Abstract:
An iris camera for capturing an image of an iris of a user is described. The iris camera comprises: a lens; an image sensor for capturing an image produced by the lens; a processor for analyzing the degree of focus of successive images produced by the sensor and for controlling a lens position adjustment motor to move the lens to a position where a subsequent captured image potentially has an increased degree of focus; and a proximity sensor for providing an initial measurement of the distance between the lens and the user. The processor is arranged to user the initial distance measurement obtained by the proximity sensor as a starting point for capturing images of the iris to effect a fine-focus adjustment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the right of priority under 35 U.S.C. §119 to GB application serial no. 1117350.7, filed Oct. 7, 2011, which is incorporated by reference in its entirety. 
     FIELD 
     This invention relates to systems, apparatus and methods for capturing iris images. The approach provides convenience of use and enables iris capture for a wide range of users. 
     BACKGROUND 
     The human iris is a muscle for controlling pupil dilation and consequently how much light enters the eye for image formation on the retina. This muscle has such rich variations in pigmentation patterns across the global population that no two people ever have the same patterns. Even twins and eyes of the same individual are greatly different and this provides an opportunity for powerful identity checking technology. It is a result of the vast number of degrees of freedom inherent in iris patterns compared with the size of the global population, or even compared with the number of humans that have ever or will ever exist, that iris recognition offers such a powerful opportunity for identity verification. Iris recognition technology is much better at differentiating between individuals than traditional methods such as presenting original photo documentation, signing, using chip and PIN and fingerprint matching. 
     Systems based on iris recognition are applicable in a range of applications including commercial and official contexts where identity checks are important for commercial, legal, security or other reasons. For example, certain airports currently employ the use of iris cameras for verifying the identity of individuals crossing national boarders. 
     Iris cameras for use in iris recognition technology must obtain images that can be checked against reference images forming part of a user profile for that individual. The iris camera is therefore one element of the whole iris recognition system, where other elements include a database storing reference images, or at least data derived from such images, as part of a remote user profile for registered users of iris recognition. If the user profile contains reference data derived from an initial reference image, the reference data acts like a barcode uniquely identifying that user. This saves on memory required to store the material against which future identity checks are made. 
     In order to register for iris recognition a user must therefore have reference images of their irises taken for their user profile. An iris camera is clearly required for this registration process, as well as for subsequent instances when the technology is being used by the user to access various rights or authorise a transaction. However, the camera used in the registration step is not necessarily the same camera as the one used in subsequent instances because of course a user may register, for example, at a bank and subsequently require ATM services elsewhere gaining access by iris recognition. 
     In order to be practically useful, iris cameras must be able to perform well with a high volume of users. This means firstly that the system must be straightforward and convenient to use for each individual using the camera. One of the most important practical considerations in terms of convenience of use is to make it easy for the user to position their head correctly for the camera to acquire images of both irises. Different users will naturally adopt varying positions in the forwards-backwards dimension and the camera system must be able to cope with this, rapidly acquire both irises and be ready to repeat the process for the next user. The depth of the volume in which each of a user&#39;s eyes can be positioned for successful iris capture —i.e. the depth of the capture box—should therefore be reasonably generous. 
     The requirement that the camera must cope with a high volume of users means, secondly, that no classes of iris types or positions should present problems to the camera system. One aspect of this issue is related to iris positions based on the distance between a user&#39;s pupils. Certain racial heritages are associated with particularly wide or particularly narrow inter-pupillary distances compared with the majority in the global population. The iris camera&#39;s field of view should be adequate to accommodate the full range of inter-pupillary distances with which it might be presented. 
     In order to provide a deep capture box and a wide field of view, some known methods use iris cameras with high curvature lenses associated with a high depth of field. The high depth of field provides a deep capture box and the high curvature lens surface captures incident light at wide angles. However, in order to mitigate defocusing effects near the edges of the lens which are most pronounced in high curvature lenses, diaphragms are placed in front of the lens the reduce the aperture and effectively refocus the resultant image. This reduces the overall amount of light that can pass through the lens and form an image on the other side. Consequently, image resolution is poor and typically the optical resolution of the image falls short of the equivalent resolution—in pixels per unit area—of the camera&#39;s image sensor. In this way, the optics of many known iris cameras degrade the quality of the image which could be obtained by the image sensor. 
