CN112394507A - Iris imaging depth of field extension method based on liquid lens - Google Patents
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- G—PHYSICS
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- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/0075—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 with means for altering, e.g. increasing, the depth of field or depth of focus
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B13/00—Optical objectives specially designed for the purposes specified below
- G02B13/001—Miniaturised objectives for electronic devices, e.g. portable telephones, webcams, PDAs, small digital cameras
- G02B13/0055—Miniaturised objectives for electronic devices, e.g. portable telephones, webcams, PDAs, small digital cameras employing a special optical element
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- G—PHYSICS
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- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/004—Optical devices or arrangements for the control of light using movable or deformable optical elements based on a displacement or a deformation of a fluid
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- G06V40/18—Eye characteristics, e.g. of the iris
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Abstract
The invention discloses an iris imaging depth of field extension method based on a liquid lens, which comprises initializing the liquid lens, a scene camera and an iris camera, wherein the scene camera detects a target and sends a starting signal to the liquid lens, then the liquid lens carries out coarse scanning according to a preset coarse step length to obtain a sequence iris image, and a coarse current value I is determined through quality evaluation0In I0Fine scanning is carried out within a certain range before and after according to preset fine step length to obtain a sequence iris image, and a coarse current value i is determined through quality evaluation0Then change the liquid lens current to i0At the mostAnd then sending a trigger signal to obtain one or more frames of clear iris images, and extracting and comparing iris features of the images to obtain a recognition result. The invention can effectively solve the problems of limited imaging depth of field and short acquisition distance of the iris recognition equipment, has high speed, large depth of field, long service life, low cost, strong stability and convenient integration and installation, is suitable for the static and moving states of the target and can realize multi-user real-time acquisition.
Description
Technical Field
The invention relates to the technical field of iris imaging and computational imaging, in particular to an iris imaging depth of field extension method based on a liquid lens.
Background
The human eye structure is composed of sclera, iris, pupil lens, retina and other parts, wherein the iris is an annular part positioned between a black pupil and a white sclera, occupies 65% of the eye, contains rich texture information, has a plurality of mutually staggered spot, filament, crown, stripe, crypt and other detailed characteristics, and is one of the most unique structures in the human body.
Unlike other biometric identification, the iris has high stability and uniqueness. One iris has about 244 oppositional variables, and fingerprints and faces have only ten or dozens of independent variables, so that the amount of information carried by the iris is one magnitude, the identification accuracy is higher, and the iris identification error rate is as low as one millionth. The complex detailed nature of the iris makes forgery almost impossible, and the biometric features of the iris can only be recognized in vivo, and replacing iris images of living subjects with photographs, video, etc. does not work. Iris recognition is one of the most stable, most accurate and highest safety factor biometric technologies at present.
At present, a plurality of iris recognition products and a plurality of application cases exist in the market, but the current iris recognition technology is still limited by the problem that the depth of field of recognition equipment is small, long-distance recognition application cannot be realized, and even under the condition of short-distance recognition, the iris recognition equipment is required to be actively matched with a station at a specified position, so that the interaction feeling is poor. Most of the current methods for increasing the depth of field use multiple different focus lens combinations (extended depth of field is maximally equal to the sum of the depths of field of each lens) or zoom lenses. However, the multiple optical lens systems are complex in design, high in manufacturing cost and large in size, and the target to be recognized can not be guaranteed to be just in the range of the depth of field of the clear focus; the zoom lens usually adopts mechanical motion focusing controlled by a motor, has slow speed, poor stability and expensive long-focus lens cost, cannot realize high-speed accurate zooming and has limited service life. Due to the limitation of the optical imaging principle, it is not practical to increase the original depth of field by changing the optical path of the lens assembly.
