EP1815394A1

EP1815394A1 - Fingerprint imaging

Info

Publication number: EP1815394A1
Application number: EP05818860A
Authority: EP
Inventors: Cristian Philips Int. Prop. & Standards PRESURA; A. H. M. Philips Int. Prop. & Standards AKKERMANS
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2004-11-11
Filing date: 2005-10-31
Publication date: 2007-08-08
Also published as: WO2006051440A1; JP2008519633A; GB0424889D0

Abstract

A fingerprint imaging device has an array of optical waveguides (113) each having an optical axis extending between a source end (130) and a reflection end (135). The reflection ends form a detection surface (136) that is oblique to the optical axis so that light (100, 101) propagating along the optical axis is reflected internally to the optical waveguide when a target medium (e.g. air) having a refractive index less than that of the waveguide is present at the reflection end and such that light is transmitted through the reflection surface when the target medium (e.g. skin of a finger) has a refractive index greater than that of the waveguide. Illumination sources are optically coupled to respective optical waveguides. At least one detector (123) is provided proximal to the detection surface for receiving transmitted light after scattering from the target medium. The transmitted light can be used to determine whether the target medium belongs to an animate object by plethysmographic analysis, pulse oximetry analysis or Doppler blood perfusion analysis, for example.

Description

DESCRIPTION

FINGERPRINT IMAGING

The present invention relates to fingerprint detection, and in particular to methods and apparatus for imaging fingerprints in conjunction with detection of a biological property of the finger that confirms the integrity of the finger being imaged.

The expression 'confirmation of integrity' in this context is intended to encompass confirmation that the finger is not a fake finger (i.e. a plastic or latex replica) and/or that the finger belongs to an animate object (i.e. not a dead body or severed finger). The expression 'finger' is used throughout the present specification for convenience, but it is intended to encompass any body part that provides a highly characteristic 'print' based on a non-planar surface pattern and which is capable of delivering a measurable biological property that confirms its integrity as defined above. Examples may include palm prints, hand prints, footprints and the like.

Authenticating the identity of a person is becoming an increasingly frequent requirement in many contexts, e.g. when travelling, accessing buildings and other secure areas, and when accessing or using electronic systems such as computers and data storage devices.

A number of prior art documents have addressed the electronic acquisition of fingerprint images, of which an example is US 4785171. With reference to figure 1 , this document proposes using a bundle 10 of optical fibres 14 having a first end 38 that provides a combined interface to a light source 40 for transmitting light down the fibres 14 and an imaging device 42 for receiving light returned along the fibres. At the opposite (second) end 36 of the fibre bundle, the fibres 14 each terminate with an end face 16 that is at an oblique angle to the longitudinal axis of the fibre. The end faces 16 of the fibres in the bundle are disposed so that the angled faces together provide a detector surface 36 upon which a finger 12 may be placed. The optical fibres 14 have a refractive index of n1 ; the medium present at the detector surface has a refractive index of n2.

In use, the medium present at the detector surface 36 may be air (n2 < n1) or skin of the finger (n2 > n1). The oblique angle is selected such that when the medium beyond the end of the fibre 14 is air, total internal reflection 32 occurs and light is internally scattered and returned down the optical fibre to the first end. If the medium beyond the end of the fibre 14 is skin, the light from the fibre is transmitted into the finger 12 where it is scattered and not generally returned down the fibre 14 from which it emerged. The finger 12 provides a non-planar surface having ridges and valleys corresponding to the fingerprint pattern. The individual fibres 14 are sufficiently small in diameter that some fibres will provide a detector surface 16 that falls between ridges and will therefore have air at the interface (n2 < n1) while others will provide a detector surface 16 on which a ridge falls (n2 > n1). Thus, light transmitted back down the fibre bundle will be a function of the fingerprint pattern placed on the detector surface. The device in US '171 cannot make use of the light that is transmitted into, and scattered within, the finger because there is no specific optical detector for this light.

To collect information relating to a biological property of the finger it is necessary to capture the scattered light. US 2003/0090650 describes, with particular reference to figure 8 therein, a system in which there is provided a first optical detector 120 for collecting light transmitted into and scattered from the finger F as well as a second detector 2Of for collecting light returned by internal reflection from a planar surface of a microprism element. The first optical detector 120 collects all light scattered from the finger and returned via the microprism element 50 to facilitate plethysmography measurements, while the second detector is a two dimensional detector that preserves spatial information for imaging the fingerprint pattern.