     Other known methods use a lower curvature lens so that defocusing effects at the lens periphery are less and a diaphragm with a larger aperture can be used to let more light in. This improves image resolution. However, the shallow lens curvature tends to restrict field of view, which in turn restricts the width of the capture box, making it inconvenient for users and reducing the range of inter-pupillary distances that can be accommodated. Furthermore, the shallow curvature lens is associated with a small depth of field (shallow focus) which does not extend across the full range of the capture box. In order to produce in-focus images of irises in the capture box, the lens itself must therefore be moved to suit the position of the user&#39;s eyes. Automatic fine-focus systems are typically employed having motors controlled by various electronic feedback loops, which move the lens so that the in-focus region corresponds to the position of the irises. 
     The feedback loops form part of an iterative autofocus system in which successive captured images are digitally analysed to assess their level of focus. Image analysis software provides a measure of focus for successive images, successive measurements of improving or worsening focus are used to make decisions as to where the lens should be moved next. The images gradually approach focus and once an acceptable level of focus has been achieved iris images can be captured. 
     However, this process is time consuming If the lens starts far away from the in-focus position, the iterative procedure of fine-focus will contribute significantly to the total time from start to finish for a user to position him or herself at the device and for dual iris acquisition to be completed. This time-consuming feature is clearly disadvantageous where the camera is to be used in a high throughput application, for example scanning people&#39;s irises as a security measure at an airport, for example. 
     The present invention seeks to address some or all of the above issues. 
     SUMMARY 
     In a first aspect, the present invention provides an iris camera for capturing an image of an iris of a user, the iris camera comprising: a lens; an image sensor for capturing an image produced by the lens; a processor for analysing the degree of focus of successive images produced by the sensor and for controlling a lens position adjustment motor to move the lens to a position where a subsequent captured image potentially has an increased degree of focus ; and a proximity sensor for providing an initial measurement of the distance between the lens and the user; wherein the processor is arranged to user the initial distance measurement obtained by the proximity sensor as a starting point for capturing images of the iris to effect a fine-focus adjustment. 
     The key advantage of this arrangement is that the focussing of each image to be captured by the camera is faster as the proximity sensor provides a very good estimate of where the iris is located and this can be used as the starting point of the fine-focus adjustment of the lens. 
     The proximity sensor may be adapted to output an electromagnetic pulse, preferably towards a user&#39;s forehead, and measure the distance to the object by measuring how long it takes for the pulse to be reflected back. This is a very simple, fast and non-intrusive way of determining the proximity of the user&#39;s forehead (and hence the likely position slightly further away of the user&#39;s iris). 
     The iris camera is preferably adapted to capture at least one iris image for translation into an identity check template for a biometric matching engine in under five seconds. 
     The capture box of the iris camera is preferably 100 mm deep, and advantageously accommodates user inter-pupillary distances of between 49 mm and 79 mm. The camera field of view is preferably about 11°. 
     Advantageously, the lens of the iris camera has a resolution of 22 lines per mm for 98% of the lens surface, an anti-reflective coating—preferably in the wavelength range 700 mm to 900 mm, and three glass elements (i.e. the lens is preferably a triplet lens). Preferably, the iris camera is optically matched to at least a 1.3 Mpixel image sensor, and adapted to capture an image between 200 and 300 pixels across the iris (according to the ISO standards). 
     According to a second aspect of the present invention, there is provided a method of capturing an image of an iris of a user using an iris camera, the method comprising: capturing an image produced by a lens of the iris camera; analysing the focus level of successive captured images; controlling a lens position adjustment motor to move the lens to a position where a subsequent captured image potentially has an increased focus level; obtaining an initial proximity measurement of the distance between the lens and the user; and using the initial proximity measurement as a starting point for capturing images of the iris and moving the lens to effect a fine-focus adjustment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic block diagram showing an iris camera according to an embodiment of the invention, the iris camera being in communication with a remote biometric matching engine via a server; 
         FIG. 2  is a schematic block diagram showing the server of  FIG. 1  in greater detail; 
         FIG. 3  is a schematic block diagram showing the remote biometric matching engine of  FIG. 1  in greater detail; 
         FIG. 4  is a schematic sectional view of the lens system of the iris camera of  FIG. 1 ; 
         FIG. 5  is a table showing quantitative aspects of the optics of the iris camera of  FIG. 1 ; 
         FIG. 6  is flowchart showing a focussing procedure of the iris camera of  FIG. 1 ; and 
         FIG. 7  is a flowchart showing an image acquisition procedure of the iris camera of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The elements of an iris camera  122  embodying the present invention are shown in  FIG. 1 , together with a server  142  with which, in use, the camera  122  communicates, and a remote biometric matching engine  162  in communication with the server  142 . The iris camera  122  supplies quality assured iris images to the server  142  for use in biometric identification checks that are run by checking against reference data stored in the matching engine  162 . 