The liquid lens is a device for changing the curvature of a film by utilizing the shape of liquid so as to change a focus, has high response speed, is stable to adjust, is convenient to integrate, and is widely applied to the field of microscopes. The liquid lens has three main using modes, namely, the liquid lens is arranged at the front end of a common imaging lens, arranged at the rear end of the common imaging lens, namely in front of the lens and a camera, and embedded in the imaging lens. The third mode has high manufacturing cost, cannot be disassembled, has poor reusability and is not easy to maintain, and the mode is rarely used; the first mode has strong universality, the existing lens is not required to be changed basically during installation, but under the condition of the mode, the focusing can only be realized at a short distance or a micro distance, the use is wide in the field of microscopic imaging, the depth of field of a large target can be expanded, such as face imaging, but the requirement (tested) of iris large depth of field imaging cannot be met; the second mode mainly needs to meet the requirement of the rear intercept of the imaging lens, and because the thickness of the standard C-port liquid lens is 17.5mm, and the standard rear intercept of the C-port lens is only 17.526mm, the rear intercept required by simply increasing the liquid lens is 35.026mm, which completely exceeds the focusing capability of the C-port lens and cannot focus, a non-standard customized lens must be used in the mode. The Optoture has a corresponding application case, so that clear iris imaging can be realized only by changing the back intercept of the iris imaging lens. This is also the way the invention takes, with a lens back intercept of 38.0 mm.
Disclosure of Invention
The invention aims to solve the problem of depth of field limitation in long-distance iris imaging based on the quick response characteristic of a liquid lens, improve the acquisition speed and the acquisition distance of an iris acquisition system, improve the close-distance iris identification interactive experience, break through the prior art, realize the requirement of farther-distance iris identification and widen the application scene and range of the iris identification technology, thereby providing an iris imaging depth of field expansion method based on the liquid lens.
In order to achieve the aim of the invention, the invention provides a liquid lens-based iris imaging depth of field extension method,
the method comprises the following steps:
step S0, initializing a scene camera, a liquid lens and an iris camera;
step S1, determining the target coarse focusing current value I by the coarse scanning algorithm0；
Step S2, according to I0Value, determining the focus current value i of the target fine focus by a fine scanning algorithm0；
Step S3, adjusting the current of the liquid lens (2) to i0Changing diopter to a focus position, and acquiring one or more frames of clear iris images by using a trigger signal after reaching a stable state;
and step S4, detecting, positioning, segmenting, extracting and comparing the features of the iris image to obtain an identification result.
Wherein,
the rough scanning algorithm in step S1 is specifically as follows (current unit mA):
setting the maximum current of the liquid lens as Max, the minimum current as Min and the maximum current of the coarse scanning as Imax(ImaxMax) and the minimum limiting current is Imin(Min≤Imin) If the coarse step is H, the number of scanning points N is equal to (I)max-Imin) /H, default from IminTo ImaxScanning, and if N is a decimal number, adding 1; the temporary stay time of the scanning point is T (T is less than or equal to 7 ms);
triggering to acquire a frame of iris image after T every time a scanning point passes through, and scanning to form a coarse sequence iris image; utilizing the synchronous thread to evaluate the quality of the iris image in the coarse sequence, and finally determining the scanning point current value corresponding to the highest image quality, namely the coarse focusing current value I0。
Wherein,
the fine scanning algorithm in step S2 is specifically as follows:
thin searchHas a maximum current of imax＝I0+ H (if Max < i)maxThen i isminMax), minimum limiting current imin＝IcH (if i)minMin, then iminMin, fine step length H (H < H), scanning point number n ═ imax-imin) H, default from iminTo imaxScanning, and if n is a decimal, rounding and adding 1; the temporary stay time of the scanning point is t (7ms is less than or equal to t);
after t passes through each scanning point, triggering to acquire a frame of iris image, and scanning to form a thin sequence iris image; using synchronous thread to evaluate the quality of iris image in thin sequence, and determining the scanning point current value corresponding to the highest image quality, that is, the thin focus current value i0。
Wherein,
the image quality evaluation algorithm adopted for quality evaluation of the iris image needs dimension reduction of the iris image, face positioning based on random sampling and pixel gray threshold, and finally gradient of the face image in x and y directions, namely an image quality value, is calculated.
The liquid lens is arranged between the iris image sensor and the iris imaging lens, the distance from the target surface of the sensor to the rear end face of the imaging lens is equal to the rear intercept of the lens, the response time is 7ms, the diopter is-10 dpt, and the working waveband is 850-1500 nm based on the electrowetting effect.
Compared with the prior art, the invention has the advantages that,
1. the depth of field is greatly expanded. The extended distance of the original depth of field for iris recognition can reach more than dozens of times by the disclosed method. According to a test experiment, the focal length of the near-infrared lens is 350mm, the focusing point is 5m, the original depth of field is 95mm (4960-5055 mm), the depth of field can be identified to be 4000mm (3800-7800 mm) after expansion, and the expansion is about 42 times.
