SECURITY DEVICE

• US 20100230615A1
• Filed: 03/11/2009
• Published: 09/16/2010
• Est. Priority Date: 02/27/2009
• Status: Active Grant

• Current Assignee: Authentx Incorporated
• Original Assignee: Nanotech Security Corp.
• Inventors: Vendette, Denis Gerard, Stone, A. Oliver, Girouard, Gilles, MacPherson, Charles Douglas
• Granted Patent: US 9,170,417 B2
• Time in Patent Office: Days
• US Class Current: 250/488.100
• CPC Class Codes: B42D 2033/18 Reflecting material
• CPC Class Codes: B42D 2033/20 Coloured material; Pigments or inks
• CPC Class Codes: B42D 2033/26 Liquid-crystal material
• CPC Class Codes: B42D 2033/32 Other material
• CPC Class Codes: B42D 2035/20 Optical effects
• CPC Class Codes: B42D 2035/24 Colours
• CPC Class Codes: B42D 2035/26 Halftone or interference patterns
• CPC Class Codes: B42D 2035/34 Markings visible under particular conditions or containing coded information
• CPC Class Codes: B42D 25/29 Securities; Bank notes
• CPC Class Codes: B42D 25/324 Reliefs
• CPC Class Codes: B42D 25/328 Diffraction gratings; Holograms
• CPC Class Codes: B42D 25/364 Liquid crystals
• CPC Class Codes: B42D 25/378 Special inks
• CPC Class Codes: B42D 25/382 absorbing or reflecting infrared light
• CPC Class Codes: B42D 25/387 absorbing or reflecting ultraviolet light
• CPC Class Codes: B42D 25/47 using adhesives
• CPC Class Codes: B44F 1/10 Changing, amusing, or secret pictures
• CPC Class Codes: D21H 21/48 Elements suited for physical verification, e.g. by irradiation
• CPC Class Codes: G02B 26/001 based on interference in an adjustable optical cavity interference filters G02B5/28; devices or arrangements using multiple reflections in spectrometry or monochromators G01J3/26
• CPC Class Codes: G02B 5/18 Diffraction gratings holographic optical elements G02B5/32, G03H; integrally combined with optical fibres G02B6/02057; for coupling light guides G02B6/34; integrally combined with optical integrated light guides G02B6/12; grating systems G02B27/44
• CPC Class Codes: G02B 5/1861 : Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials G02B5/1809, G02B5/1828, G02B5/1833, G02B5/1838 and G02B5/1847 take precedence
• CPC Class Codes: G02B 5/201 : in the form of arrays
• CPC Class Codes: G02B 5/285 : comprising deposited thin solid films G02B5/281 - G02B5/289 take precedence; multilayered film filters for fibre optic multiplexing G02B6/29361
• CPC Class Codes: G02B 5/32 : Holograms used as optical elements
• CPC Class Codes: G03H 1/0011 : for security or authentication holograms on information-bearing cards B42D25/328; testing papers with holograms G07D7/0032
• CPC Class Codes: G03H 1/0244 : Surface relief holograms replicating hologram without interference recording G03H1/0276
• CPC Class Codes: G03H 1/0256 : having specific functional layer
• CPC Class Codes: G07D 7/003 : using security elements using digital security elements G07D7/004
• CPC Class Codes: G07D 7/1205 : Testing spectral properties

Abstract

A security device for authenticating bank notes, documents and other items, comprises a luminescent material for producing luminescent radiation of first and second wavelengths. The security device includes an optically variable structure for controlling emission of luminescent radiation of at least one of the first and second wavelengths from the security device, the security device being arranged to permit, from an area of the optically variable structure, emission of luminescence of the first and second wavelengths from the security device. The optically variable structure causes the relative emissivity of the security device for luminescent radiation of the first and second wavelengths to change with a change in emission angle, so that the security device produces an angle-dependent colour shift in the emitted luminescent radiation. The optically variable structure may comprise an optical interference stack that controls transmission of luminescent radiation therethrough in response to the wavelength of luminescent radiation.

