CN111385497A - Fixed mode noise elimination method applied to pulse type image sensor - Google Patents

Fixed mode noise elimination method applied to pulse type image sensor Download PDF

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CN111385497A
CN111385497A CN201811609552.7A CN201811609552A CN111385497A CN 111385497 A CN111385497 A CN 111385497A CN 201811609552 A CN201811609552 A CN 201811609552A CN 111385497 A CN111385497 A CN 111385497A
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pixel
dark current
discharge time
coefficient
time
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徐江涛
张培文
王延昭
高静
聂华峰
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Tianjin University Marine Technology Research Institute
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Tianjin University Marine Technology Research Institute
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/60—Noise processing, e.g. detecting, correcting, reducing or removing noise
    • H04N25/67—Noise processing, e.g. detecting, correcting, reducing or removing noise applied to fixed-pattern noise, e.g. non-uniformity of response
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/60—Noise processing, e.g. detecting, correcting, reducing or removing noise
    • H04N25/63—Noise processing, e.g. detecting, correcting, reducing or removing noise applied to dark current

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Abstract

A fixed mode noise elimination method applied to a pulse type image sensor obtains the existence mode of noise through theoretical analysis, represents the noise introduced by a photodiode, a transistor and dark current respectively, adopts a uniform light source under the condition that a reset voltage and a reference voltage are fixed, obtains a dark current coefficient matrix by using two groups of dark light irradiation, and obtains a coefficient matrix for correcting the noise introduced by the photodiode and the transistor by using one group of strong light irradiation. When complex scenes are really shot, noise elimination can be realized by introducing the two coefficient matrixes, and the imaging quality of the sensor is improved.