2. High speed dynamic response. Current response time <7ms, steady state time <40 ms.
3. And the man-machine interaction is good. The focus position can be quickly adjusted through the lens for identification without actively matching a person standing at a specified position.
4. And (4) ultra-long distance identification. The distance limit of the current commercial iris recognition product is broken through, and the 10m long-distance iris recognition is realized under the condition that the volume of the imaging equipment is basically unchanged.
5. And (4) multi-target collection. In the same field range, the fast adjustment of a plurality of current focuses can be realized, and the multi-target clear iris can be obtained and identified.
The invention can obviously improve the depth of field of iris imaging, expands the application scene of iris recognition, can realize clear imaging and recognition of multi-target irises in a visual field by using large depth of field, can realize continuous clear imaging and recognition of dynamic irises in progress by using the quick focusing characteristic of the liquid lens and a reasonable control strategy, is particularly suitable for places with large passenger flow, such as airports, railway stations, hospitals and the like, and can effectively improve the passenger flow volume.
Drawings
FIG. 1 is a schematic diagram of a liquid lens based iris recognition depth of field extension method;
FIG. 2 is a schematic diagram of the liquid lens coarse scan S1 algorithm;
FIG. 3 is a schematic diagram of the liquid lens fine scan S2 algorithm;
FIG. 4 is a schematic diagram of liquid lens current adjustment;
FIG. 5 is an overall schematic of a liquid lens feedback-less focusing algorithm;
FIG. 6 is a schematic view of a liquid lens installation;
fig. 7 is a schematic diagram of a liquid lens based camera imaging.
Detailed Description
It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.
The invention is described in further detail below with reference to the figures and specific examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Referring to fig. 1-6, a liquid lens based iris identification depth of field extension method according to an embodiment of the present invention is shown. Specifically, the method comprises the following steps: s0: initialization, S1: coarse scanning, S2: fine scan, S3: current change and S4 iris recognition; wherein, the hardware involved includes: a scene camera 1, a liquid lens 2 and an iris camera 3. The algorithm must be based on the hardware described or similar. The scene camera 1 can be a common camera, an industrial camera or similar image acquisition equipment, the scene camera in the embodiment is a 200-thousand RGB industrial camera, and is mainly used for monitoring and identifying whether an object to be identified exists in a scene, and a lens is selected according to the size of a scene range needing to be monitored; the liquid lens 2 is a core component for realizing the extension of the depth of field of iris recognition, and because the front end of the imaging lens of the iris camera 3 cannot realize the requirements of iris remote focusing and depth of field extension, the liquid lens 2 must be arranged between the image sensor and the imaging lens, and it needs to be specially explained that the imaging lenses all have fixed back intercept, after the liquid lens 2 is added, the distance from the back end surface of the lens to the target surface of the image sensor still needs to be ensured to be equal to the back intercept, otherwise, the depth of field extension cannot be realized, which is also a necessary precondition for implementing the method of the invention; the iris camera 3 is a key part for acquiring iris image data.
The initialization step S0 is to set and open the communication serial port parameters of the liquid lens 2 and to be in a state to be operated; initializing the scene camera 1, creating a data stream thread, and starting to work; initializing the iris camera 2 and keeping in a soft trigger state.
Fig. 2 is a detailed flow chart of the liquid lens coarse scanning S1 algorithm.
After the scene camera 1 detects the object to be recognized, it issues a start command to the liquid lens 2, the coarse scanning S1 is started, and then the image acquisition thread S11 and the quality evaluation thread S12 are simultaneously started. The image acquisition thread S11 sets the minimum current IminStarting searching by step length H, and judging whether each calculated current value is greater than maximum current ImaxNo, then adjust the liquid lens 2 to the present current IcThen a frame of iris image is obtained from the iris camera 3 through soft triggering,and storing the data into a shared data buffer. When the current value I of the scanningcIs greater than ImaxWhen so, the S11 thread exits.
