CN112312125B - Multi-tap EMCCD (Electron multiplying Charge coupled device) non-uniformity comprehensive correction method - Google Patents
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Abstract
The invention discloses a multi-tap EMCCD (electron-multiplying charge coupled device) nonuniformity comprehensive correction method. The method can be roughly divided into four steps, namely: determining background non-uniformity correction parameters; determining a light response linearity non-uniformity correction parameter; step three, determining a multiplication gain non-uniformity correction parameter; and step four, comprehensive correction operation of the three types of non-uniformity correction parameters. The comprehensive multi-tap EMCCD heterogeneity correction method provided by the invention can greatly eliminate the influence of the three types of heterogeneity on the EMCCD output image, and improve the quality of the EMCCD output image.
Description
Technical Field
The invention relates to the field of EMCCD (electron multiplying charge coupled device) non-uniformity correction, in particular to the field of comprehensive correction of a multi-channel EMCCD.
Background
The EMCCD is an electron multiplication CCD, an internal multiplication register can amplify photo-generated charges exponentially in the transfer process of the photo-generated charges, and compared with a common CCD, the EMCCD has higher detection sensitivity in the process, and is widely applied to the field of low-light detection of astronomy, biology, medicine and the like. The non-uniformity of the output image of the EMCCD camera directly affects the use of the user, so the output image uniformity of the camera is a key factor of the imaging performance of the EMCCD. The nonuniformity of the ordinary CCD is mainly caused by dark field background nonuniformity and photoresponse nonuniformity of the detector. The EMCCD has background non-uniformity, photoresponse non-uniformity, and also introduces electron multiplication non-uniformity during electron multiplication.
The electron multiplication non-uniformity is divided into two categories, inter-channel non-uniformity and inter-pixel non-uniformity. The multi-tap output structure of the EMCCD detector can greatly improve the output speed of the EMCCD detector, but the difference of multiplication processes among different channels can also introduce non-uniformity of multiplication gain among EMCCD channels. The multiplication process of photo-generated charges for different pixels in the same channel is a random event, which also introduces non-uniformity of multiplication gain among pixels.
Different from the nonuniformity correction of a common CCD, the method needs to correct the background nonuniformity and the photoresponse nonuniformity existing in the common CCD and also needs to correct the multiplication gain nonuniformity. Background non-uniformity and photoresponse non-uniformity are introduced into multiplication gain non-uniformity, so that the relation between the three types of non-uniformity needs to be comprehensively analyzed, the influence of different non-uniformity is eliminated, and the three types of non-uniformity in the EMCCD can be effectively removed.
Disclosure of Invention
The invention aims to provide a correction method integrating background non-uniformity, photoresponse non-uniformity and multiplication gain non-uniformity by analyzing non-uniformity components and mutual relations thereof in an EMCCD (electron-multiplying charge coupled device).