Other fingerprint imaging systems have been variously described in US 6175641 , US 2003/0118219, US 2003/0044051 and WO 03/063065. It is an object of the present invention to provide a method and apparatus for imaging fingerprints and for detecting at least one biological property of the finger that confirms the integrity of the finger being imaged. It is a further object of the invention to provide a method and apparatus for fingerprint imaging that can readily incorporate detection of a number of possible animate biological properties of the finger. It is a further object of the invention to facilitate verification of the finger as belonging to an animate object using Doppler blood perfusion measurements. It is a further object of the invention to provide a highly compact apparatus for fingerprint imaging that can conveniently be incorporated within other electronic devices.

According to one aspect, the present invention provides a fingerprint imaging device comprising: an array of optical waveguides each having an optical axis extending between a source end and a reflection end, the reflection end comprising a detection surface that is oblique to the optical axis such that light propagating along the optical axis is reflected internally to the optical waveguide when a target medium having a refractive index less than that of the waveguide is present at the reflection end and such that light is transmitted through the reflection surface when the target medium has a refractive index greater than that of the waveguide; an array of illumination sources, each illumination source optically coupled to a respective one of the optical waveguides; and at least one detector, proximal to the detection surfaces of the reflection ends of the optical waveguides, for receiving said transmitted light after scattering from the medium.

According to another aspect, the present invention provides a fingerprint imaging device comprising: an array of optical waveguides each having an optical axis extending between a source end and a reflection end, the reflection end comprising a detection surface that is oblique to the optical axis such that light propagating along the optical axis is reflected internally to the optical waveguide when no skin is in contact with the detection surface and such that light is transmitted through the reflection surface when skin is in contact with the detection surface; an array of illumination sources, each illumination source optically coupled to a respective one of the optical waveguides; and at least one detector, proximal to the detection surfaces of the reflection ends of the optical waveguides, for receiving said transmitted light after scattering from the skin.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 shows schematic views of a prior art fingerprint acquisition system in which figure 1(a) is side view of the optical components including an optical fibre bundle; figure 1 (b) is a cross-sectional side view of an optical fibre within the bundle of figure 1(a); and figure 1(c) is a perspective view of the optical fibre bundle;

Figure 2 is a schematic diagram of detection principles according to the present invention in which figure 2(a) shows an optical path for light scattered from a target medium; and figure 2(b) shows an optical path for light reflected within an optical waveguide;

Figure 3 is a perspective view of optical elements of a fingerprint and biological property acquisition system according to a presently preferred configuration; and

Figure 4 is a schematic diagram of a fingerprint and biological property acquisition system according to a presently preferred configuration; and

Figure 5 is a schematic diagram of an alternative optical scanning system that can replace a switched array of optical sources.

The prior art principle of using a plurality of optical fibres each with a detection surface that is oblique to the optical axis of the fibre has been discussed in connection with the figure 1. With reference to figures 2 and 3, the present invention generally provides a plurality of optical waveguides 113 which may be optical fibres or waveguides within an integrated optical element 110 such as can be formed using known optical semiconductor device fabrication processes. The optical waveguide array 110 comprises a plurality of waveguides 113 each of which has a first (source) end 130 that acts an input facet for a light source 122, for transmitting light down the waveguide 113. At the opposite (reflection) end 133 of each waveguide 113, the waveguide terminates with a reflection end face 135 that is at an oblique angle to the longitudinal axis of the waveguide 113. The reflection end faces 135 of the waveguides 113 are disposed so that the angled faces together provide a detection surface 136 upon which a finger 111 may be placed. The non-planar surface 105 of the finger 111 is evident in figure 2.

The waveguides 113 have a refractive index of n1 ; the target medium present at the detection surface 136 has a refractive index of n2. In the example of figure 2(a), the target medium present at the detection surface 136 is skin of the finger 111 (n2 > n1) while in the example of figure 2(b) the target medium present at the detection surface 136 is air 106 (n2 < n1). The oblique angle of the detection surface 136 relative to the optical axes of the waveguides 113 is selected such that when the target medium at the detection surface 136 is air 106 (figure 2(b)), input light 100 undergoes total internal reflection resulting in a reflected beam as illustrated, internally scattered and returned down the optical waveguide to the first end or dissipated in the walls of the waveguide. Conversely, if the target medium at the detection surface is skin (figure 2(a)), input light 101 is transmitted into the finger 111 where it is scattered as shown by paths 112 to detector elements 123 and not generally returned down the waveguides.