Claims

1. A device comprising:

luminescent means, which when stimulated, produces luminescent radiation of first and second wavelengths, the second wavelength being different to said first wavelength, said luminescent means including a luminescent material; and

an optically variable structure for controlling emission of the luminescent radiation of at least one of said first and second wavelengths from said device, the device being arranged to permit, from within an area of said optically variable structure, emission of luminescent radiation of said first wavelength from said device at a first angle of emission, and to permit, from said area, emission of luminescent radiation of said second wavelength from said device at one or both of (1) said first angle of emission and (2) a second angle of emission, different to said first angle of emission, and said optically variable structure is arranged to cause, from said area, the emissivity of said device for luminescent radiation of said first or second wavelengths to change relative to the emissivity of the device for luminescent radiation of the other of the first or second wavelengths with a change in emission angle of luminescent radiation from said device from said first emission angle to said second emission angle.

2. A device as claimed in claim 1, wherein said optically variable structure is arranged to control emission of the luminescent radiation of both said first and second wavelengths from said device, and to permit emission of luminescent radiation of said first wavelength from said device at said first angle, and to permit emission of luminescent radiation of said second wavelength at one or both of said first and second angles.

3. A device as claimed in claim 2, wherein said optically variable structure is arranged to permit luminescent emission of said second wavelength from said device at said second angle.

4. A device as claimed in claim 3, wherein said optically variable structure is adapted to reduce the emissivity of the device for luminescent radiation of said second wavelength with a change in emission angle from said second angle towards said first angle.

5. A device as claimed in claim 1, wherein said optically variable structure is adapted to reduce the emissivity of the device for luminescent radiation of said first wavelength with a change in emission angle from said first angle towards said second angle.

6. A device as claimed in claim 1, wherein at least one of (1) at said first emission angle, the emissivity of said device for said luminescent radiation of said first wavelength is greater than the emissivity of said device for luminescent radiation of said second wavelength, and (2) at said second emission angle, the emissivity of said device for said luminescent radiation of said second wavelength is greater than the emissivity of said device for luminescent radiation of said first wavelength.

7. A device as claimed in claim 1, wherein at least one of (1) at said first emission angle, the intensity of luminescence of said first wavelength emitted by said device is greater than the intensity of luminescence from said device of said second wavelength, if any, and (2) at said second emission angle, the intensity of luminescence of said second wavelength emitted by said device is greater than the intensity of luminescence from said device of said first wavelength, if any.

8. A device as claimed in claim 1, wherein the intensity of luminescent emission of one of said first and second wavelengths changes with a change in emission angle from said first angle to said second angle and the change in intensity is greater than any change in the intensity of luminescent emission of the other of the first and second wavelengths with said change in emission angle.

9. A device as claimed in claim 8, wherein the intensity of the other of the first and second wavelengths is substantially constant with said change in emission angle.

10. A device as claimed in claim 1, wherein one or both of said first and second wavelengths are in the visible spectrum.

11. A device as claimed in claim 10, wherein the luminescent radiation emitted at said first angle is sufficiently different from the luminescent radiation emitted at said second angle to produce a change in colour in the emitted luminescent radiation with a change in emission angle from said first angle to said second angle that is detectable by the human eye.

12. A device as claimed in claim 1, wherein one or both of said first and second wavelengths are outside the visible spectrum.

13. A device as claimed in claim 1, wherein said luminescent material is responsive to electromagnetic radiation having a wavelength which is shorter or longer than said first wavelength to generate said luminescent radiation at said first wavelength.

14. A device as claimed in claim 1, wherein said luminescent material is responsive to electromagnetic radiation having a shorter wavelength or a longer wavelength than said second wavelength to generate said luminescent radiation at said second wavelength.

15. A device as claimed in claim 13, wherein said shorter wavelength radiation is ultraviolet (UV) radiation, and said longer wavelength radiation is infrared (IR) radiation.

16. A device as claimed in claim 1, wherein said luminescent means is adapted to produce, when stimulated, luminescent radiation having a third wavelength different from said first and second wavelengths, and at least one of said device and said optically variable structure is adapted to permit emission of luminescent radiation of said third wavelength from said device at one or more of (1) said first emission angle, (2) said second emission angle, and (3) a third angle of emission different from said first and second emission angles.

17. A device as claimed in claim 16, wherein at least one of (1) the optically variable structure is adapted to cause the relative emissivity of the device for luminescent radiation of the first and third wavelengths to change with a change in emission angle from the third emission angle to the first emission angle, (2) the optically variable structure is adapted to cause the relative emissivity of the device for luminescent radiation of the second and third wavelengths to change with a change in emission angle from the third emission angle to the second emission angle and (3) the optically variable structure is adapted to permit emission of luminescent radiation of said third wavelength at one or both of said first and second emission angles.