Description

Fixed mode noise elimination method applied to pulse type image sensor
Technical Field
The invention relates to the field of CMOS image sensor testing, in particular to a fixed mode noise elimination method applied to a pulse image sensor.
Background
In recent years, image sensors are developed rapidly and are applied more and more widely in the fields of consumer electronics, automotive electronics, intelligent monitoring military reconnaissance and the like. Compared with the traditional frame-based imaging method, the data volume of the pulse interval type image sensor can be reduced to one eighth, and the bottleneck of overlarge data volume during high-speed imaging is solved. However, due to the structural characteristics of the pulse interval type image sensor, the pulse interval type image sensor is greatly influenced by noise, and the imaging quality is seriously influenced.
Due to process dimension variations, there can be mismatch problems between devices, which can lead to non-uniformity of light response and dark current response between pixel arrays. The optical response non-uniformity, i.e., device mismatch, is mainly due to the photodiode variation and transistor-transistor variation. Dark current non-uniformity is caused by photodiode mismatch. By analyzing the structure and imaging characteristics of the sensor, the cause and mode of noise generation are analyzed and compensated and eliminated respectively. In the reconstructed image, the image quality can be obviously improved, and the signal-to-noise ratio is obviously improved.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a fixed mode noise elimination method for a pulse type image sensor, which obtains the existence mode of noise through theoretical analysis, respectively represents the noise introduced by a photodiode, a transistor and dark current, measures a correction coefficient through a uniform illumination test, can realize the elimination of the noise and improve the imaging quality of the sensor.
The pulse type image sensor is constructed as shown in fig. 1, each pixel starts to integrate photo-generated charges after being reset, when the accumulation amount reaches a set threshold value, the pixel generates pulses, the pixel resets after the pulses are output, the integration of the photo-generated charges is restarted, the interval time between two pulses generated by the pixel is determined by the intensity of light, and the light intensity information can be restored by measuring the interval of two times; in the above-mentioned operation mode, the photocurrent is measuredI phAnd discharge time of pixeltInversely proportional and the relationship can be expressed as:
Figure 662631DEST_PATH_IMAGE001
due to the mismatch between the capacitances, the amount of charge actually stored by different photodiodes in the array is non-uniform, which introduces a coefficient at the discharge time t in equation (1), which can be expressed as:
Figure 755221DEST_PATH_IMAGE002
wherein,
Figure 789036DEST_PATH_IMAGE003
in order to include a discharge time of the capacitance mismatch,
Figure 189056DEST_PATH_IMAGE004
is a value of the change in the capacitance,
Figure 444457DEST_PATH_IMAGE005
for the introduced coefficient, its value is
Figure 520997DEST_PATH_IMAGE006
；
As the core part of the pixel, when the transistor has deviation, an offset voltage is introduced at the input end of the comparator, the reset tube will also cause the deviation of the reset voltage of the photodiode, and it can also be considered that an offset voltage is introduced at the input end of the comparator, and the offset voltage introduced by the reset tube and the offset voltage introduced by the comparator are combined together to define the total offset voltageV OS,totThis will be at the discharge time of equation (1)tThe above introduces a coefficient:
Figure 462015DEST_PATH_IMAGE007
wherein,
Figure 164260DEST_PATH_IMAGE008
in order to introduce the coefficients of the coefficients,
Figure 775633DEST_PATH_IMAGE009
is the exposure time interval when the offset voltage is included;
during discharge due to device mismatchMay vary by a factor of
Figure 210025DEST_PATH_IMAGE010
To show that the deviation of the photodiode is except at the discharge timetThe introduction of a coefficient will also result in dark currentI darkThe non-uniformity of dark current will introduce a factor related to the light intensity, which can be expressed as:
Figure 952853DEST_PATH_IMAGE011
Figure 709324DEST_PATH_IMAGE012
in order to include the discharge time affected by the dark current,