As shown in figure 3, preferably, the detector elements 123 are provided in an array 140 that corresponds in dimensionality to the array of waveguides 113 so that each waveguide has at least one detector element 123. The detector elements 123 are each positioned proximal to respective reflection end faces 135 of the waveguides 113, on an adjacent part of the detection surface 136. Thus, the detector elements 123 can receive light scattered by the finger 111 directly without requiring return passage of the scattered light through the waveguide array. Preferably, the proximity of the detector elements 123 to the respective reflection end faces is of similar order of spacing to the separation of the waveguides 113 in the array 110, but could generally be from 100 microns up to the order of a few millimetres.

Each waveguide 113 is provided with an illumination source such as semiconductor laser 122. Preferably, the illumination sources are provided in an array of such semiconductor lasers as shown. For reasons which will be described later, preferably, the individual illumination sources in the array are separately switchable.

Various methods are possible for detection of a biological property of the target medium (e.g. finger) that confirms the integrity of the object being imaged, e.g. that it belongs to an animate entity such as a living human. Photoplethysmography is based on the changing optical properties of a selected area of skin, e.g. the finger. When light is transmitted into the body, more or less light is absorbed depending on the blood volume present within the body in the illuminated area. Consequently, monitoring backscattered light from the illuminated skin area corresponds with any variation of the blood volume. Blood volume changes can thus be determined by measuring variations in the amount of reflected light and using the optical properties of tissue and blood.

In one embodiment of the invention shown in figure 4, the detector elements 123 are coupled to a suitable processing device 150 that is adapted to determine whether the target medium corresponds to an animate object based on plethysmographic analysis of the signals received by the detector elements 123.

Pulse oximetry is a technique for monitoring the level of oxygenation of the blood. Pulse oximetry makes use of the phenomenon that blood changes colour depending on whether haemoglobin in red blood cells is oxygenated or deoxygenated. Deoxygenated blood is darker red in colour than oxygenated blood. It is therefore possible to deduce the degree of oxygen saturation of blood from its colour. Consequently, measuring the absorption of certain parts of the colour spectrum in transmitted or scattered light from an illuminated area of skin enables a determination of the oxygenation of the blood. Typically, relative absorption of different frequencies of light is monitored in order to compensate for other light absorptive characteristics of the body. Further, the pulse oximetry technique distinguishes between arterial blood which is oxygenated and venous blood which is deoxygenated by measuring the differences in light absorption characteristics between the high and low points of pulse fluctuations (systolic and diastolic) so as to cancel out effects of steady venous flow and measure only the colour of the pulsating arterial blood.

In another embodiment of the invention also represented in figure 4, the detector elements 123 are coupled to a suitable processing device 150 that is adapted to determine whether the target medium corresponds to an animate object based on pulse oximetry analysis of the signals received by the detector elements 123. In this example, it is preferable that the illumination sources 122 are adapted to provide optical output at at least two different wavelengths of light such that absorption coefficients at different wavelengths can be determined.

In one configuration, the illumination sources 122 are broad spectrum sources encompassing the at least two different wavelengths so that absorption can be measured for each. In this instance, preferably a pair of detector elements 123, each responsive to one of the different wavelengths, is provided for each waveguide.

In another configuration, the illumination sources 122 are each substantially monochromatic but comprise at least two groups, each group covering the at least two different wavelengths. In this instance, detector elements 123 are preferably optimised for sensitivity to their respective illumination source and waveguide.

Doppler blood perfusion measurement is a technique for measuring the circulation of blood within the body. When light impinges on the skin or other perfused tissue, some of the light is scattered by moving red blood cells and some is scattered by static tissue. The light scattered by moving red blood cells will have minute changes in frequency due to the Doppler effect. By detecting these small changes in frequency in the scattered light using a sensitive optical detector, an indication of blood perfusion can be derived. To facilitate such measurements, highly monochromatic light sources such as semiconductor laser sources are required.

