GB2276732A

GB2276732A - Fingerprint imaging

Info

Publication number: GB2276732A
Application number: GB9306776A
Authority: GB
Inventors: Alastair Sibbald
Original assignee: Central Research Laboratories Ltd
Current assignee: Central Research Laboratories Ltd
Priority date: 1993-03-31
Filing date: 1993-03-31
Publication date: 1994-10-05
Anticipated expiration: 2013-03-31
Also published as: GB2276732B; GB9306776D0

Abstract

An apparatus for producing an image of a fingerprint includes a right-angled prism 2, wherein the finger 6 is placed an the hypotenuse face 4 thereof. First and second polarising elements 12, 14 on die other two faces 8, 10 polarise light A entering the prism, and light B emerging to form the image respectively. Since light impinging on the prism/air boundary between the skin ridges is totally internally reflected maintaining polarisation whereas light, impinging an a prism/skin boundary is scattered, the second element 14 provides a screen to one of the types of light depending upon the relative orientation of the elements. This improves the contrast ratio of the image obtained. <IMAGE>

Description

FINGERPRINT IMAGING This invention relates to apparatus for producing an usage of a fingerprint or other skin ridge pattern, comprising a light transmissive member having a first surface region for receiving skin to be image thereon, and a light source arranged such that in operation light from the source enters tlle light transmissive member through a second surface region and is totally intemally reflected at the boundary between the first surface region and air, so as to emerge through a third surface region.

A commonly encountered apparatus of this type involves the use of a rightangle prism as the light transmissive member, where the finger is placed on to the hypotenuse face and illuminated tlirougll the prism by a light source through one of the other faces, sucll that the angle of incidence is greater than the critical angle. This causes the light to be totally internally reflected at the points where the skin frictionridges do not contact the glass, and partially absorbed where the ridges do contact the glass, since the refractive index of the skin locally changes the critical angle conditions. The reflected light emerging from the prism can then be focused on to an extemal surface so as to fonn an image of the fingerprint having a light field (i.e.

black-on-white) appearance, with "black" regions where the skin is in direct contact with the prism surface.

Prior art illuniillation methods include broad-area illumination, for example, from a LED or through a diffuser, and a scanning, fan-beam system for use in conjunction with a linear photosensor array. In a system disclosed in our copending patent application WO 92/11608, a relatively broad, collimated beam is used.

In practice, however, this method does not produce high-contrast images, because the light which is incident on to the areas of glass which are in contact with the friction ridges of the skin does not undergo total absorption; much of it is scattered and reflected diffusely back into tlle prism. As a consequence, images of fingerprints obtained using prior art teclmiques possess poor contrast; that is they appear 'pitlk/grey-on-white', rather than 'black-on-white'. In addition, any residues of skin grease left over on the prism surface tend to scatter sufficient light so as to cause latent images, which carry over to subsequent images and degrade their quality.

It is an object of the present invention to alleviate these problems.

Accordingly, the present invention consists in an apparatus of the type defiled in the first paragraph which is characterised by polarising means provided in the path of the light entering the light transniissive member and in the path of the light emerging therefrom, the direction of polarisation of the polarising means relative to each other being other than midway between crossed and aligned with respect to light from the source which is totally internally reflected.

The direction of polarisation of the polarising means relative to each other may be substantially aligned with respect to light from the source which is totally internally reflected.

In this way substantially all of the totally internally reflected light emerges from the apparatus whereas only about half of any randomly polarised light emerges.

The randomly polarised light includes the light scattered from the finger ridges. Thus in theory a two-fold increase in contrast can be obtained.

As an attemative, the direction of polarisation of the polarising means relative to each other may be substantially crossed with respect to light from the source which is totally internally reflected. Thus, light which has been totally internally reflected does not emerge, and the randomly polarised scattered light, soine of which emerges, can be used to form an image. In this manner, a still greater increase in contrast ratio can, in tlleory, be obtained.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of an apparatus in accordance with the invention; Figures 2 and 3 are enlarged diagrammatic views of part of the apparatus of Figure 1 for different embodiments of the invention.

Referring to Figure I, a light transmissive member in the form of a prism 2, for example a right-angle type, has a first surface region 4, or ilnaging surface, which is arranged to receive a finger 6. Light from a collimated or diffuse light source (not shown) is projected, as indicated by arrows A, into the prism 2 through a second surface region 8, or entrance surface, such that it strikes the intemal imaging surface 4. The angle of incidence is greater than the critical angle so as to cause total internal reflection of the light beam out of the prism through a third surface region 10, or exit surface, in the direction indicated by arrows B, towards an imaging system such as a video camera (not shown).

