WO2001067746A1

WO2001067746A1 - Digital camera with autofocus

Info

Publication number: WO2001067746A1
Application number: PCT/GB2001/000896
Authority: WO
Inventors: Paul Roy Charles Stanborough; Andrew Michael Holland
Original assignee: Idiris Limited
Priority date: 2000-03-04
Filing date: 2001-03-02
Publication date: 2001-09-13
Also published as: GB0005194D0; GB0105228D0; GB2361826A; AU2001240776A1

Abstract

An imaging apparatus comprises an adaptive focus optical system, a digital image acquisition system, and an image analyser, and is arranged to set the focus of the optical system at a predetermined distance, acquire an image and pass the image to the image analyser, filter the image to identify an area containing an edge, analyse the sharpness of edge detail in that area, repeat a number of times using different focal distances, and identify a focal distance which corresponds to a peak image sharpness. A series of scans are therefore taken, one of which should correspond to an ideal distance or close thereto. Each scan is analysed for sharpness and the best can be selected. The adaptive focus optical system can include a reflective surface formed on a first conductive member spaced from a second conductive member and a means for applying a voltage across the two conductive members thereby to deform the first conductive member. The first conductive member is suitably silicon-based, such as silicon nitride, preferably with an aluminium surface layer. The second conductive member can be any suitable material, such as a surface of a printed circuit board. The mirror is preferably mounted at an angle to the horizontal (when in use) so as to cast an image towards the digital image acquisition system along an axis which is not directed towards the user.

Description

DIGITAL CAMERA WITH AUTOFOCUS

FIELD OF THE INVENTION

The present invention relates to an imaging system. It is particularly (but not exclusively) useful for use in acquiring an image for use in biometric analysis. An example of such an image is the iris pattern of a person for identification purposes or for use in security applications such as granting or refusing access to a computer network, and therefore the invention will be described in the context of iris imaging. However, the optical system of this invention may well be of value in other contexts.

BACKGROUND OF THE INVENTION

The use of biometric analysis for identifying individuals dates from the identification of fingerprints in order to identify suspects in criminal cases. Since then, it has been proposed to use automated fingerprint recognition for other purposes, such as governing access to computer networks. However, there are difficulties in doing so accurately which primary relate to the image acquisition system. It has therefore been proposed to rely upon an image of the user's iris instead of fingerprint. The pattern of an iris is as distinctive, if not more so, than a fingerprint and trials based on iris recognition show an excellent level of accuracy. Iris recognition systems depend on the fact that the human iris contains a pattern which is unique to that individual and which does not substantially vary over time. However, iris recognition relies on the ability to obtain a clear and analysable image of the iris pattern concerned.

SUMMARY OF THE INVENTION

A principal difficulty is in focussing the image. A image which is out of focus will be inadequate for iris recognition. Existing iris recognition systems require the user to vary the actual distance between the device and the eye until the fixed focal distance of the device matches the actual distance. The image is analysed continuously and a match will occur at that time. This is inconvenient for the user.

It would be preferred if the recognition system could focus itself. However, this. has not been practical to date since auto-focus systems typically operate over greater distances than those required to image the iris. Range-finding systems are also unsuitable since the topography around the eye can be of a significant height as compared to the distance to be identified. Spectacles are also apt to confuse rangefinders.

The present invention therefore provides an imaging apparatus comprising:

-an adaptive focus optical system

-a digital image acquisition system

-an image analyser; the apparatus being arranged to:

(a) set the focus of the optical system at a predetermined distance;

(b) acquire an image and pass the image to the image analyser;

(c) filter the image to identify an area containing an edge;

(d) analyse the sharpness of edge detail in that area; (e) repeat at least steps (a) (b) and (d) a number of times using different focal distances;

(f) identify a focal distance which corresponds to a peak image sharpness.

According to the present invention, a series of scans are therefore taken, one of which should correspond to an ideal distance or close thereto. Each scan is analysed for sharpness and the best can be selected. It is preferred that each individual image is filtered to identify edge detail within the image, and sharpness of the edge analysed. However, if the individual images are captured very quickly, it is possible that the subject will not move significantly, allowing the same region of each image to be analysed for sharpness and eliminating a process step for the second and subsequent images.