After the quality evaluation thread S12 is started, continuously acquiring the iris image from the data buffer shared with the S11, and continuously and circularly inquiring if the buffer is empty; if the iris image is not empty, acquiring a frame of iris image, firstly reducing the dimension of the image to improve the face detection speed, then rapidly determining the approximate position and size of the face through random sampling, brightness clustering and screening, segmenting according to the positioned face parameters, finally performing first-order gradient calculation, weighting and averaging on the x direction and the y direction of the face image to obtain the image quality value of the iris image, storing the value in the image quality value buffer area, and establishing a corresponding relation with the current label; if the human face cannot be detected and positioned, the value is 0. Recording the processing quantity of the iris images by using the Nc, when the processing quantity is less than the quantity N of the known images needing to be acquired, acquiring the images from the buffer again, starting the next cycle, abandoning the previous frame, otherwise, ending the current cycle, acquiring all values from the quality evaluation value buffer at one time, searching the maximum image quality through iteration, and determining the optimal coarse current I through the corresponding relation0And exiting the thread.
Fig. 3 is a detailed flow chart of the liquid lens fine scan S2 algorithm.
After the coarse scanning S1 is finished, a coarse current I can be obtained0Then the range of the fine scan S2 is I0-H～I0+ H, the step length H is preset, and then the image acquisition thread S21 and the quality evaluation thread S22 are started synchronously. The image acquisition thread S21 sets the minimum current iminStarting searching by step length h, and judging whether each calculated current value is larger than maximum current imaxIf No, adjusting the liquid lens 2 to the current icAnd then acquiring a frame of iris image from the iris camera 3 by soft triggering and storing the iris image in a shared data buffer area. Current value i when scanningcGreater than imaxWhen so, the S21 thread exits.
After the quality evaluation thread S21 is started, continuously acquiring the iris image from the data buffer shared with the S21, and continuously and circularly inquiring if the buffer is empty; if the iris image is not empty, acquiring a frame of iris image, firstly reducing the dimension of the image to improve the face detection speed, then rapidly determining the approximate position and size of the face through random sampling, brightness clustering and screening, segmenting according to the positioned face parameters, finally performing first-order gradient calculation, weighting and averaging on the x direction and the y direction of the face image to obtain the image quality value of the iris image, storing the value in the image quality value buffer area, and establishing a corresponding relation with the current label; if the human face cannot be detected and positioned, the value is 0. Using said ncRecording the processing number of the iris images, and when the processing number is less than the number n of the known images needing to be acquired, acquiring the images from the buffer again, starting the next cycle, and abandoning the previous frame; otherwise, ending the current cycle, acquiring all values from the quality evaluation value buffer area at one time, searching the maximum image quality through iteration, and determining the optimal trickle current i through the corresponding relation0And exiting the thread. The current value i0I.e. the focus current of the target.
Fig. 4 is a detailed flow chart of liquid lens current adjustment.
First of all the focus current i0Firstly, converting the data into a 2-bit hexadecimal number through table lookup and normalization, and then solving a check bit to obtain a complete dynamic instruction; the command is sent to the liquid lens driver buffer area through the USB serial port line, after the command is received, the driver can change the current of liquid in the lens film cavity according to the set current, when the current changes, the shape of the liquid in the cavity can be changed, the shape of the film focused at two ends of the corresponding liquid can be changed (becomes convex or concave), further diopter change is caused, the focus of the target iris falls on the target surface of the image sensor of the iris camera 3, and focusing is completed.
Fig. 5 is a flowchart showing detailed operations of the coarse scanning S1, the fine scanning S2, and the current adjustment S3.
After focusing is completed, automatic soft triggering is carried out to acquire a frame of clear iris image from the iris camera 3, and then an iris recognition result is obtained through the iris recognition S4. The next cycle is started after the recognition is completed.
Fig. 6 is a schematic view showing the installation of the liquid lens 2 and the iris camera 3. The 3.1 is the image sensor of the iris camera 3, the 3.2 is the imaging lens of the iris camera 3, and the liquid lens 2 is arranged between the iris image sensor 3.1 and the iris imaging lens 3.2. If the standard back intercept of the iris imaging lens 3.2 is s and the thickness of the liquid lens 2 is l, s must be larger than l to ensure imaging.
Fig. 7 is a schematic diagram of the imaging principle of a camera based on a liquid lens.