Figure 673737DEST_PATH_IMAGE013
a coefficient introduced for dark current, having a value of
Figure 780496DEST_PATH_IMAGE014
；
Considering that the image sensor operates continuously in the time domain, the influence of the non-uniformity of the light response can be reflected in the form of time accumulation, first, with uniform lightI ph1Irradiating the sensor for a fixed timeT totalIn the case of the number of pulses per pixel in the arrayN i,j The statistics are carried out, and the statistics are carried out,T totalthe time must be long enough to eliminate the effects of temporal noise; selecting the pulse number of any one pixel as a reference valueNBy passing throughNDivided by the number of pulses of other pixelsN i,j Obtaining a coefficient matrix
Figure 276199DEST_PATH_IMAGE015
The coefficient matrix is used for suppressing the non-uniformity of the light response of the pixel array;
for the reference pixel or pixels it is possible to refer to,Nandtthe relationship between can be expressed as:
Figure 992352DEST_PATH_IMAGE016
Figure 637703DEST_PATH_IMAGE017
is the capacitance value of the reference pixel PD node, and
Figure 226947DEST_PATH_IMAGE018
for the reference pixel to release the voltage, for the convenience of processing, the selected reference pixel may be considered as having no device mismatch, and for other pixels, due to the device mismatch, in fact, the difference of the device mismatch between the pixel and the reference pixel, may be obtained:
Figure 131318DEST_PATH_IMAGE019
Figure 214943DEST_PATH_IMAGE020
is in an array ofi,j) PD node capacitance of a pixel, instead
Figure 966999DEST_PATH_IMAGE021
The voltage to be discharged for the pixel is divided by equation (6) using equation (5) to obtain the coefficient matrix of the photoresponse non-uniformity:
Figure 976412DEST_PATH_IMAGE022
considering the current reference voltageV refAnd the power supply voltage of the reset tube is fixedV DDpixTime, dark currentI darkThe effect on each discharge time is fixed, and for the purpose of analysis, it is assumed thatV refAndV DDpixwhen the fixing is carried out, the fixing device,I darkis constant, in the absence of light, the discharge timet darkCan be expressed as:
Figure 181128DEST_PATH_IMAGE023
dividing equation (8) by equation (4) yields the ratio of dark current to photocurrent:
Figure 9057DEST_PATH_IMAGE024
then, it can be calculated
Figure 864886DEST_PATH_IMAGE025
Has a value of
Figure 484348DEST_PATH_IMAGE026
And further in the formula (3-9)tAnd
Figure 176361DEST_PATH_IMAGE027
the relationship of (d) can be rewritten as:
Figure 365903DEST_PATH_IMAGE028
according to the above formula, the dark current discharge time can be used
Figure 826971DEST_PATH_IMAGE029
The matrix corrects for dark current non-uniformity.
A fixed mode noise elimination method applied to a pulse type image sensor analyzes the type of noise and the influence on the performance of the image sensor on the basis of a sensor framework, and specifically expresses the noise of a photodiode, a transistor and a dark current part by a formula; different correction algorithms are provided for different noises, the signal-to-noise ratio of the image is obviously improved, and the imaging quality of the sensor is greatly improved.
Drawings
Fig. 1 is a diagram of a pulse-space image sensor architecture.
Detailed Description
The invention is explained in detail below with reference to the figures and examples, without however restricting the scope of protection of the invention thereto.
The noise cancellation approach designed herein requires testing the coefficient matrix of each pixel under uniform illumination. The reset voltage was set to 2.9V, the digital light source was set to illuminate the sensor with a uniform light of 10lux intensity to obtain an average trigger interval matrix D1 for each pixel, and then the dark light intensity was set to 100lux to obtain an average interval D2. The two sets of data are simultaneous to obtain the correction coefficient matrix of dark current. In order to reduce the interference of dark current, the light intensity of the digital light source is set to be 400lux of strong light, if the overexposure phenomenon occurs, the light intensity is properly reduced, each pixel is ensured to be within the upper limit of the light intensity, so that an average interval matrix D3 is obtained, the pixel with the position of (1, 1) is taken as a reference point, all the other points are normalized to be the same as the brightness of the reference point, and a coefficient matrix is obtained
Figure 614405DEST_PATH_IMAGE015
。