In a presently most preferred arrangement, also represented by figure 4, the detector elements 123 are coupled to a suitable processing device 150 that is adapted to determine whether the target medium corresponds to an animate object based on Doppler blood perfusion analysis of the signals received by the detector elements 123. In this example, it is preferable that the illumination sources 122 are all adapted to provide substantially monochromatic optical output at a wavelength of light such that Doppler shifts in light scattered by moving red blood cells is readily identifiable.

It will be understood that the invention as so far described can be implemented with a two dimensional array of illumination sources 122, waveguides 113 and detector elements 123 in order to obtain the required two dimensional image of the fingerprint as well as the biological property indication. A particular advantage of providing an array of detector elements 123 corresponding in dimension to the array of waveguides 123 and illumination sources 122 is that the biological property indication and fingerprint image information can be collected by the same detector elements 123. This avoids the need for two sets of detectors and also avoids the need for scattered light to be passed back into the waveguides. Still further, it avoids the need for the reflected light within the waveguides to be captured and detected.

To achieve this, it is necessary that a mechanism for preserving spatial locality of detected light is required, as will now be described. In addition, it becomes possible to implement the system as corresponding one-dimensional arrays of illumination sources, waveguides and detectors. In the preferred embodiment of figures 3 and 4, a one-dimensional array

120 of illumination sources 122 is optically coupled to a corresponding one- dimensional array 110 of waveguides 113. A semiconductor laser array 120 and corresponding optical waveguide array 110 can be integrated onto a common substrate for a compact device. The detector elements 123 preferably also comprise a corresponding one-dimensional array of corresponding dimensions. Although two detector elements 123 are depicted per waveguide 113 in figure 3, it will be understood that this configuration represents two linear arrays of detector elements in which each linear array comprises detectors sensitive to a specific wavelength, useful for example when performing pulse oximetry measurements or Doppler perfusion measurements. In the linear array configuration of figure 3, the array dimension extends in the 'x' direction and a user's finger is drawn across the detection surface in the direction of arrow 125 which represents the 'y' direction. The finger could equally be drawn in the opposite (-y) direction. The detected light is sampled over short periods each corresponding to a location in y-space. Each waveguide 113 corresponds to a location in x-space. It will be noted that light scattered in the finger will be detected by multiple detector elements 123 due to its loss of directionality. To compensate for this, the illumination sources 122 are preferably each fired in turn, for a short period of time that does not overlap with the firing period of any other illumination source. In a preferred arrangement, the firing period, or 'on-time' of each illumination source is of the order of 0.025 ms. During the firing time for an illumination source, one or more detector elements 123 check to see whether light has entered the finger 111 and been scattered. If so, the location of medium in contact with the detection surface 136 is established (e.g. a ridge of a fingerprint). If no light is detected by the detector elements 123, it is determined that air is present at the detection surface for that location (e.g. a fingerprint valley overlies the detection surface) and all light from the illumination source has been internally reflected within the waveguide.

The process is repeated for each successive illumination source 122 and waveguide 113 in the array until an entire x-scan is completed. Subsequent x-scans are performed at predetermined time intervals as the finger is drawn across the detection surface 136 in order that a two- dimensional image of the fingerprint can be established.

Measurement of the animate biological property typically requires a longer period of illumination than an image scan as described above. For example, Doppler blood perfusion analysis typically requires data acquisition times of the order of 1 ms. Therefore, between one or more selected x-scans, it is possible to switch on at least one illumination source for an extended period, e.g. 2.5 ms, in order to capture data adequate for perfusion analysis. For better signal to noise ratio, and because spatial information is not necessarily required for perfusion analysis, it may be desirable to switch all illumination sources 122 on together for the perfusion analysis reading.

Alternatively, one illumination source 122 in the array 120 and its respective detectors may be selected to have a wavelength different than the other illumination sources selected for fingerprint imaging or mapping. In this case, providing that the respective sources and detectors operating at different wavelengths do not interfere with each other, it is possible for the selected source and detector to be dedicated full time to biological property measurement.