As an alternative to retrieving one of the original images, the apparatus can set the optical system to the focal distance determined as ideal and capture a final image. Interpolation can be used to calculate an ideal distance.

The adaptive optical system can be a collection of lenses and/or mirrors which are moveable relative to each other so as to vary the focal length. Preferably, however, the optical system includes an active component whose focal length is variable. A suitable element is a flexible mirror whose curvature is variable.

The present invention also relates to a computer monitor or computer base unit incorporating such an imaging apparatus.

An example of a suitable mirror is shown in "Quick focusing of imaging optics using micromachined adaptive mirrors", Gleb Vdovin, Optics Communications, 140 (1 997) 1 87-1 90 published on 1 August 1 997. This describes mirrors machined from bulk silicon with a reflective surface formed by a circular 0.5-1 μm-thick silicon-rich nitride membrane coated with a reflective aluminium layer. This is mounted 75|Jm over a metallized PCB. The radius of curvature of the membrane can be controlled between infinity and 0.5m by applying a variable high voltage between the membrane and the PCB. A suitable level is between 0 and 1 60V .

Accordingly, it is preferred that the adaptive focus optical system includes a reflective surface formed on a first conductive member spaced from a second conductive member and a means for applying a voltage across the two conductive members thereby to deform the first conductive member. The first conductive member is suitably silicon-based, such as silicon nitride, preferably with an aluminium surface layer. The second conductive member can be any suitable material, such as a surface of a printed circuit board.

The mirror is preferably mounted at an angle to the horizontal (when in use) so as to cast an image towards the digital image acquisition system along an axis which is not directed towards the user. It is then preferable for the mirror to be oval, to eliminate astigmatism. A suitable angle is 45 ° .

According to the invention, the system can capture a sharp image in real time without requiring any user intervention. This enables iris recognition systems to identify a person swiftly, accurately and transparently.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example with reference to the accompanying figures, in which;

Figures 1 and 2 illustrate the variation in sharpness with focus distance;

Figure 3 shows a method of interpolation; Figure 4 is a schematic illustration of an example of the invention;

Figure 5 is a block diagram of a closed loop micro-controller design;

Figures 6 and 7 are illustrations of a suitable encapsulation of the example, viewed from the front and side respectively;

Figure 8 shows a closed loop micro-controller design; and

Figure 9 shows a long range example of the invention.

DETAILED DESCRIPTION OF THE EXAMPLES

Figure 1 illustrates a means of selecting an in-focus image from a collection of images of the same view at varying focal distances. For ease of illustration, an image of a simple tone change will be considered, shown in figure 1 (x). Figures 1 (a) to 1 (e) are images of that tone change at increasing focal lengths over a range which spans the ideal focal length at figure 1 (c). Each of figures 1 (a) to 1 (e) show the captured image and a graph of the pixel darkness along the line (L) marked. On each graph is marked the tangent of the curve at its steepest point.

It will be apparent that as the focal length approaches the ideal, the tangent and hence the rate of change of pixel darkness increases to a maximum, then falls off as the ideal focal length is passed. This is shown in figure 2 where peak rate of change of pixel darkness is plotted against focal length. From this graph it is evident that the focal length corresponding to figure 1 (c) offers peak sharpness.

Accordingly it is possible to determine the optimum focal length from a series of captured images. Once that focal length is chosen, the closest image can be selected. Alternatively, the image acquisition system can be set to that focal length and a fresh image captured. This has the advantage that the intermediate images need not be stored, but must be done swiftly before the subject moves. It may be possible (according to the application) to achieve this speed if images are analysed for rate of change of pixel darkness in real time while the next intermediate image is being captured which is synchronised to the incoming video. This latter possibility also allows for determination of the ideal focal length by interpolation.

Interpolation is shown in figure 3, an enlarged portion of a graph corresponding to figure 2. The two points P1 and P2 are shown between which the gradient of the graph changes. The estimated ideal focal length F corresponds to a point along the line joining the two points. A point can be chosen positioned along the line such that ratio between the distances P1 -F and F-P2 equals the ratio of the gradients at P1 and P2. This will not (in general) be the precise peak of the curve, but will be closer than either P1 or P2 and in most applications will give an image which is indistinguishable given the resolution of the imaging system.

