GB-A-2174831 describes a method and device for analysing a skin pattern from a finger, toe or palm in order to verify an input skin pattern with a reference. The method described is very successful but depends on the presented skin patterns causing a well defined image to be produced. ^' In practice, a small percentage of the population have relatively dry fingers which do not generate satisfactory images for subsequent authentication or verification. Additionally it is known that there is normally a period following presentation during which the image improves. This applies to the whole population and the improvement period is affected by environmental conditions, especially temperature and humidity.

One method which has been proposed to deal with this is described in EP-A-0169496 in which a portion of a presented fingerprint is reviewed and an image of the finger is only taken once the intensity of the reviewed portion reaches a predetermined threshold. This proposal recognises the fact that the image produced by a dry finger will improve with time but the method described in fairly unsophisticated and is unable to cope with a number of practical situations. For example, in a practical device, a person will provide some indication

to the device of his alleged identity to enable a reference skin pattern to be obtained. In some cases following that initial presentation, the person will not, in fact, present his own skin pattern for comparison and in the method described in EP-A-0169496 this would be construed as being indicative of a dry finger and could produce a false result.

In accordance with one aspect of the present invention, a method of monitoring the suitability of an image of a skin pattern for use in a subsequent verification or identification process comprises a) irradiating a region in which a skin pattern is expected; b) detecting the intensity of radiation from the irradiated region; c) comparing the detected intensity with a first threshold; d) if the detected intensity is greater than the first threshold, detecting the intensity after a predetermined time interval and comparing the second detected intensity with a second threshold defining an intensity greater than that of the first threshold; and, e) generating at least a preliminary acceptance signal if the second intensity exceeds the second threshold.

In accordance with a second aspect of the present invention, apparatus for monitoring the suitability of an image of a skin pattern for use in a subsequent verification or identification process comprises a skin pattern imaging means including a radiation source for irradiating a region in which a skin pattern is expected; and processing means for carrying out the steps of a method according to the first aspect of the invention.

The use of at least two thresholds in accordance with the invention leads to a much more sophisticated and

practical method of monitoring the suitability of an image of a skin pattern. In particular, not only can the problems of dry fingers be accommodated but also the problems of non-presentation of a skin pattern and the wiping of a finger over a support. Thus, step c carries out a preliminary step of detecting whether a skin pattern has been presented and only if this is the case are the subsequent steps carried out.

Typically, the first threshold can be equal to the sum of a background intensity level (which may be locally detected) and a noise level. The second threshold, however, will be greater than the first threshold since even though a skin pattern may be detected in step c, the image may not be suitable for subsequent processing where a higher intensity level is desirable.

Preferably, if in step d the second intensity is found to be less than the second threshold, step d is repeated one or more times. This enables any image to improve to an approximately constant standard before it is captured for further processing, although in general a limit will be placed on the number of repeats of step d.

In some cases, if the detected intensity _; in step d does not exceed the second threshold, the image of the skin pattern will be found not to be suitable for subsequent processing. However, in certain verification processes (where the level of accuracy can be reduced) , if at the completion of step d the second intensity is less than the second threshold, a signal may be generated indicating the presence of a dry skin pattern. In such a situation the skin pattern itself may be subsequently processed although account will be taken of the fact that it has resulted from an image which never exceeded the second threshold.

Preferably, the method further comprises, subsequent to step e,

f) detecting the intensity of radiation from the irradiated region after a predetermined interval (e.g. 500ms) ; and, g) comparing the third detected intensity with a third threshold less than the second threshold and generating an acceptance signal if the third detected intensity is greater than the third threshold.

This is particularly useful in enabling the method to ignore false images caused by a person wiping the presentation surface in an attempt to clean it prior to presenting a correct skin pattern. If the third threshold is less then the second threshold then step g detects the problem of transient false presentation as the image intensity usually falls with time in such cases.

In the preferred method, the method further comprises after step a, the step of obtaining a background intensity from the irradiated region, comparing the background intensity with the background threshold, and if the background intensity is greater than the background threshold, immediately generating the preliminary acceptance signal while otherwise proceeding to step b.