With further reference to figure 4, an exemplary control system for the fingerprint imaging device includes a control module 160 for providing signal timing for the array 120 of illumination sources 122 and for the array 140 of detectors 123. In one arrangement as shown, the control module addresses each relevant illumination source 122 via a multiplexer 161 to ensure that each illumination source fires for the appropriate on-time. Signals from each of the detectors 123 are sampled and passed to a signal processor 150 by way of demultiplexer 162.

The signal processor 150 receives the detected signals. With knowledge of the firing timing of the illumination sources from the control module 160, the signal processor 150 is operative to determine the spatial fingerprint image 151 which may be output as a real time displayed image, hardcopy and or data file for further processing. The signal processor is also operative to perform suitable signal processing, e.g. by plethysmographic analysis, pulse oximetry analysis or Doppler blood perfusion analysis to determine whether the received data has been obtained from an animate object. For processing the detected signals using Doppler perfusion analysis, as described above, it is desirable that light enters a single specific optical fibre for a much longer time than the scan frequency of the other optical fibres. In the preferred arrangement, the Doppler perfusion sampling signal is of duration of approximately 2.5 ms, whereas the other sampled signals for fingerprint imaging have duration of approximately 0.025 ms each as scanned along the one dimensional array. This ensures that the total measurement time for the fingerprint is kept within reasonable limits to enables line scans as the finger moves across the array. For 100 line scans, the total measurement time can be limited to less than half a second, sufficient to capture the moving finger with sufficient spatial resolution. The preferred technique of figures 3 and 4 above uses electronically controlled scanning by way of separately switchable laser diodes in an array.

Figure 5 illustrates an alternative scanning arrangement that requires only a single illumination source 231 to provide an effective 'virtual array' of illumination sources for each waveguide. A specially configured mirror 234, 235 rotates continuously at a frequency of, for example, 200 Hz and is illuminated by an input beam 232 from the illumination source 231. The mirror has a first surface 234 that is planar and a second surface 235 that is convex. Half of the time (e.g. 5 ms), light from the illumination source 231 is reflected from the planar side 234 as shown in figure 5(b). Since the illumination source 231 is positioned orthogonally to the axis 237 of rotation of the mirror, the reflected beam 236 is scanned in the direction indicated by arrow 233, thereby creating a scanning beam for scanning across the array of waveguides 110. The other half of the time, the input beam 232 is reflected from the convex side 235 as shown in figure 5(c). In this instance, the reflected beam 236 remains stationary and can be directed at the waveguide used for biological property measurement. In this way, sufficient time is provided for Doppler measurements, for example. It will be noted that this scanning illumination source provides a mechanical alternative to the separately switchable array of illumination sources.

Modifications to the waveguide properties are possible. As described above, based on the arrangement of waveguides of figure 1 , the waveguides 113 have a refractive index of n1 ; the target medium present at the detection surface 136 has a refractive index of n2. In the example of figure 2(a), the target medium present at the detection surface 136 is skin of the finger 111 (n2 > n1) while in the example of figure 2(b) the target medium present at the detection surface 136 is air 106 (n2 < n1). In practical application, this relies on a selection of waveguide material such that the refractive index of the waveguide material lies between that of skin (approximately 1.3 to 1.4), and that of air (1.0).

This imposes some restrictions on selection of waveguide material, which can be avoided. In an alternative configuration, a glass material is used for the waveguides, having a refractive index in the range 1.5 to 1.7. The oblique angle of the detection surface 136 relative to the optical axes of the^' waveguides 113 is selected such that when the target medium at the detection surface 136 is air 106 (figure 2(b)) with a refractive index of approximately 1.0, input light 100 undergoes total internal reflection resulting in a reflected beam as illustrated, internally scattered and returned down the optical waveguide to the first end or dissipated in the walls of the waveguide. The oblique angle is also selected such that if the target medium at the detection surface is skin (figure 2(a)), with a refractive index in the range 1.3 to 1.4, input light 101 is transmitted into the finger 111 where it is scattered as shown by paths 112 to detector elements 123 and not generally returned down the waveguides.

In a general aspect, the oblique angle is selected according to the relative refractive indices of the waveguide material, the skin, and air. The oblique angle of the detection surface relative to the optical axis is selected such that light propagating along the optical axis is reflected internally to the optical waveguide when no skin is in contact with the detection surface and such that light is transmitted through the reflection surface when skin is in contact with the detection surface.