A first polarising element 12 is provided in the optical path of the entrance beam A adjacent the entrance surface 8, and a second polarising element 14 is provided in the path of the exit beam B adjacent the exit surface 10. The polarising elements 12,14 may comprise polarising films adhered to the surfaces 8,10 of the prism. The first element 12 polarises the light which is incident on to the internal imaging surface 4. The light from direction A which strikes the internal imaging surface 4 is either totally intemally reflected if there is no skin contact at that point, or it is scattered and diffusely reflected if there is local skin contact. If the light does become totally intemally reflected (i.e. no skin contact), then the polarisation plane is maintained.

Figure 2 shows the situation in which the elements are oriented in a crossed matter. In this case, light impinging on the boundary of the imaging surface 4 and air, that is, for example, adjacent a gap 16 between the finger ridges, is totally intemally reflected and absorbed by the second element 14 immediately after leaving the prism 2. Conversely, light impinging on the boundary between the imaging surface 4 and skin; that is, for example, adjacent a ridge 18, does not become totally internally reflected but becomes scattered and diffusely reflected, causing a significant reduction in the degree of polarisation which enables a substantial portion of the light leaving the prism 2 to pass through the second element 14 to the imaging system.

Thus, a dark-field, or white-on-black image is formed.

Typical parameters for low-cost, plastic, polarising material are given in catalogue &num;19WF (1989) of the Edmund Scientific Co., p.44, as follows: Polarising Efficiency = 99.5%; Transmission: 24%. This indicates that when unpolarised light is passed through the material, the exit beam possesses 24% of the intensity of the incident beam, 99.5% of which is plane polarised. In other words, for an unpolarised source such as the light reflected from skin ridges, the effective transmission coefficient for light polarised parallel to the polarisation plane of the material is 99.5% x 24% = 0.239. For light polarised perpendicularly to the polarisation plane of the material, such as the light reflected where there is no skin contact, it is 0.5% x 24% = 0.001. Us, the ratio of light intensities for light reflected from a skin ridge, assuming reflectivity of 40%, and light reflected from an area in between ridges can be calculated as follows: skin contact: reflected intensity = 40%; final intensity = 40x0.239 = 9.56% no skin contact: reflected intensity = 100%; final intensity = 100x0.001 = 0.1% contrast ratio: 9.56/0.1 = 95.6 (i.e. 95.6:1) The apparatus shown in Figure 1 may also be used to form the image using the total internal reflected light as shown in Figure 3. This is achieved by orienting the second polarising element 14 in substantially the same way as the first 12; that is such that it is orientated optimally to pass as much totally internally reflected light as possible whilst passing only about half as much randomly polarised light, which includes the diffusely reflected or scattered light from the finger-ridges. As a consequence, one-half of the "unwanted" light which is not fully absorbed by the finger-ridges 18, but which is scattered forwards is eliminated without reducing the "wanted" light reflected from the gaps 16 between the ridges, thus providing a twofold increase in contrast as compared with prior art methods.

The inclusion of the crossed polarising elements reduces the intensity of light tllrough the system, but this is relatively ullimportant because present detector technologies are more than adequately sensitive. Using the same assumptions as previously, then the following contrast ratio can be calculated: skin contact: reflected intensity = 40%; final intensity = 40x0.239 = 9.56% no skin contact: reflected intensity = 100%; final intensity = l00xO.478 = 47.8% contrast ratio: 47.8/9.56 = 5 (i.e. 5:1) It will be appreciated tllat the relative orientation of the polarisation directions of the elements 12 and 14 may be other than aligned or crossed with respect to light from the source which has been totally intemally reflected, provided only that this orientation is not chosen to be midway between crossed and aligned. Deviation from crossed or aligned will, of course, result in less contrast improvement than would otherwise be obtained, whereas no improvement at all would be obtained with the "midway" orientation.

It will be appreciated that although reference has been made to fingerprints, the invention is equally applicable to other skin ridge pattems such as thumb prints, palm prints or toe prints.

Claims

1. Apparatus for producing an image of a skin ridge pattern comprising a light transmissive member having a first surface region for receiving a skin to be imaged thereon, and a light source arranged such that hl operation light from the source enters the light transmissive member through a second surface region and, in the absence of skin, is totally intemally reflected at a boundary between the first surface region and air, so as to emerge through a third surface region, characterised by polarising means provided in the path of the light entering the light transmissive member and in the path of the light emerging therefrom, tlle direction of polarisation of the polarising means relative to each other being other than midway between crossed and aligned with respect to light from the source which is totally internally reflected.

2. Apparatus as claimed in claim 1, wherein the direction of polarisation of the polarising means relative to each other is substantially crossed with respect to light from the source with is totally internally reflected.

3. Apparatus as claimed in claim 1, wherein the direction of polarisation of the polarising means relative to each other is a substantially aligned with respect to light from the source which is totally internally reflected.

4. Apparatus as claimed in any preceding claim, wherein the first, second and third surface regions each comprise a planar face of the light transmissive member which faces are inclined relative to each other.

5. Apparatus substantially as described herein with reference to Figs. 1 and 2 or Figs 1 and 3 of the drawings.