In this case, an initial check is made to see whether the presented skin pattern is immediately sufficient for processing without the need to delay capture of the image.

Preferably, in this case, the method further comprises after generating the preliminary acceptance signal, detecting the intensity of radiation from the irradiated region, comparing the third detected intensity with the background threshold, and generating a final acceptance signal if the detected intensity is greater than the background threshold. As before, this

accommodates problems which can arise due to movement of a skin pattern.

Typically, if the first detected intensity does not exceed the first threshold after a predetermined time interval, the method is immediately terminated. This is one way in which non-presentation of a skin pattern can be dealt with.

The intensity of the skin pattern referred to above can be determined in a variety of ways. For example, the skin pattern from which the intensity is determined can comprise a portion of a larger skin pattern which is eventually processed (either as a reference or input pattern) . In addition, the value or values which are determined for that intensity can be obtained in a variety of ways. In the preferred method, the skin pattern is divided into a number of regions each constituted by a number of pixels and the intensity of each pixel within each region is determined. For each region, a median intensity value is determined and then the sum of the medians is found and used to constitute the intensity value. Of course, variations on _. this method are possible such as averaging the sum of the medians; obtaining average values from each region instead of the medians; or simply averaging the individual pixel intensities.

An example of a fingerprint verification system incorporating a method and apparatus according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 illustrates schematically the optical arrangement of the apparatus;

Figure 2 is a block diagram of the apparatus; and,

Figure 3 is a flow diagram illustrating operation of the image preprocessor/discriminator shown in Figure 1.

The invention can be applied to many conventional forms of finger print reading apparatus including, for example, the apparatus described in GB-A-2174831. Furthermore, the verification technique described in that patent application is particularly suited for use with this invention. However, Figure 1 illustrates a preferred form of optical arrangement in which a primary housing 1 supports a right-angled prism 2 which is positioned with its hypotenuse surface 3 exposed. A polymeric elastomer layer 4 is glued onto the hypotenuse surface 3 with a wetting agent 5 therebetween. One adjacent surface 6 of the prism is provided with a non-reflective surface to absorb incident light and enhance contrast. set of LEDs 7 (two of which are shown) are mounted to the housing 1 and generate respective light beams which are focussed onto the hypotenuse surface 3 of the prism 2 after passing through a face 10 of the prism.

When a finger is placed on the exposed surface of the elastomer 4, it causes the elastomer to deform which better accommodates users with the so called "dry-finger" problem. The ridges of the skin pattern of the finger causes incident light to scatter and some .of that incident light passes back through the face 10 towards a lens 8 which generates a substantially parallel beam passing onto an image sensor 9 such as a photodetector. Any light reflected by the adjacent surface 10 of the prism is prevented from reaching the lens 8 by means of a mask (not shown) . The apparatus is described in more detail in a copending International patent application of De La Rue

Systems Limited entitled "Skin Pattern Sensing Apparatus"

(Agents Ref:30/2960/03) claiming priority from British patent application No. 8918697.7 and incorporated herein by reference.

It will be seen therefore that in the absence of any contact between a fingerpad 1 and the elastomer 4, the image viewed through face 10 of the prism is a dark image of the face 6, internally reflected by the face 3. When the ridges of the fingerpad contact the elastomer 4 however, the process of internal reflection is disturbed and incident light from any direction is scattered at the points of contact. This gives rise to a bright image of the ridge pattern of the fingerpad as viewed through the face 10. It should be noted that it is not essential for the incident light to be internally reflected in the absence of a finger.

The photo-detector 9 receives a fingerprint image on a photo-sensitive surface thereof and feeds an electrical signal representative of the image to an image pre-processor/discriminator unit 21 (Figure 2) . The output of the unit 21 can be used as a record of the individual providing the fingerprint (in which case it is processed in a template generator 22 and retained in a store 23) or as a representation of a real time image for comparison with a record in the template store 23. The comparison is effected in a unit 24. Calling-up information from the store 23 and controlling the units 21 and 24 is performed by a system controller 25 actuated (at least as shown in Figure 1) by a keypad/card reader 26 and supplying an output signal (indicating recognition/or non-recognition) to an indicator 27.