Figure 4 shows an adaptive optical system for use in the present invention. A partially silvered mirror 1 0 allows light rays 1 2 from the user to enter without showing the internal parts and also allows the user to line up with the device by looking at a reflected image of their iris. Within the device, the light 1 2 is reflected by an adaptive mirror 1 4 mounted at 45 ° to the vertical. To eliminate astigmatic errors, the mirror is elliptical such that it appears circular when viewed at 45 ° . The incoming light 1 2 then travels downward to a CCD sensor 1 6 mounted on a PCB 1 8 which contains the analysis circuitry for the image and driver circuitry for the adaptive mirror 1 4.

Figure 5 shows a schematic illustration of the control circuitry in a closed loop form. The CCD camera 20 acquires an image which will in practice be a continuous moving image. This is made available at an output 22 of the device and can be displayed, for example for feedback to the user to assist in positioning. The image is digitised by an analogue to digital converter 24 and fed to a frame grabber 26 which extracts individual frames as digital images. These are stored as necessary in one of two available memory banks 28, 30 and fed as required to a micro controller 32 from where they can be stored in a working memory 34. The micro controller 32 can then command the mirror driver 36 to set the mirror 38 at a different focal length and the process can repeat. A press-button switch 40 is available for the user to initiate the scanning process and a lamp 42 can illuminate the user. The micro controller 32 will scan the switch 40 periodically and when a start signal is detected will illuminate the lamp 42 and begin the process.

A USB interface 44 and a serial (RS232) interface 46 are provided to allow the micro controller 32 to pass data to an external computer (not shown) . It is intended that the focal length selection process will be carried out by the micro controller 32 and a chosen image will be passed to the external computer which can then compare the acquired iris image or an iris code derived therefrom with stored images or codes held in a database and corresponding to users with security access and decide whether to permit the user to proceed further.

Figures 6 and 7 show a suitable encapsulation for the device, from the front and side respectively. A handheld form is illustrated although it would also be beneficial to attach the device to a suitably positioned support, such as the side of a computer monitor. The partially silvered mirror 1 0 is placed in the centre of the device and a "ready" LED 50 is placed to one side to confirm activity. The lamp 42 is placed below the mirror 10. The start switch 40 is accessible. A cable 52 leads to the computer under control.

In a further development, the encapsulation could be dispensed with and the device integrated with the monitor or another suitable part of the computer such as the base unit in a tower configuration. The latter possibility would give rise to exceptional security as the electronics shown in figure 4 could be placed on the motherboard and hence render improper access to the system still more difficult. Figure 8 shows an open loop example of the present invention. A CCD or CMOS video camera 1 00 is located in the device and its video out connections 102, 104 are passed direct to a computer for analysis. A micro controller is provided with power form a 5volt power supply 108 and receives incoming lamp 1 1 0 and start 1 1 2 signals. An illuminating lamp 1 14 and a driver unit 1 1 6 powering an adaptive mirror 1 1 8 are connected to outputs of the micro controller 1 06. A user-operable start switch 1 20 is connected to an input.

When the start switch 1 20 is operated, or when a start signal is received via the input 1 1 2, the cycle begins. If this is by way of the user activating the start switch 1 20, a signal is sent via switch output 1 22 to inform the connected computer. The lamp 1 1 4 is illuminated. The micro controller 1 06 begins a scan at a first focal length, which is achieved by commanding the mirror driver 1 1 6 to power the mirror 1 1 8 to that length. The mirror driver is synchronised with the video frame start and is altered from a flat to a fully deformed mirror within a frame time. After holding at that focal length for a preset time, the micro controller adopts a different length, and so on. At all times the image from the camera 100 is available. Individual frames from this source will be a successive focal distances and the best can be selected as described previously.