The technical scheme adopted by the invention is as follows: a multi-tap EMCCD non-uniformity comprehensive correction method comprises the following specific steps:
step (1), an EMCCD is installed on a test support of a darkroom, a camera is powered on, a refrigeration function is started, an integrating sphere is powered on, and after the camera reaches a preset refrigeration temperature, the illumination radiated by the integrating sphere is stable;
step (2), opening the aperture of the integrating sphere to enable the camera to be in a bright field environment, and setting the minimum exposure time t which can be set by the camera through upper computer softwaremin(ii) a Determining the maximum exposure time t when all pixels are not saturated according to the output image of the cameramax(ii) a Obtaining the linear light response exposure time interval of the camera under the illumination intensity radiated by the current integrating sphere as tmin,tmax]；
Step (3), closing the aperture of the integrating sphere, closing the EMCCD multiplication gain function, and setting the minimum exposure time t of the cameraminObtaining m0(m0Not less than 2000) frame dark field background image, calculating dark field background average value imagebenDiAnd its whole picture gray average imagebenDiAve；
Step (4) in the exposure time [ tmin,tmax]Uniformly selecting n in interval1(n1Not less than 20) exposure time points, closing the integrating sphere to enable the camera to be in a dark field environment, and acquiring m of the camera at each exposure time point1(m1Not less than 100) dark field images; opening the integrating sphere, and acquiring m of the camera at each exposure time point according to the same steps1(m1Not less than 100) bright field images; calculating the difference between the bright field mean image and the dark field mean image of each exposure time point to obtain n1(n1Not less than 20) mean light and shade difference images;
step (5) ofn1(n1Not less than 20) exposure time points are taken as X, and n is taken as1(n1Not less than 20) taking the image of the mean value light-dark difference as Y, and carrying out linear fitting to obtain linear light response coefficients k (i, j) and b (i, j) of each pixel point. Where (i, j) represents the pixel coordinates of the current pixel in the image. Averaging all pixel points to obtain linear light response coefficient mean value k of whole imageave、bave；
Step (6), in the exposure time interval [ t ]min,tmax]Internally selecting a certain exposure time t0The multiplication gain function of the EMCCD is turned on, and the gain voltage is volt when the multiplication gain of the camera is 1minMaximum gain voltage volt that no pixel in the output image of the camera reaches saturationmax；
Step (7), exposure time t of camera0Keeping the gain constant, closing the multiplication gain function, and obtaining bright field images and dark field images of the current state of the camera2(m2Not less than 100), calculating a mean value light and shade difference image, namely the original signal quantity image of the camera at the current exposure timebrightDarkDiff0(i, j). Removing the influence of background non-uniformity and linear non-uniformity on the original signal quantity through the following formula to obtain the corrected original signal quantity sign before multiplicationorigin(i,j)。
Step (8) exposing time t of camera0Keeping the gain constant, turning on the multiplication gain function, in the interval [ voltmin,voltmax]Selecting p (p is more than or equal to 10) gain voltages, and collecting m bright field images and m dark field images of the EMCCD at different gain voltage points2(m2Not less than 100). Calculating the signal quantity image of the camera after multiplication and amplification under the current exposure time and the gain voltagebrightDarkDiffVoltX(i, j). After the influence of background nonuniformity and linear nonuniformity is removed, multiplied semaphore sign is obtainedvoltX(i,j)。
Step (9) according to the corrected original semaphore signorigin(i, j) and the multiplication semaphore signvoltX(i, j) different gain voltages volt can be obtainedXTrue multiplication gain of (i, j) and full graph mean multiplication gain ofave。
Here, hang and lie represent the total number of rows and the total number of columns of the current EMCCD output image.
Step (10) using p gain voltages volt (p is more than or equal to 10)XAnd taking X as the reference value, taking p (p is more than or equal to 10) real multiplication gains Gain (i, j) as Y, and performing exponential fitting according to the following formula to obtain multiplication Gain correction coefficients c (i, j) and d (i, j).
gain(i,j)＝exp[c(i,j)*voltx d(i,j)]
Using p (p is more than or equal to 10) gain voltages voltXMaking X to obtain p gain voltages voltXReal multiplication gain mean value gain of lower full imageaveTaking Y, and performing exponential fitting according to the following formula to obtain the average gain correction coefficient c of the whole graphaveAnd dave。
And (11) the whole non-uniformity correction process of the original image output by the EMCCD camera is as follows:
gain(i,j)＝exp[c(i,j)*voltx d(i,j)]
the invention has the beneficial effects that:
the invention comprehensively corrects the background nonuniformity, the photoresponse linearity nonuniformity and the multiplication gain nonuniformity which mainly exist by analyzing the nonuniformity components in the EMCCD. The method provides a new idea for the non-uniformity comprehensive correction of the EMCCD, greatly reduces the non-uniformity of the EMCCD output image, and can improve the quality of the output image of the EMCCD camera under different illumination intensities, different exposure times and different multiplication gains.
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Fig. 1 is a flow chart of a multi-tap EMCCD non-uniformity comprehensive correction method of the present invention.
