band corresponding to the measured spectral band k of the integrating sphere received by channel c of the color area array CCD camera; in a specific implementation, if only the R channel and the B channel are calculated, the average irradiance within the band corresponding to the integrating sphere received by the G channel does not need to be considered, so Equation (2) can take the average of the average irradiance within the band of only the R channel and the B channel, reducing the calculation amount;

[0092] Step 3.9: The computer performs a least-squares linear fitting on the mean response of the central pixels of channel c of the color area array CCD camera corresponding to the minimum, typical, and maximum spectral radiances under the currently turned-on color LED in sequence, in the spectral band k and integration time t, so as to obtain the absolute radiation response coefficient matrix of the R, B, and G channels at three spectral bands r, b, g and integration time t and outputs it, where, represents the absolute radiation response coefficient of channel c at the spectral band k and integration time t, that is, the fitting slope;

[0093] Step 3.10: The computer uses Equation (3) to establish a relationship between the response DN values of the pixels of the R, B, and G channels of the color area array CCD camera based on the Bayer array after background subtraction at the integration time T and the average irradiance values within the bands at three spectral bands r, b, g:

[0094]

[0095] In Equation (4): The subscripts R, B, and G respectively represent the R, B, and G channels of the Bayer array; r, b, and g respectively represent the corresponding spectral bands; DN R,T 、DN B,T 、DN G,T respectively represent the response DN values of the pixels of the R, B, and G channels of the color area array CCD camera based on the Bayer array at the integration time T; respectively represent the average irradiance values within the bands of the measured spectral bands r, b, and g; respectively represent the absolute radiation response coefficients of the R channel of the Bayer array to the spectral bands r, b, and g at the integration time t, respectively represent the absolute radiation response coefficients of the B channel of the Bayer array to the spectral bands r, b, and g at the integration time t, respectively represent the absolute radiation response coefficients of the G channel of the Bayer array to the spectral bands r, b, and g at the integration time t.

[0096] In this embodiment, as Figure 3 shown, the relative spectral response function r c (λ) of the color area array CCD camera in step 3.8 is obtained as follows:

[0097] Step 3.8.1: A relative spectral response function test system is composed of a computer, a color area array CCD camera, a monochromator, a light source, a beam splitter, and a standard detector. The monochromator is connected to the light source and preheated to a stable state;

[0098] A beam splitter is arranged at the light output port of the monochromator, and the output light energy of the monochromator is separated into two output lights with the same light intensity and spectral components;

[0099] Step 3.8.2: The color area array CCD camera is installed on another optical platform and its relative position with the beam splitter is adjusted so that the output light of the monochromator is located at the central pixel of the color area array CCD camera;

[0100] Step 3.8.3: According to the working spectral range and bandwidth of the color area array CCD camera, set the lower limit of the wavelength working range of the monochromator to λ min , the upper limit of the wavelength working range to λ max and the stepped interval wavelength to λ inter ;

[0101] Step 3.8.4: Turn on the standard detector for preheating, set the integration time of the color area array CCD camera so that the color area array CCD camera is within the best response interval, and set the wavelength λ of the output light of the monochromator to λ min ;

[0102] Step 3.8.5: The standard detector continuously collects U data of the output light of the monochromator at the wavelength λ. At the same time, the color area array CCD camera continuously collects V images of the output light of the monochromator, and then collects V background images of the color area array CCD camera, where V≥10;

[0103] Step 3.8.6: The computer performs data preprocessing on the DN value measured by the central pixel of the color area array CCD camera according to Equation (5) to obtain the response mean value of the central pixel of channel c of the color area array CCD camera at the wavelength λ

[0104]

[0105] In Equation (5): v is the current acquisition sequence of the color area array CCD camera; represents the response DN value of the pixel at the i-th row and j-th column of the radiation source image collected for the v-th time by channel c of the color area array CCD camera at wavelength λ; represents the background DN value of the pixel at the i-th row and j-th column of the background image collected for the v-th time by channel c of the color area array CCD camera at wavelength λ;

[0106] Step 3.8.7: The computer performs data preprocessing on the data measured by the standard detector according to Equation (6) to obtain the output value of the standard detector at wavelength λ

[0107]

[0108] In Equation (6): u is the current acquisition sequence of the standard detector; P ref,u,λ represents the output value collected for the u-th time by the standard detector at wavelength λ;

[0109] Step 3.8.8: The computer calculates the spectral responsivity of channel c at wavelength λ using Equation (7)

[0110]

[0111] Step 3.8.9: After assigning λ min +λ inter to λ, return to Step 3.8.5 and execute sequentially until λ ≥ λ max At this point, spectral responsivity data under different wavelength conditions are obtained. The computer normalizes the spectral responsivity data under different wavelength conditions and then performs interpolation processing to obtain the relative spectral response function r c (λ).