To determine an appropriate oblique angle of the detection surface 136,

Snell's law is applied to determine the critical angle at which total internal reflection occurs. This defines that, for a refractive index of waveguide n1 and a refractive index of target medium n2, the critical angle at which total internal reflection occurs,

Θcrit = sin-1(n1 / n2).

We have two conditions to be satisfied. First, we want light to have total internal reflection if the target medium is air. Thus, n2 = 1.0, and the requirement for total internal reflection is sin(a) > 1 / nw, where a is the angle of incidence relative to the normal to the detection surface and nw is the refractive index of the waveguide.

Second, we want light to pass through the detection surface if the target medium is skin. The condition for this is sin(a) < nf / nw, where a is the angle of incidence relative to the normal to the detection surface, nf is the refractive index of the finger or skin, and nw is the refractive index of the waveguide. By combining the two relations, we obtain the range of desired angles of incidence a, and thus the range of possible angles for the detection surface, as a function of the refractive indices of the waveguide and skin:

1 / nw < sin(a) < nf / nw

or equivalent^

1 < nw sin(a) < nf.

Thus, preferably, the angle of the detection surface relative to the optical axis is selected such that it lies in the range 1 < nw sin(a) < nf where a is the angle of the optical axis relative to the normal to the detection surface. In a preferred embodiment, nf = 1.3 and nw = 1.5, and a lies in the range:

41.81° < a < 60.07°.

If the value of nf is taken as 1.4, then the upper limit for a would be approximately 70°.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims

1. A fingerprint imaging device comprising: an array (110) of optical waveguides (113) each having an optical axis extending between a source end (130) and a reflection end (135), the reflection end comprising a detection surface (136) that is oblique to the optical axis such that light (100, 101) propagating along the optical axis is reflected internally to the optical waveguide when a target medium (106, 111) having a refractive index less than that of the waveguide is present at the reflection end and such that light is transmitted through the detection surface when the target medium has a refractive index greater than that of the waveguide; an array (120) of illumination sources (122), each illumination source optically coupled to a respective one of the optical waveguides; and at least one detector (123), proximal to the detection surfaces (136) of the reflection ends (135) of the optical waveguides, for receiving said transmitted light after scattering from the medium.

2. The device of claim 1 in which the array (110) of optical waveguides (113) is a fibre optic bundle.

3. The device of claim 1 in which the array (120) of illumination sources (122) is a semiconductor laser array.

4. The device of claim 1 or claim 3 in which the illumination sources (122) are each substantially monochromatic.

5. The device of claim 1 in which the illumination sources (122) are separately switchable.

6. The device of claim 5 in which a first plurality of illumination sources (122) are switched on and off in a consecutive manner for non- overlapping on-times and at least one illumination source is switched on for an extended period substantially longer than the on-times of the first plurality of illumination sources.

7. The device of claim 4 in which illumination sources (122) in the array have different wavelength optical output.

8. The device of claim 1 in which the at least one detector (123) comprises an array (140) of detectors each corresponding to one or more of the optical waveguides.

9. The device of claim 8 in which detectors (123) in the array (140) are responsive to different optical wavelengths.

10. The device of claim 1 in which the array (110) of optical waveguides (113) and the array of illumination sources (122) are one- dimensional arrays.

11. The device of claim 1 further including a microprocessor (150), coupled to receive electrical output signals from each detector (123) indicative of the scattered light, and adapted to perform plethysmographic analysis on the output signals.

12. The device of claim 11 further adapted to indicate whether a target medium forms part of an animate object based on the plethysmographic analysis.

13. The device of claim 1 further including a microprocessor (150), coupled to receive electrical output signals from each detector (123) indicative of the scattered light, and adapted to perform pulse oximetry analysis on the output signals.

14. The device of claim 13 further adapted to indicate whether a target medium forms part of an animate object based on the pulse oximetry analysis.

15. The device of claim 4 or claim 7 further including a microprocessor (150), coupled to receive electrical output signals from each detector (123) indicative of the scattered light, and adapted to perform Doppler blood perfusion analysis on the output signals.

16. The device of claim 15 further adapted to indicate whether a target medium forms part of an animate object based on the Doppler blood perfusion analysis.

17. The device of claim 1 in which the array of illumination sources is a virtual array comprising a single illumination source (231 ) having an output

(236) scanned across the source ends of the optical waveguides.