Detailed Description
The following detailed description of the invention refers to the accompanying drawings.
The invention relates to the field of EMCCD non-uniformity correction, in particular to the field of comprehensive correction of a multi-channel EMCCD.
The invention discloses a multi-tap EMCCD (Electron multiplying Charge coupled device) heterogeneity comprehensive correction method which can be roughly divided into four steps, wherein the four steps are as follows: determining background non-uniformity correction parameters; determining a light response linearity non-uniformity correction parameter; step three, determining a multiplication gain uniformity correction parameter; and step four, comprehensive correction operation of the three types of non-uniformity correction parameters.
Step one, determining background nonuniformity correction parameters.
1.1, an EMCCD is installed on a test bracket of a darkroom, a camera is powered on, a refrigeration function is started, an integrating sphere is powered on, and after the camera reaches a preset refrigeration temperature, the illumination intensity radiated by the integrating sphere is stable;
1.2 shielding an external light source, and opening an integrating sphere diaphragm to enable the camera to be in a bright field environment. Through upper computer softwareSetting a camera to a settable minimum exposure time tmin(ii) a Determining a maximum exposure time t from the camera output image when all pixels have not reached saturationmax(ii) a Obtaining the linear light response exposure time interval of the camera under the illumination intensity radiated by the current integrating sphere as tmin,tmax]；
1.3 turning off the aperture of the integrating sphere, turning off the EMCCD multiplication gain function, and setting the minimum exposure time t of the cameraminObtaining m by the upper computer control software of the camera0(m0Not less than 2000) background image of dark field, calculating m0(m0More than or equal to 2000) imagesbenDi(i, j) and its mean imagebenDiAve；
And step two, determining the optical response linearity non-uniformity correction parameters.
2.1 in the Linear photoresponse Exposure time Interval [ t ]min,tmax]Uniformly selecting n1(n1Not less than 20) exposure time points.
2.2 turning off the integrating sphere to make the camera in dark field environment, obtaining m of the camera at each exposure time point1(m1Not less than 100) dark field images; opening the integrating sphere, and acquiring m of the camera at each exposure time point according to the same steps1(m1Not less than 100) bright field images.
2.3 calculating n1(n1Not less than 20) exposure time points, namely obtaining the difference between the bright field mean image and the dark field mean image1(n1Not less than 20) mean light and shade difference images;
2.4 by n1Taking exposure time points as X, n1(n1Not less than 20) image of the mean light and shade difference is taken as Y, and linear fitting is carried out to obtain linear light response coefficients k (i, j) and b (i, j) of each pixel point. Averaging all pixel points to obtain the linear light response coefficient mean value k of the whole imageave、bave；
And step three, determining a multiplication gain uniformity correction parameter.
3.1 Exposure time interval [ t ]min,tmax]Internally selecting a certain exposure time t0Is kept unchangedWhen the multiplication gain function of the EMCCD is turned on and the multiplication gain of the camera is 1, the gain voltage is voltminMaximum gain voltage volt that no pixel in the output image of the camera reaches saturationmaxObtaining the current illumination and the exposure time t of the EMCCD0The lower multiplication gain voltage interval is [ volt ]min,voltmax]；
3.2 Exposure time t of Camera0Keeping the state unchanged, closing the multiplication gain function, and acquiring bright field and dark field images m of the current state of the camera2(m2Not less than 100), calculating a mean value light-dark difference image, namely the original semaphore image of the camera at the current exposure timebrightDarkDiff0(i, j). Removing the influence of background non-uniformity and linear non-uniformity on the original signal quantity through the following formula to obtain the corrected original signal quantity sign before multiplicationorigin(i,j)。
3.3 Exposure time t of Camera0Keeping the gain constant, turning on the multiplication gain function, in the interval [ voltmin,voltmax]Selecting p (p is more than or equal to 10) gain voltage points, and collecting m bright field images and m dark field images of the EMCCD at different gain voltage points2(m2Not less than 100). Calculating the signal quantity image of the camera after multiplication and amplification under the current exposure time and the gain voltage according to the method in 3.2brightDarkDiffVoltX(i, j). After the influence of background nonuniformity and linear nonuniformity is removed, the multiplied semaphore sign is obtainedvoltX(i,j)。
3.4 according to the corrected original semaphore signorigin(i, j) and the multiplication semaphore signvoltX(i, j) different gain voltages volt can be obtainedXTrue multiplicative gain (i, j) and full-image mean multiplicative gainave。
Wherein hang represents the total number of lines of the current EMCCD output image; lie represents the total number of columns of the current EMCCD output image.