18. A device as claimed in claim 16, wherein at least one of (1) at said first emission angle, the emissivity of said device for said third wavelength is greater than the emissivity of said device for said second wavelength and (2) at said second emission angle, the emissivity of said device for luminescent radiation of said third wavelength is greater than the emissivity of said device for luminescent radiation at said first wavelength.

19. A device as claimed in claim 16, wherein the optically variable structure causes the emissivity of the device for luminescent radiation of said third wavelength at said third angle to be greater than the emissivity of said device for luminescent radiation of said third wavelength at at least one of said first and second angles.

20. A device as claimed in claim 16, wherein said third wavelength is in the visible spectrum.

21. A device as claimed in claim 20, wherein the luminescent radiation emitted at said third angle is sufficiently different from the radiation emitted at at least one of said first and second angles to produce a change in colour of the emitted luminescent radiation with a change in emission angle from said third emission angle to a respective one of said first and second emission angles that is detectable by the human eye.

22. A device as claimed in claim 16, wherein said luminescent material is responsive to electromagnetic radiation of a shorter or a longer wavelength than said third wavelength to generate said luminescent radiation at said third wavelength.

23. A device as claimed in claim 22, wherein said shorter wavelength radiation is ultraviolet (UV) radiation and said longer wavelength radiation is infrared (IR) radiation.

24. A device as claimed in claim 16, having a solid-to-air interface for emitting said luminescent radiation and which extends along a first direction, and wherein two or more of said first, second and third emission angles lie in the same quadrant defined by said first direction and an axis normal to said first direction.

25. A device as claimed in claim 16, wherein said luminescent means is adapted to generate said luminescent radiation at two or more of said first, second and third wavelengths in response to a common wavelength of stimulating radiation.

26. A device as claimed in claim 1, having a solid-to-air interface for emitting said luminescent radiation and which extends along a first direction, and said first and second angles lie in the same quadrant defined between a first line extending along said first direction and a second line extending outwardly of said interface and orthogonal to said first line.

27. A device as claimed in claim 1, wherein said luminescent means is adapted to generate luminescent radiation at said first and second wavelengths in response to a common wavelength of stimulating radiation.

28. A device as claimed in claim 1, wherein said optically variable structure is at least partially transparent or transmissive to stimulating radiation for stimulating said luminescent material.

29. A device as claimed in claim 28, wherein said optically variable structure is at least partially transparent or transmissive to said stimulating radiation for stimulating radiation directed in first and second directions wherein the first and second directions are angled relative to one another by an angle equal to the difference between said first and second angles.

30. A device as claimed in claim 1, having an interface for defining a solid-to-air interface and wherein said optically variable structure is positioned between said luminescent material and said interface.

31. A device as claimed in claim 30, wherein the extent of said optically variable structure is defined by a second interface opposite said first interface, and said luminescent material is external of said optically variable structure defined between said first and second interfaces.

32. A device as claimed in claim 31, wherein said luminescent material is in the form of a luminescent layer.

33. A device as claimed in claim 31, wherein said luminescent material is immediately adjacent said second interface or is spaced therefrom by one or more intermediate layers.

34. A device as claimed in claim 33, wherein said one or more intermediate layers comprises an adhesive material.

35. A device as claimed in claim 33, wherein said luminescent material is in an adhesive layer.

36. A device as claimed in claim 1, wherein luminescent material which luminesces at at least one of said first and second wavelengths is disposed within the optically variable structure.

37. A device as claimed in claim 1, wherein said optically variable structure controls transmission of said luminescent radiation therethrough based on the wavelength of said luminescent radiation.

38. A device as claimed in claim 1, wherein said optically variable structure is adapted to control the direction of transmission of said luminescent radiation therethrough based on the wavelength of said luminescent radiation.

39. A device as claimed in claim 37, wherein said luminescent material is disposed at a first position on a substrate, said optically variable structure is disposed at a second position on said substrate spaced apart from said first position, and said optically variable structure can be made to overlap said luminescent material by folding said substrate, or otherwise moving said optically variable structure relative to said luminescent material.