18. The device of claim 17 in which the illumination source output (236) is scanned across the source ends (130) of the optical waveguides (113) such that the illuminated periods of a first plurality of the waveguides are provided in a consecutive manner of non-overlapping illumination periods and at least one optica! waveguide is illuminated for an extended period substantially longer than the illumination periods of the first plurality of waveguides.

19. The device of claim 17 or claim 18 in which the virtual array comprises said single illumination source (231 ) optically directed to a rotatable mirror, the mirror having a first reflective surface (234) that is substantially planar and a second reflective surface (235) that is convex.

20. A fingerprint imaging device comprising: an array (110) of optical waveguides (113) each having an optical axis extending between a source end (130) and a reflection end (135), the reflection end comprising a detection surface (136) that is oblique to the optical axis such that light (100, 101) propagating along the optical axis is reflected internally to the optical waveguide when no skin is in contact with the detection surface and such that light is transmitted through the reflection surface when skin (111 ) is in contact with the detection surface; an array (120) of illumination sources (122), each illumination source optically coupled to a respective one of the optical waveguides; and at least one detector (123), proximal to the detection surfaces (136) of the reflection ends of the optical waveguides, for receiving said transmitted light after scattering from the skin.

21. The device of claim 20 in which the array (120) of illumination sources (122) is a semiconductor laser array.

22. The device of claim 20 or claim 21 in which the illumination sources (122) are each substantially monochromatic.

23. The device of claim 20 in which the illumination sources (122) are separately switchable.

24. The device of claim 23 in which a first plurality of illumination sources (122) are switched on and off in a consecutive manner for non- overlapping on-times and at least one illumination source is switched on for an extended period substantially longer than the on-times of the first plurality of illumination sources.

25. The device of claim 22 in which illumination sources (122) in the array have different wavelength optical output.

26. The device of claim 20 in which the at least one detector (123) comprises an array (140) of detectors each corresponding to one or more of the optical waveguides.

27. The device of claim 26 in which detectors (123) in the array (140) are responsive to different optical wavelengths.

28. The device of claim 20 in which the array (110) of optical waveguides (113) and the array (120) of illumination sources (122) are one- dimensional arrays.

29. The device of claim 20 further including a microprocessor (150), coupled to receive electrical output signals from each detector (123) indicative of the scattered light, and adapted to perform plethysmographic analysis on the output signals.

30. The device of claim 29 further adapted to indicate whether a target medium forms part of an animate object based on the plethysmographic analysis.

31. The device of claim 20 further including a microprocessor (150), coupled to receive electrical output signals from each detector indicative of the scattered light, and adapted to perform pulse oximetry analysis on the output signals.

32. The device of claim 31 further adapted to indicate whether a target medium forms part of an animate object based on the pulse oximetry analysis.

33. The device of claim 22 or claim 25 further including a microprocessor (150), coupled to receive electrical output signals from each detector (123) indicative of the scattered light, and adapted to perform Doppler blood perfusion analysis on the output signals.

34. The device of claim 33 further adapted to indicate whether a target medium forms part of an animate object based on the Doppler blood perfusion analysis.

35. The device of claim 20 in which the array of illumination sources is a virtual array comprising a single illumination source (231 ) having an output (236) scanned across the source ends of the optical waveguides.

36. The device of claim 35 in which the illumination source output (236) is scanned across the source ends (130) of the optical waveguides (113) such that the illuminated periods of a first plurality of the waveguides are provided in a consecutive manner of non-overlapping illumination periods and at least one optical waveguide is illuminated for an extended period substantially longer than the illumination periods of the first plurality of waveguides.

37. The device of claim 35 or claim 36 in which the virtual array comprises said single illumination source (231) optically directed to a rotatable mirror, the mirror having a first reflective surface (234) that is substantially planar and a second reflective surface (235) that is convex.

38. The device of claim 20 in which the angle of the detection surface (136) relative to the optical axis lies in the range 1 < nwsin(a) < nf where nw is the refractive index of the waveguide, nf is the refractive index of the skin, and a is the angle of the optical axis relative to the normal to the detection surface.

39. The device of claim 38 in which nw lies in the range 1.3 to 1.4.

40. The device of claim 39 in which nw lies in the range 1.5 to 1.7.