The recognition process is described in more detail in GB-A-2174831 but briefly involves comparing small regions of an input image with reference regions from the template store 23 to see if a match can be found.

The present invention is concerned with obtaining and presenting to the recognition process 24 or the template generator 22 an image of a skin pattern which is suitable for recognition or storage. This suitability

is determined by the image preprocessor/discriminator 21 which operates in accordance with the flow diagram shown in Figure 3. In all the steps which involve obtaining an intensity value for the skin pattern, this is achieved by dividing the image of the skin pattern into a number of regions (for example 16) and for each region which in itself constitutes a square array of pixels, determining the intensity of each pixel and then determining a median intensity value. Median values are then summed to generate a resultant intensity value.

In Stage 1, once a user has indicated to the system his presence, the image preprocessor/discriminator obtains a background intensity value Ibg from the image region (step 100) and then compares this intensity value with a background threshold value bg_thresh (step 101) . If the background intensity Ibg is greater than this threshold then the skin pattern is deemed to be already present and processing jumps to Stage 2. Otherwise, the processor initiates a ten second time out timer (step 102) . If the timer times out (step 103) then this indicates that a skin pattern has not been presented and preprocessing terminates with a "timed out" status (step 104) .

During the ten second time interval, processing enters Stage 2 and the intensity I of the irradiated region is then repeatedly obtained and compared with a first threshold bg_noise + Ibg (step 106) . If I is greater than this threshold then this indicates that a skin pattern has been presented and Stage 2 is started (step 107) . Otherwise, processing returns and step 105 is repeated.

During Stage 2 a dry finger timer is started (step

107) and image frames are continuously captured (step

108) and the intensity of each image frame I is compared with a second threshold pp__thresh (step 109) representing

an intensity value which is sufficient for subsequent processing. If this threshold is not exceeded processing returns to step 108, otherwise processing continues to Stage 3. If the dry finger timer times out (step 110) without an acceptable intensity being achieved then a dry finger flag DF is set (step 111) , a further image is captured (step 114) and processing continues to Stage 3.

In Stage 3, following step 109, a print image is captured after a delay 112a (eg. 500ms) to ensure that the skin pattern or print has not been removed between detection and capture. This could occur due to permanent removal of the skin pattern or if the user wipes the pattern. The algorithm shown in Figure 3 enables this wiping to be accommodated without causing the system to malfunction. The intensity I of the further image frame is obtained (step 112) and this intensity is compared with a third threshold pp_thresh2 constituting a "confirmation" threshold. This threshold must be lower than the second threshold pp_thresh as the intensity of the image can vary and thus could be less than the intensity at which it was initially detected. If the intensity is greater than the third threshold (step 113) then this indicates that a satisfactory image has been captured and template storage or image recognition can be performed. If this condition is not satisfied then processing returns to the beginning of Stage 1.

If Stage 2 has been reached due to the setting of the dry finger flag (step 111) or because the original image exceeded the background threshold then the further image captured in step 114 is compared with Ibg + bg_noise. If the intensity of this further image is greater than bg_noise (step 115) then storage or

recognition can be undertaken otherwise processing returns to the beginning of Stage 1.

Typical values for the thresholds may be: bg noise = 50. This is a very low relative threshold to detect instantly if anything at all is presented. bg_thresh = 800. This absolute threshold detects if a print is already present at the start of the method. If so, proceed directly to Stage 2. If a dry print is already present, then it will still be developing, so Ibg + bg_noise will detect it in Stage 1 and proceed to Stage 2. pp_thresh = 10,000. This is the ideal value for all images before capture. pp_thresh2 = 8,000. An image will be accepted above this after the image intensity has exceeded pp_thresh. If the intensity is any lower, something is wrong and it is assumed the user was wiping the surface or has removed his finger too early.