It can be known from the figure that when the current of the liquid lens is 0mA, the liquid lens is only equivalent to a plane lens, the imaging of the light path is not influenced, and the distance d that the target iris can ensure clear imaging under the current is the original depth of field; when the distance is closer or farther, imaging blur occurs, although certain adaptation can be added to manual focusing of an imaging lens, the actual equipment is fixed at one focus in use, and the possibility of manual secondary focusing does not exist, so that the contrast in the image is under a certain fixed focus; when the current (positive or negative) is changed by the liquid lens driver, the voltage at two ends of the liquid is changed, the surface shape of the liquid surface is changed, the diopter of the lens film is changed, the imaging light path is changed, and the discrete target can be refocused on the target surface of the sensor. The minimum current or diopter (10dpt) corresponds to the nearest focus of the extended depth of field, the maximum current or diopter (-10dpt) corresponds to the farthest focus of the extended depth of field, and the distance difference between the two focuses is the extended depth of field D. The iris imaging depth of field extension method based on the liquid lens disclosed by the invention has the advantages of extremely large extension range, high speed, no image distortion and stable regulation and control.
The technical means not described in detail in the present application are known techniques.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (5)
1. An iris imaging depth of field extension method based on a liquid lens is characterized in that,
the method comprises the following steps:
step S0, initializing a scene camera (1), a liquid lens (2) and an iris camera (3);
step S1, determining the target coarse focusing current value I by the coarse scanning algorithm0；
Step S2, according to I0Value, determining the focus current value i of the target fine focus by a fine scanning algorithm0；
Step S3, adjusting the current of the liquid lens (2) to i0Changing diopter to a focus position, and acquiring one or more frames of clear iris images by using a trigger signal after reaching a stable state;
and step S4, detecting, positioning, segmenting, extracting and comparing the features of the iris image to obtain an identification result.
2. The liquid lens based iris imaging depth of field extension method of claim 1,
the rough scanning algorithm in step S1 is specifically as follows:
the maximum current of the liquid lens (2) is Max, the minimum current is Min, and the maximum current of coarse scanning is Imax(ImaxMax) and the minimum limiting current is Imin(Min≤Imin) If the coarse step is H, the number of scanning points N is equal to (I)max-Imin) /H, default from IminTo ImaxScanning, and if N is a decimal number, adding 1; the temporary stay time of the scanning point is T (T is less than or equal to 7 ms);
triggering to acquire a frame of iris image after T every time a scanning point passes through, and scanning to form a coarse sequence iris image; and utilizing a synchronous thread to evaluate the quality of the iris image in the coarse sequence, and finally determining the scanning point current value corresponding to the highest image quality, namely the coarse focusing current valueValue I0。
3. The liquid lens based iris imaging depth of field extension method of claim 1,
the fine scanning algorithm in step S2 is specifically as follows:
let the maximum current of the fine search be imax＝I0+ H (if Max < i)maxThen i isminMax), minimum limiting current imin＝IcH (if i)minMin, then iminMin, fine step length H (H < H), scanning point number n ═ imax-imin) H, default from iminTo imaxScanning, and if n is a decimal, rounding and adding 1; the temporary stay time of the scanning point is t (7ms is less than or equal to t);
after t passes through each scanning point, triggering to acquire a frame of iris image, and scanning to form a thin sequence iris image; using synchronous thread to evaluate the quality of iris image in thin sequence, and determining the scanning point current value corresponding to the highest image quality, that is, the thin focus current value i0。
4. The liquid lens-based iris imaging depth of field extension method according to claim 2 or 3,
the image quality evaluation algorithm adopted for quality evaluation of the iris image needs dimension reduction of the iris image, face positioning based on random sampling and pixel gray threshold, and finally gradient of the face image in x and y directions, namely an image quality value, is calculated.
5. The liquid lens-based iris imaging depth of field extension method according to claim 1, characterized in that the liquid lens (2) is installed between an iris image sensor (3.1) and an iris imaging lens (3.2), and the distance from the target surface of the sensor to the rear end surface of the imaging lens is equal to the rear lens intercept.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN114415364A (en) * | 2022-02-08 | 2022-04-29 | 南京邮电大学 | Time division based multi-focus imaging system |
| EP4403974A4 (en) * | 2021-09-14 | 2024-10-16 | Nec Corp | Information processing system, information processing device, information processing method, and recording medium |
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