Classifications

• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
  • G06V40/00—Recognition of biometric, human-related or animal-related patterns in image or video data
  • G06V40/10—Human or animal bodies, e.g. vehicle occupants or pedestrians; Body parts, e.g. hands
  • G06V40/12—Fingerprints or palmprints
  • G06V40/13—Sensors therefor
  • G06V40/1324—Sensors therefor by using geometrical optics, e.g. using prisms

Definitions

• the present invention relates to apparatus and a method for fingerprint imaging.
• European Patent Application publication number 0,348,182.
• a method for forming a direct image of a fingerprint comprising; introducing collimated light into a light transmitting member having first and second parallel major surfaces such that the light interacts with the first major surface thereof, in contact with which is a fingerprint to be imaged, the light then reflecting from the first major surface towards the second major surface, optically coupled with which is a direct image producing means, thereby to form a direct image of the fingerprint.
• both the fingerprint and image thereof lie in parallel planes by virtue of the parallel major surfaces of the light transmitting means, there is no requirement for compensation of'kerbstone' distortion and no need for a large depth-of-field in the image detector means as is the case with prior-art non-direct image formation methods.
• 'kerbstone' distortion (which is introduced by obliquely viewing an object) will be assumed to be understood by persons skilled in this art.
• apparatus for forming a direct image of a fingerprint comprising a light transmitting member having a first major surface for receiving a fingerprint and a second major surface, arranged substantially parallel to the first major surface and in optical cooperation with a direct image producing means, the apparatus arranged for receiving collimated light via the second major surface and reflecting from the first major surface thereby to produce a direct image of a fingerprint.
• the direct image producing means comprises a light. scattering surface such as a ground glass plate member or the like or a very thin layer of paint which is non-opaque, or the like.
• the direct image producing means is optically-coupled with the light transmitting member, that is, their respective refractive indices match, or they are coupled together using known refractive index matching means, such as a suitable gel, oil or adhesive, then the image producing means is receptive to light impinging thereon and acts as a back- projection viewing screen or "secondary" light source.
• the direct image producing means comprises fluorescent material. This may take the form of a dye-doped plastics material.
• the collimated light is introduced into the light transmitting member via a prism means.
• Advantageously light is reflected from the first major surface toward the second major surface by total internal reflection.
• Figure 1 shows a schematic representation of a prior art fingerprint imaging device
• Figure 2 shows a schematic representation of a first embodiment of the present invention
• Figure 3 shows a schematic representation of a further embodiment of the present invention
• Figure 4 shows a schematic illustration of yet a further embodiment of the present invention
• Figure 5 shows a schematic illustration of yet a further embodiment of the present invention.
• a light source 2 is effective to illuminate a transparent plate 4.
• a fingerprint characterised by ridges 6 and recesses 8 In contact with the plate 4 is a fingerprint characterised by ridges 6 and recesses 8, the ridges 6 being in contact with the surface of plate 4 and the recesses 8 being spaced from the surface of plate 4 by a series of air gaps.
• Light entering the plate 4 is randomly reflected by the ridges 6 and is thereby scattered in all directions, since an air gap is not formed at ridges 6.
• Scattered light beams 10 which satisfy the total internal reflection condition, that is, have an angle of incidence greater than the critical angle, are totally reflected by the lower surface of plate 4. Such beams, only one of which is shown in Figure 1, are then totally reflected by the lower surface of plate 4 and then, following one or more further total internal reflections at upper and lower surfaces, pass through some form of optical element 12, for example a hologram or lens, to be detected by an image sensor 14 arranged outside of the transparent plate 4. This is necessary because the system does not form a direct image of the fingerprint.
• optical element 12 for example a hologram or lens
• FIG. 2 illustrates one embodiment of the present invention wherein relatively poor contrast images, as are obtained by such devices as shown in Figure 1, are at least alleviated.
• the embodiment in Figure 2 comprises a light source 16 and collimating lens 18 arranged to produce a broad collimated light beam 20.
• This light 20 is introduced into a light transmitting member 22, which is in the form of a transparent substrate such as a glass or plastics plate, via a prism means 24 such as a glass prism optically coupled to one major surface 26 of the light transmitting member 22 by a refractive index coupling medium.
• a prism means 24 such as a glass prism optically coupled to one major surface 26 of the light transmitting member 22 by a refractive index coupling medium.