Figure 9 shows a longer range form of the invention. Several adaptive mirrors 1 22, 1 24 and 1 26 as described above are placed in the optical path prior to the imager 1 28 and controlled to provide greater focussing capability. This method of operation could be used for access control where 'in focus' grabbing of images needs to commence at a greater distance. Each mirror is adapted to provide accurate variation of focal length over a particular range of distances. To obtain a set distance, the other mirrors are set to a preset focal length which maps the range of the remaining active mirror to that range. That mirror can then be controlled to provide accurate and optically reliable focus throughout that range. As an example, the three mirrors of figure 9 could be set to zones of 1 20mm each covering a total of 360mm, sufficient to enable use of the invention in an access control field.

It will be appreciated that many variations to the above-described embodiment are possible without departing from the present invention. For example, other adaptive optics could be employed although at present it is thought that a silicon adaptive mirror is most suitable. Equally, the physical format of the device could be altered according to the particular level of physical security needed. The scanning distances could be chosen to cover the range of distances likely in a particular location and the interval chosen to give an adequate number of images within a reasonable time. Other possible variations will be apparent to the skilled person.

Claims

1 . An imaging apparatus comprising:

-an adaptive focus optical system -a digital image acquisition system -an image analyser; the apparatus being arranged to:

(a) set the focus of the optical system at a predetermined distance;

(b) acquire an image and pass the image to the image analyser;

(c) filter the image to identify an area containing an edge;

(d) analyse the sharpness of edge detail in that area;

(e) repeat at least steps (a) (b) and (d) a number of times using different focal distances;

(f) identify a focal distance which corresponds to a peak image sharpness.

2. An imaging apparatus according to claim 1 in which each individual image is filtered to identify edge detail within the image, and sharpness of the edge analysed.

3. An imaging apparatus according to claim 1 in which the same region of each image is analysed for sharpness.

4. An imaging apparatus according to any one of the preceding claims in which the apparatus is adapted to set the optical system to the focal distance which corresponds to a peak image sharpness and capture a final image.

5. An imaging apparatus according to claim 4 in which an interpolation method is used to calculate the focal distance.

6. An imaging apparatus according to any one of the preceding claims in which the adaptive focus optical system is a collection of lenses and/or mirrors which are moveable relative to each other so as to vary the focal length.

7. An imaging apparatus according to any one of claims 1 to 5 in which the adaptive focus optical system includes an active component whose focal length is variable.

8. An imaging apparatus according to claim 7 in which the active component is a flexible mirror whose curvature is variable.

9. An imaging apparatus according to claim 8 in which the mirror is machined from bulk silicon.

1 0. An imaging apparatus according to claim 9 in which the reflective surface is formed by a circular silicon-rich nitride membrane coated with a reflective aluminium layer.

1 1 . An imaging apparatus according to claim 1 0 in which the nitride membrane is between 0.5 and 1 m thick.

1 2. An imaging apparatus according to any one of claims 9 to 1 1 in which the reflective surface is mounted over a planar conductive member.

1 3. An imaging apparatus according to any one of claims 1 to 5 in which the adaptive focus optical system includes a reflective surface formed on a first conductive member spaced from a second conductive member and a means for applying a voltage across the two conductive members thereby to deform the first conductive member.

14. An imaging apparatus according to claim 1 3 in which the first conductive member is silicon-based.

1 5. An imaging apparatus according to claim 14 in which the first conductive member is silicon nitride.

1 6. An imaging apparatus according to claim 1 5 in which the silicon nitride has an aluminium surface layer.

1 7. An imaging apparatus according to any one of claims 1 3 to 1 6 in which the second conductive member is the surface of a printed circuit board.

1 8. An imaging apparatus according to any one of the preceding claims in which the mirror is mounted at an angle to the horizontal (when in use) so as to cast an image towards the digital image acquisition system along an axis which is not directed towards the user.

1 9. An imaging apparatus according to claim 1 8 in which the mirror is non- circular.

20. An imaging apparatus according to claiml 9 in which the mirror is oval.

21 . An imaging apparatus according to any one of claims 1 8 to 20 in which the mirror lies at an angle of 45 ° .

22. A computer monitor or computer base unit incorporating an imaging apparatus according to any one of the preceding claims.

23. An imaging apparatus substantially as any one described herein with reference to and/or as illustrated in the accompanying figures.