     Referring now to  FIG. 2 , the server  142  is shown in greater detail. The server  142  may be a client personal computer (PC), a client computer network or other computing infrastructure of or for the client. Communication with the server  142  may be indirect via a host of the iris camera and/or the client, including through a host switch which enables communication between a camera domain  120  and a client domain  140 . The client domain  140  may communicate with a network of iris cameras through such a switch. A calling application  144  for requesting sets of iris images may be provided in the client software. Applications  146  for extracting unique codes from incoming images may also be provided here, so that data files smaller than whole images may be sent to the matching engine  162  for faster matching. In this regard a match request application  148  is provided in the server for constructing a matching request and communicating with the matching engine  162 . 
     The client is any organisation requiring that the identity of individuals be verified before certain permissions or access can be granted or certain transactions can be carried out. Examples include banks, point of sale (POS) terminals, auto-telling machine (ATM) terminals, high security departments or facilities and national border controls. A range of clients may be served by a single matching domain  160 . 
     Referring now to  FIG. 3 , the biometric matching engine  162  comprises computing infrastructure housing a database  164  containing data files  166  relating to users for the purpose of verifying their identity in biometric security checks. The matching domain  160  may be remote from both the camera and client domains  120 ,  140  and may communicate with the client domain  140  either directly or via one or more hosts, including via a switch so that it may communicate with a network of clients. 
     Some aspects of the present embodiment relate to the elements inside the camera domain  120 . More specifically, as shown in  FIG. 1 , the camera  122  comprises left and right lens systems  123 ,  131 , one for each iris, and a central processing unit (CPU)  136  which may, for example, be an embedded chip or a PC. In the embodiment shown, each lens system  123 ,  131  comprises a triplet lens (described further below) with associated automatic fine-focus capability and an image sensor  126 ,  132 . The image sensors  126 ,  132  digitise images formed by the camera optics and provide them to the CPU  136  for processing, for which various applications  138 ,  139  are used to analyse the images and send feedback instructions for making adjustments. An image analysis application  138  for making adjustments as part of an automatic fine-focus system is provided, as is an iris tracking application  139  for performing iris-tracking in which adjustments are made to keep the pupil in-focus and centred as the user invariably moves their head during the image capture process. 
     In another embodiment of the present invention, there is a single image sensor in the image plane that spans both the left and right images, and the images are only separated out at a later stage using image analysis software. 
     In the embodiment of  FIG. 1 , the image sensors have a resolution of 1.3 M pixels, and the optics of the iris camera are matched to the resolution of the image sensors. 
     The lens system  123 ,  131  is shown in more detail in  FIG. 4 . Each of the right and left lenses  123 ,  131  is a triplet lens having three glass elements  400   a ,  400   b  and  400   c , each glass element being manufactured from high grade glass, and having an anti-reflective coating  401   a ,  401   b ,  401   c  in the optimum wavelength range for iris recognition (around 700 nm to 900 nm). The lens has an optical resolution of 22 lines per mm over 98% of its surface. This high illumination gives rise to an image of high optical resolution which means that the image incident on the image sensor is information rich. There is sufficient information in the image to make proper use of all the pixels of the image sensor, and in this way the optics of the camera are matched to the full resolution capability of the image sensor. This means that each pixel of the image sensor conveys information about the image which is useful in the matching process. 
     Production of a high-resolution image increases the reliability in general of the iris camera for verifying identity. This embodiment is also adapted for high applicability across the range of iris colourings in the population. Different colourings require different frequencies of illumination for the pigmentation features of the iris to be detected. For example, darker eyes absorb more higher frequencies than blue eyes, and if only these frequencies are used the dark iris cannot be localised. To illuminate the iris for correct feature extraction, illumination angular spread and power are required to be matched to both the optical path (lens) and the dynamic sensitivity of the image sensor. All these criteria need to be correct so that the irises from humans with different ethnic background are not precluded from using their iris biometric trait to identify themselves. The differences in the pigmentation and melanin of the iris structure from a light to a dark-coloured iris require different wavelengths of NIR (Near InfraRed) illumination to allow correct feature extraction. 