Claims

1. An absolute radiometric calibration method for a color area array CCD camera based on a Bayer array, characterized in that It is applied to a device composed of an LED light source, an integrating sphere, a standard irradiance meter, a computer, and a color area array CCD camera based on a Bayer array to be measured; the LED light source includes a red LED, a blue LED, and a green LED; The color area array CCD camera based on a Bayer array includes: a lens, a filter, and a area array CCD detector based on a Bayer array; the absolute radiometric calibration method includes the following steps: Step 1: Build an absolute radiometric calibration system for the color area array CCD camera; Install the color area array CCD camera, the standard irradiance meter, and the integrating sphere on an optical platform, and adjust the angles of the color area array CCD camera and the standard irradiance meter so that they are respectively aligned with the center of the light outlet of the integrating sphere, so that the radiation source exit surface of the integrating sphere fills the entire field of view of the color area array CCD camera; Step 2: Adjust the current magnitude of the LED light source and set the spectral irradiance; Adjust the current magnitudes of the red LED, blue LED, and green LED in the LED light source in sequence, and only keep one color of LED on during the adjustment process, and the rest of the LEDs are in the off state; Determine the minimum value, typical value, and maximum value of the spectral irradiance according to the spectral irradiance of the incident light at the required spectral band, adjust the current of one color of LED in the LED light source so that the spectral irradiance reaches the minimum value, typical value, and maximum value respectively. After the LED light source is stable, record the corresponding current values of the red LED, blue LED, and green LED in the LED light source; Denote the minimum value, typical value, and maximum value of the spectral radiance of the red LED at the spectral band r as respectively ; Denote the minimum value, typical value, and maximum value of the spectral radiance of the blue LED at the spectral band b as respectively ; Denote the minimum value, typical value, and maximum value of the spectral radiance of the green LED at the spectral band g as ; Step 3: Computer-controlled measurement; Step 3.1: Under the control of the computer, turn on the color area array CCD camera and the standard irradiance meter, and set the integration time t of the color area array CCD camera so that the camera is within the normal working range for the minimum value, typical value, and maximum value of all spectral irradiances, and preheat to a stable state; Step 3.2: Under the control of the computer, adjust the current magnitude of the red LED in the LED light source, and only keep the red LED on during the adjustment process, and the rest of the LEDs are in the off state; let the red LED be the currently on LED of the current color; Step 3.3: After the currently on LED of the current color is stable, make the radiation source exit irradiance of the integrating sphere be the minimum value of the currently on LED of the current color; Step 3.4: The standard irradiance meter continuously collects M data of the radiation source of the integrating sphere, and at the same time, the color area array CCD camera collects N images of the radiation source of the integrating sphere, and then collects N background images of the color area array CCD camera, where N≥10; Step 3.5: Under the control of the computer, make the radiation source exit irradiance of the integrating sphere be the typical value and maximum value of the currently on LED of the current color in sequence, and return to Step 3.4 respectively, so as to obtain data under three different conditions of the currently on LED of the current color; Step 3.6: Let the blue LED and the green LED be the LEDs of the current turned-on color in sequence, and process them according to the process of Step 3.3 - Step 3.5, so as to obtain data of nine different cases; Step 3.7: The computer calculates the response mean of the central pixel at the p-th row and q-th column of the channel c of the color area array CCD camera at the spectral band k and integration time t according to Equation (1). : (1) In Equation (1): n is the order of the current acquisition by the color area array CCD camera; represents the response DN value of the pixel at the i-th row and j-th column of the radiation source image acquired for the n-th time by channel c of the color area array CCD camera under the spectral band k and integration time t; represents the background DN value of the pixel at the i-th row and j-th column of the background image acquired for the n-th time by channel c of the color area array CCD camera under the spectral band k and integration time t; Step 3.8: The computer calculates the average radiance value within the band of the integrating sphere corresponding to the measured spectral band k according to Equation (2). : (2) In formula (2): , , respectively represent the average radiant luminance within the band of the corresponding spectral band k of the integrating sphere received by channels R, B, and G of the color area array CCD camera, and there is: (3) In formula (3): and respectively represent the upper wavelength and the lower wavelength of the measured spectral band k; m is the order currently collected by the standard irradiance meter; represents the spectral irradiance of the integrating sphere radiation source collected by the standard irradiance meter for the m-th time, and is a function of the wavelength ; represents the relative spectral response function of the color area array CCD camera channel c, represents the in-band average irradiance of the integrating sphere corresponding to the measured spectral band k received by the color area array CCD camera channel c; Step 3.9: The computer linearly fits the mean center pixel responses of the channels c of the color area array CCD camera corresponding to the minimum, typical, and maximum spectral radiance values under the currently turned-on LEDs of the current color by the least squares method, so as to obtain the absolute radiation response coefficient matrix of the R, B, and G channels under the three spectral bands r, b, g, and the integration time t and outputs it, where represents the absolute radiation response coefficient of channel c under the spectral band k and the integration time t, that is, the fitting slope; Step 3.10: The computer uses Equation (3) to establish the relationship between the response DN values of the R, B, and G channel pixels of the color area array CCD camera based on the Bayer array after background subtraction at the integration time T and the average in-band radiance values in the three spectral bands r, b, and g respectively: (4) In Equation (4): Subscripts R, B, and G respectively represent the R, B, and G channels of the Bayer array; r, b, and g respectively represent the r spectral band, b spectral band, and g spectral band. , , respectively represent the response DN values of the R, B, and G channel pixels of the color area array CCD camera based on the Bayer array under the integration time T. , , respectively represent the in-band average radiance values of the measured r spectral band, b spectral band, and g spectral band. , , respectively represent the absolute radiation response coefficients of the R channel of the Bayer array to the r spectral band, b spectral band, and g spectral band under the integration time t. , , respectively represent the absolute radiation response coefficients of the B channel of the Bayer array to the r spectral band, b spectral band, and g spectral band under the integration time t. , , respectively represent the absolute radiation response coefficients of the G channel of the Bayer array to the r spectral band, b spectral band, and g spectral band under the integration time t.