Claims (1)

1. A fixed mode noise elimination method applied to a pulse type image sensor is characterized in that: the pulse type image sensor is characterized in that each pixel starts to integrate photo-generated charges after being reset, when the accumulation amount reaches a set threshold value, the pixel generates pulses, the pixel resets after the pulses are output, the integration of the photo-generated charges is restarted, the interval time between two pulses generated by the pixel is determined by the intensity of light, and light intensity information can be restored by measuring two intervals; in the above-mentioned operation mode, the photocurrent is measuredI phAnd discharge time of pixeltInversely proportional and the relationship can be expressed as:
Figure 1323DEST_PATH_IMAGE001
due to the mismatch between the capacitances, the amount of charge actually stored by different photodiodes in the array is non-uniform, which introduces a coefficient at the discharge time t in equation (1), which can be expressed as:
Figure 563455DEST_PATH_IMAGE002
wherein,
Figure 40573DEST_PATH_IMAGE003
in order to include a discharge time of the capacitance mismatch,
Figure 609220DEST_PATH_IMAGE004
is a value of the change in the capacitance,
Figure 984837DEST_PATH_IMAGE005
for the introduced coefficient, its value is
Figure 592405DEST_PATH_IMAGE006
；
As the core part of the pixel, when the transistor has deviation, an offset voltage is introduced at the input end of the comparator, the reset tube will also cause the deviation of the reset voltage of the photodiode, and it can also be considered that an offset voltage is introduced at the input end of the comparator, and the offset voltage introduced by the reset tube and the offset voltage introduced by the comparator are combined together to define the total offset voltageV OS,totThis will be at the discharge time of equation (1)tThe above introduces a coefficient:
Figure 2658DEST_PATH_IMAGE007
wherein,
Figure 536014DEST_PATH_IMAGE008
in order to introduce the coefficients of the coefficients,
Figure 398928DEST_PATH_IMAGE009
is the exposure time interval when the offset voltage is included;
the discharge time variation due to device mismatch can be a factor
Figure 810186DEST_PATH_IMAGE010
To show that the deviation of the photodiode is except at the discharge timetThe introduction of a coefficient will also result in dark currentI darkThe non-uniformity of dark current will introduce a factor related to the light intensity, which can be expressed as:
Figure 74946DEST_PATH_IMAGE011
Figure 47712DEST_PATH_IMAGE012
in order to include the discharge time affected by the dark current,
Figure 584873DEST_PATH_IMAGE013
a coefficient introduced for dark current, having a value of
Figure 284975DEST_PATH_IMAGE014
；
Considering that the image sensor operates continuously in the time domain, the influence of the non-uniformity of the light response can be reflected in the form of time accumulation, first, with uniform lightI ph1Irradiating the sensor for a fixed timeT totalIn the case of the number of pulses per pixel in the arrayN i,j The statistics are carried out, and the statistics are carried out,T totalthe time must be long enough to eliminate the effects of temporal noise; selecting the pulse number of any one pixel as a reference valueNBy passing throughNDivided by the number of pulses of other pixelsN i,j Obtaining a coefficient matrix
Figure 157903DEST_PATH_IMAGE015
The coefficient matrix is used for suppressing the non-uniformity of the light response of the pixel array;
for the reference pixel or pixels it is possible to refer to,Nandtthe relationship between can be expressed as:
Figure 550838DEST_PATH_IMAGE016
Figure 637612DEST_PATH_IMAGE017
is the capacitance value of the reference pixel PD node, and
Figure 141406DEST_PATH_IMAGE018
for the reference pixel to release the voltage, for the convenience of processing, the selected reference pixel may be considered as having no device mismatch, and for other pixels, due to the device mismatch, in fact, the difference of the device mismatch between the pixel and the reference pixel, may be obtained:
Figure 865910DEST_PATH_IMAGE019
Figure 429747DEST_PATH_IMAGE020
is in an array ofi,j) PD node capacitance of a pixel, instead
Figure 3816DEST_PATH_IMAGE021
The voltage to be discharged for the pixel is divided by equation (6) using equation (5) to obtain the coefficient matrix of the photoresponse non-uniformity:
Figure 45722DEST_PATH_IMAGE022
considering the current reference voltageV refAnd the power supply voltage of the reset tube is fixedV DDpixTime, dark currentI darkThe effect on each discharge time is fixed, and for the purpose of analysis, it is assumed thatV refAndV DDpixwhen the fixing is carried out, the fixing device,I darkis constant, in the absence of light, the discharge timet darkCan be expressed as:
Figure 887382DEST_PATH_IMAGE023
dividing equation (8) by equation (4) yields the ratio of dark current to photocurrent:
Figure 622120DEST_PATH_IMAGE024
then, it can be calculated
Figure 417907DEST_PATH_IMAGE025
Has a value of
Figure 529082DEST_PATH_IMAGE026
And further in the formula (3-9)tAnd
Figure 228179DEST_PATH_IMAGE027
the relationship of (d) can be rewritten as:
Figure 320769DEST_PATH_IMAGE028
according to the above formula, the dark current discharge time can be used
Figure 354584DEST_PATH_IMAGE029
The matrix corrects for dark current non-uniformity.
CN201811609552.7A 2018-12-27 2018-12-27 Fixed mode noise elimination method applied to pulse type image sensor Withdrawn CN111385497A (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN112188127A (en) * 2020-09-29 2021-01-05 中国科学院高能物理研究所 Method, device, medium and equipment for calibrating integral pixel array detector
CN113473048A (en) * 2021-06-16 2021-10-01 天津大学 Non-uniformity correction method for pulse array image sensor

Citations (3)

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US20090046177A1 (en) * 2007-08-14 2009-02-19 Samsung Electronics Co., Ltd. Method and apparatus for canceling fixed pattern noise in CMOS image sensor
US20090046180A1 (en) * 2006-02-10 2009-02-19 Sharp Kabushiki Kaisha Fixed-Pattern Noise Elimination Apparatus, Solid-State Image Sensing Apparatus, Electronic Appliance, and Fixed-Pattern Noise Elimination Program

Patent Citations (3)

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CN1984242A (en) * 2005-12-16 2007-06-20 富士通株式会社 Image data processing circuit and image data processing method
US20090046180A1 (en) * 2006-02-10 2009-02-19 Sharp Kabushiki Kaisha Fixed-Pattern Noise Elimination Apparatus, Solid-State Image Sensing Apparatus, Electronic Appliance, and Fixed-Pattern Noise Elimination Program
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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN112188127A (en) * 2020-09-29 2021-01-05 中国科学院高能物理研究所 Method, device, medium and equipment for calibrating integral pixel array detector
CN112188127B (en) * 2020-09-29 2021-08-24 中国科学院高能物理研究所 Method, device, medium and equipment for calibrating integral pixel array detector
CN113473048A (en) * 2021-06-16 2021-10-01 天津大学 Non-uniformity correction method for pulse array image sensor

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