     The glass elements  400   a ,  400   b ,  400   c  are supported by a lens mount assembly  440  and a spacer  460 . A motor  420  controls the position of the glass elements as part of the camera&#39;s automatic fine-focus system. 
     In the embodiment of  FIG. 1 , the capture boxes of the left and right lens systems each have a field of depth of 100 mm. This provides a generous region in which a user can be positioned so that images of his or her irises can be captured. As a result, there is no problem if different users adopt different positions within the generous capture box, and slight changes in a person&#39;s position during the process will not matter. Images acquired from anywhere in the capture box of this embodiment satisfy the ISO standard requirement that an iris image must be between 200 and 300 pixels across (see  FIG. 5 ). 
     The capture boxes are also sufficiently wide to cope with the full range of interpupillary distances in the global population. The minimum and maximum interpupillary distances, 49 mm and 79 mm respectively, are indicated in  FIG. 1 . As shown, the irises of an individual at a nominal distance of 300 mm from the lens aperture fit inside the capture boxes, whether they have the minimum, the maximum or an intermediate interpupillary distance (IPD). This is made possible by the field of view of 11.06° of each of the lens systems of the embodiment. 
     The proximity sensor device  129  shown in  FIG. 1  is used at the beginning of the process of iris capture to measure the distance to a user&#39;s forehead. This provides an approximate measure of the distance from the lenses to the irises and is used to put the camera in approximate focus. This gives the fine-focus system a head start so that fewer iterations of the fine-focus protocol (method)—and consequently less time—are required before complete focus is achieved. 
     Following this approach, a method for rapidly focussing an iris camera will now be described with reference to  FIG. 6 . 
     An approximation of the distance to the irises is measured at Step  600  using the proximity sensor which outputs an electromagnetic pulse and measures how long it takes for the pulse to be reflected back from the user&#39;s forehead. The proximity sensor comprises a pulse generator for generating the output pulse and a sensor which detects the return signal. The distance measured is only an approximation of the distance from the lenses to the irises because the iris and forehead are not at exactly the same distance, and variations in user cranial size, as well as scarves and other headgear being worn make this measurement a less accurate approximation. 
     Based on this initial distance measurement, the lens is moved at Step  602  to a position which provides a first approximation to focus. This rough focus is then tested by acquiring at Step  604  an image and determining at Step  606  the level of focus of the image. To further improve focus, the first approximation to focus and the improved focus are compared together with their associated lens positions and lens is accordingly moved at Step  608  to a third position. The new focus level is determined (step  610 ) and, if it is still out of focus, the camera cycles through further iterations, moving the lens based on focus information learnt from previous cycles and re-measuring focus at Steps  614 ,  616 ,  610 ). Each time the CPU  136  controlling the process using tries to determine the degree of blur in the image (sharpness of the image and inversely proportional to focus) and looking at the history of previous positions and levels of blur determine where to move the lens to best minimise blur (and hence get the image into focus). 
     Once focus has been achieved, a further check is performed before a final set of images is captured and output. The focussed image is analysed at Step  618  for saturation level and if it is above a pre-determined threshold, saturation is decreased at Step  620  before re-checking focus at Steps  610 ,  612 . This ensures that the in-focus state of the image is not merely a result of high saturation of the image, which can sometimes lead to a false positive for focus. If the image is found to be in focus and at the same time saturation is not too high (below the pre-set threshold level), the camera zooms in to the iris and acquires and stores at Step  622  a set of images to be output to the server. 
     Referring to  FIG. 7 , before zooming, the image analysis software differentiates the intensity profile of the image in order to locate the edges of the iris and pupil. The pupil centre can then be located at Step  720  and the camera optically zooms in at Step  730  on the iris with the iris centred and tracked. To complete the session, and with the pupil centre still tracked, a video graphics array (VGA) resolution image is streamed to acquire at Step  740  a series of focussed, centred images. The process occurs for both eyes simultaneously and the total time required is between 1 and 5 seconds to acquire a set of quality checked images for coding into iris templates for presentation to a biometric matching engine to be stored as reference identification data for that particular user. Subsequent identity checks against this stored reference data, from iris capture to delivery of a result, take around  1  second. 
     Having specifically described embodiments of the present in detail, it is to be appreciated that the above described embodiments are exemplary only and that modifications will occur to those skilled in the art without departure from the spirit and scope of the present invention. For example, even though a specific pulsed proximity sensor has been described any form of proximity sensor which give and accurate reading relatively quickly could be used.