3.5 with p (p ≧ 10) gain voltages voltXTaking X as the reference, taking the real multiplication gain (i, j) of a single pixel under p (p is more than or equal to 10) gain voltages as Y, and carrying out exponential fitting according to the following formula to obtain multiplication gain correction coefficients c (i, j) and d (i, j):
gain(i,j)＝exp[c(i,j)*voltx d(i,j)]
using p (p is more than or equal to 10) gain voltages voltXMaking X to make p gain voltages voltXReal multiplication gain mean value gain of lower full-frame imageaveTaking Y, and performing exponential fitting according to the following formula to obtain the average gain correction coefficient c of the whole graphaveAnd dave：
And step four, comprehensive correction operation of the three types of non-uniformity correction parameters.
4.1 the process of non-uniformity correction of the raw image output by the EMCCD camera is as follows:
gain(i,j)＝exp[c(i,j)*voltx d(i,j)]
wherein, (i, j) represents the ith row and the jth column of pixels in the graph; imageorigin(i, j) is the original image output by the EMCCD; imagebenDi(i,j)、imagebenDiAve(i, j) is the background non-uniformity correction coefficient obtained in step 1.3; k (i, j), b (i, j), kave、baveThe linear non-uniformity correction coefficient of the photoresponse obtained in the step 2.4; c (i, j) and d (i, j) are the multiplication gain correction coefficients obtained in step 3.5; voltxIs the current applied gain voltage value of the EMCCD.
4.2 the beneficial effect of this method is, through analyzing the heterogeneous composition in EMCCD, remove the mutual influence among different heterogeneity and then carry on the heterogeneous comprehensive correction to EMCCD. The method provides a new idea for the non-uniformity comprehensive correction of the EMCCD, and greatly reduces the non-uniformity of the EMCCD output image.
The above-mentioned non-uniformity comprehensive correction step of the EMCCD is only an operation example of the present invention, and the specific content of the present invention is not limited. All changes which do not depart from the technical essence of the scheme are within the protection scope of the scheme.
Claims (1)
1. A multi-tap EMCCD non-uniformity comprehensive correction method is characterized by comprising the following steps:
step (1), an EMCCD is installed on a test support of a darkroom, a camera is powered on, a refrigeration function is started, an integrating sphere is powered on, and after the camera reaches a preset refrigeration temperature, the illumination radiated by the integrating sphere is stable;
step (2), shielding an external light source, opening an integrating sphere diaphragm to enable the camera to be in a bright field environment, and setting the minimum exposure time t which can be set by the camera through upper computer softwaremin(ii) a Determining a maximum exposure time t from the camera output image when all pixels have not reached saturationmax(ii) a Obtaining the linear light response exposure time interval of the camera under the illumination intensity radiated by the current integrating sphere as tmin,tmax]；
Step (3), closing the aperture of the integrating sphere, closing the EMCCD multiplication gain function, and setting the minimum exposure time of the cameratminObtaining m through camera upper computer control software0Calculating m from background image of dark field0Dark field background mean image of imagebenDi(i, j) and its gray-scale mean imagebenDiAve(ii) a M in the step (3)0The number of dark field background images needs to satisfy the following conditions: m is0≥2000；
Step (4), in the linear light response exposure time interval [ tmin,tmax]Uniformly selecting n1At each exposure time point, closing the integrating sphere to enable the camera to be in a dark field environment, and acquiring m of the camera at each exposure time point1A dark field image is obtained; opening the integrating sphere, and acquiring m of the camera at each exposure time point according to the same steps1A bright field image; calculating n1The difference between the bright field mean image and the dark field mean image of each exposure time point is obtained as n1An amplitude-mean difference image;