• the prism 24 can be formed integrally with the light transmitting member 22.
• a second major surface 28 of the light transmitting member is arranged parallel to major surface 26.
• a finger 30 from which a fingerprint image is to be obtained.
• fingerprint is to include all anatomical attributes characteristic of an individual which are susceptible to the present method such as thumb-prints, palm-prints and foot-prints. The finger is chosen simply as it is the most convenient by way of example.
• the angle of incidence, q, at which the collimated light 20 strikes the upper surface 28 against which is placed the finger 30 is arranged to be greater than the critical angle for the medium from which the light transmitting member 22 is formed, such as by appropriately positioning the light source 16 relative to that area of the major surface 28 designated to receive the finger.
• a direct image producing means 38 in the form of a transparent fluorescent plastics layer material, is in optical cooperation with the lower surface 26 of member 22.
• the direct image producing means 38 need not be in direct contact with surface 26, in which case some form of refractive index coupling medium may be required.
• Reflected light 36 passes through the light transmitting member 22 via lower surface 26 and impinges upon the direct image producing means 38.
• the reflected light 36 is spatially modified as compared with the incident light 20 due to the spatially dependent absorption processes which occur where the light is internally reflected at the upper surface 28 caused by the pressure of ridges 34.
• the fluorescent dye within the direct image producing means 38 absorbs the reflected light 36 incident thereon within a specific wavelength range (governed by the fluorescent dye) and then re-emits light 40 in a characteristic (of the dye) narrow, longer-wavelength range, as determined by the fluorescent dye.
• the spatial modulation at this stage is unaffected, that is, the image of the light is unchanged as between that of the fingerprint on upper surface 28 and that produced by the direct image producing means 38.
• Fluorescence of the dye in the direct image producing means 38 may be enabled not only by the wavelength characteristics of the incident light, but also by the intensity of this light.
• the image produced will have light areas (caused by fluore ⁇ cence) corresponding to those points on the upper surface 28 underlying fingerprint recesses 32; and dark areas (caused by non-fluorescence) corresponding to those points on the upper surface 28 in contact with fingerprint ridges 34. It will be appreciated that there will be high contrast between these light and dark areas as compared with prior art devices. Because the lower surface 26 and upper surface 28 of the light transmitting member 22 are parallel, then it will be seen that if the direct image producing means 38 is planar, then the plane of emission of light 40 egressing from the direct image producing means 38 is parallel to the object plane; that is, the upper surface 28.
• the collimated light 20 is introduced into the light transmitting member by way of a prism means in the form of a Fresnel prism strip 44.
• a prism means in the form of a Fresnel prism strip 44.
• Figure 3 details the image producing means being in the form of a ground glass plate 46 or thin layer of non-opaque paint applied to the surface 26. Whilst this form of image producing means does not work by fluorescence, the ground glass plate 46 scatters or diffuses the reflected light 36 incident thereon and hence also produces a direct image of the fingerprint.
• Figure 5 illustrates a further embodiment of the present invention in which the spatially modulated light 36, 36a is permitted to undergo a further total internal reflection at surface 26 before being imaged onto the ground glass plate 46.
• the direct image produced is viewed by video camera 42 from lower surface 26 via the light transmitting member 22.
• This particular embodiment allows for the finger- placement surface to be laterally displaced from the imaging screen thereby providing for a compact imaging system. It will be appreciated by those skilled in this art that the present invention provides a compact, relatively slim imaging system by suitable choice of the optical components described herebefore.
• the image plane that is the fluorescent plastics layer or ground glass or the like is accessible.
• any optical calibration which may be required, for example alignment of video camera 42 may be achieved by simply inserting some form of reference grid or "test card" over the direct image plane.
• the present invention requires no focussing or correction elements or the like due to the formation of a direct image. Furthermore, because the invention is relatively simple optically, alignment procedures and assembly costs are not extensive.

Applications Claiming Priority (2)

Publications (1)

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• 1990
  • 1990-12-20 GB GB909027718A patent/GB9027718D0/en active Pending
• 1991
  • 1991-12-20 EP EP92901705A patent/EP0563178A1/de not_active Withdrawn
  • 1991-12-20 JP JP4502149A patent/JPH06504637A/ja active Pending
  • 1991-12-20 WO PCT/GB1991/002295 patent/WO1992011608A1/en not_active Application Discontinuation
  • 1991-12-20 CA CA002094599A patent/CA2094599A1/en not_active Abandoned

Non-Patent Citations (1)

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