2. The absolute radiometric calibration method of a color area array CCD camera based on a Bayer array according to claim 1, characterized in that: The relative spectral response function in Step 3.8 is obtained according to the following steps: Step 3.8.1: A relative spectral response function test system is composed of the computer, the color area array CCD camera, the monochromator, the light source, the beam splitter, and the standard detector. Connect the monochromator to the light source and preheat it to a stable state; The beam splitter is arranged at the light output port position of the monochromator, and the output light energy of the monochromator is separated into two output lights with the same light intensity and spectral components; Step 3.8.2: Mount the color area array CCD camera on another optical platform and adjust its relative position to the beam splitter so that the output light of the monochromator is located at the central pixel of the color area array CCD camera; Step 3.8.3: According to the working spectral band and bandwidth of the color area array CCD camera, set the lower limit of the wavelength working range of the monochromator to be , the upper limit of the wavelength working range to be , and the stepped interval wavelength to be ; Step 3.8.4: Turn on the standard detector for preheating, set the integration time of the color area array CCD camera so that the color area array CCD camera is within the optimal response range, and set the wavelength of the outgoing light of the monochromator to ; Step 3.8.5: The standard detector continuously collects data U times for the light emitted by the monochromator at the wavelength Meanwhile, the color area array CCD camera continuously collects V images of the light emitted by the monochromator, and then collects V background images of the color area array CCD camera, where V ≥ 10; Step 3.8.6: The computer performs data preprocessing on the DN value measured by the central pixel of the color area array CCD camera according to Equation (5) to obtain the response mean value of the central pixel of channel c of the color area array CCD camera at wavelength under : (5) In Equation (5): v is the order of the current acquisition by the color area array CCD camera; represents the response DN value of the pixel at the i-th row and j-th column of the radiation source image collected for the v-th time by channel c of the color area array CCD camera at wavelength ; represents the background DN value of the pixel at the i-th row and j-th column of the background image collected for the v-th time by channel c of the color area array CCD camera at wavelength ; Step 3.8.7: The computer preprocesses the data measured by the standard detector according to Equation (6) to obtain the output value of the standard detector at the wavelength below : (6) In Equation (6): u is the current acquisition order of the standard detector; represents the output value of the standard detector at the wavelength for the u-th acquisition; Step 3.8.8: The computer calculates the spectral responsivity of channel c at the wavelength as follows : (7) Step 3.8.9: After assigning to , return to Step 3.8.5 and execute sequentially until ≥ . Thus, spectral responsivity data under different wavelengths are obtained. After normalizing the spectral responsivity data under different wavelengths, the computer performs interpolation processing to obtain the relative spectral response function of channel c of the color area array CCD camera.

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