step (5) with n1Taking exposure time points as X, n1Taking the image with the difference between the light and the shade of the amplitude mean value as Y, carrying out linear fitting to obtain the linear light response coefficients k (i, j) and b (i, j) of each pixel point, and averaging all the pixel points to obtain the mean value k of the linear light response coefficients of the whole imageave、bave(ii) a The number of exposure points n in the step (4) and the step (5)1Number of images m1The requirements are as follows: n is a radical of an alkyl radical1≥20、m1≥100；
Step (6), in the exposure time interval [ t ]min,tmax]Internally selecting a certain exposure time t0Keeping the EMCCD unchanged, and turning on the multiplication gain function of the EMCCD; the gain voltage when the multiplication gain of the camera is 1 is voltminThe maximum gain voltage at which the pixels in the output image of the camera do not reach saturation is voltmaxObtaining the multiplication gain voltage interval [ volt ] of the EMCCD under the current illumination and the exposure timemin,voltmax]；
Step (7), exposure time t of camera0Keeping the gain constant, closing the multiplication gain function, and obtaining the bright field image and the dark field image m of the camera in the current state2Amplitude, calculating the mean shading image, i.e. the original semaphore image of the camera at the current exposure timebrightDarkDiff0(i, j) removing the influence of background non-uniformity and linear non-uniformity on the original signal quantity through the following formula to obtain the corrected original signal quantity sign before multiplicationorigin(i,j)，
Step (8), exposure time t of camera0Keeping the gain constant, turning on the multiplication gain function, in the interval [ voltmin,voltmax]Selecting p gain voltage points, and collecting m bright field images and m dark field images of EMCCD at different gain voltage points2Calculating the signal quantity image of the camera after multiplication and amplification under the current exposure time and the gain voltage according to the method in the step (7)brightDarkDiffVoltX(i, j), removing the influence of background nonuniformity and linear nonuniformity to obtain the multiplied semaphore signvoltX(i,j)，
Step (9) according to the corrected original semaphore signorigin(i, j) and the multiplication semaphore signvoltX(i, j) different gain voltages volt can be obtainedXTrue multiplicative gain (i, j) and full-image mean multiplicative gainave，
Wherein hang represents the total line number of the current EMCCD output image; lie represents the total number of columns of the current EMCCD output image;
step (1)0) With p gain voltages voltxTaking X, taking p real multiplication gains gain (i, j) of a single pixel under the gain voltage as Y, and performing exponential fitting according to the following formula to obtain multiplication gain correction coefficients c (i, j) and d (i, j):
gain(i,j)＝exp[c(i,j)*voltx d(i,j)]
with p gain voltages voltxMaking X to obtain p gain voltages voltxReal multiplication gain mean value gain of lower full imageaveTaking Y, carrying out exponential fitting according to the following formula to obtain the average gain correction coefficient c of all pixelsaveAnd dave：
The number m of images in the steps (7), (8) and (10)2The gain voltage number p needs to satisfy: m is2≥100、p≥10；
Step (11), the non-uniformity correction process of the original image output by the EMCCD camera is as follows:
gain(i,j)＝exp[c(i,j)*voltx d(i,j)]
wherein, (i, j) represents the ith row and the jth column of pixels in the graph; imageoriginThe original image is output by the EMCCD; imagebenDi、imagebenDiAveThe background nonuniformity correction coefficient obtained in the step (3); k (i, j), b (i, j), kave、baveCorrecting coefficients for the linear non-uniformity of the photoresponse obtained in the step (5); c (i, j) and d (i, j) are multiplication gain non-uniformity correction coefficients obtained in the step (10); voltxThe value of the gain voltage currently applied for the EMCCD.
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