40. A device as claimed in claim 1, wherein said optically variable structure comprises any one or more of an optical interference structure, a liquid crystal structure, a micro electrical mechanical system, a diffraction structure and a holographic structure.

41. A device as claimed in claim 1, wherein said optically variable structure comprises an optical interference structure comprising three or more layers of material, each layer being adjacent another said layer and arranged in a stack.

42. A device as claimed in claim 41, wherein said optical interference structure comprises first, second and third layers of material in which the second layer has a refractive index which is either higher or lower than the refractive index of said first and third layers.

43. A device as claimed in claim 40, wherein said optical interference structure is in the form of a continuous sheet or comprises discrete particles or flakes, each particle or flake comprising said three or more layers.

44. A device as claimed in claim 1, wherein said optically variable structure comprises a diffraction structure and a transmissive material spaced from said diffraction structure, and said luminescent material is disposed between said diffraction structure and said transmissive material, wherein said transmissive material is adapted to reflect part of the luminescent radiation produced by said luminescent material towards said diffraction structure and to transmit part of said luminescent radiation therethrough, wherein the intensity of transmitted luminescent radiation is a function of wavelength of said luminescent radiation and angle of emission thereof from said device.

45. A device as claimed in claim 44, wherein said diffraction structure comprises a reflective material.

46. A device as claimed in claim 44, wherein said diffraction structure comprises a holographic diffraction structure.

47. A device as claimed in claim 44, wherein said diffraction structure is defined by an embossed substrate.

48. A device as claimed in claim 1, wherein said optically variable structure comprises a reflector, an absorber, and a support for enabling a spacing between the reflector and absorber to be varied, and said luminescent material is disposed between the reflector and the absorber, wherein said absorber controls the admittance of said reflector in response to changes in the spacing therebetween.

49. A device as claimed in claim 48, wherein at a first spacing, at least one of (1) the emissivity of the device for luminescent radiation of said first wavelength at said first emission angle is greater than the emissivity of said device for luminescent radiation of said first wavelength at said first angle at a second spacing, different from said first spacing and (2) the emissivity of the device for luminescent radiation of said second wavelength at said second emission angle is greater than the emissivity of said device for luminescent radiation of said second wavelength at said second emission angle at a second spacing, different from said first spacing.

50. A device as claimed in claim 48, wherein, for a first positional relationship between said reflector and absorber, the emissivity of said device for luminescent radiation of a predetermined wavelength has a first dependence on emission angle and at a second positional relationship between the reflector and absorber, the emissivity of said device for luminescent radiation of said predetermined wavelength has a reduced angular dependence relative to said first angular dependence.

51. A device as claimed in claim 1, wherein said optically variable structure comprises a member having opposed upper and lower major surfaces, and opposed first and second edges, wherein said luminescent means comprises a luminescent material on or above said upper major surface for producing luminescent radiation of said first wavelength, and a luminescent material on or below said lower major surface for producing luminescent radiation of said second wavelength.

52. A device as claimed in claim 51, wherein said optically variable structure comprises a plurality of said members, adjacent members being spaced apart to provide a gap therebetween for the passage of luminescent radiation from the luminescent material for producing luminescent radiation of said second wavelength.

53. A device as claimed in claim 52, comprising reflector means for reflecting luminescent radiation from said luminescent material for producing luminescent radiation of said second wavelength into the gap for emission from said device.

54. A device as claimed in claim 51, having a lower surface or interface, and the lower major surface of the or each member is angled upwardly relative to said lower surface or interface of said device.

55. A device as claimed in claim 51, wherein the or each member is one of (1) opaque, (2) partially opaque or (3) transparent to luminescent radiation of said second wavelength.

56. A device as claimed in claim 51, wherein the or each member has the form of a flake.

57. A device as claimed in claim 1, wherein said optically variable structure comprises a first portion having an area which faces in a first direction and a second portion having an area which faces in a second direction different from said first direction, and wherein said means comprises first means in said first area, which, when stimulated, emits luminescent radiation of said first wavelength and second means in said second area, which when stimulated, emits luminescent radiation of said second wavelength.

58. A device as claimed in claim 57, wherein the device extends along a plane, said first area faces away from one side of a line orthogonal to said plane and the second area faces away from one of (1) the other side of said line and (2) the same side of said line.

59. A device as claimed in claim 58, wherein said optically variable structure further comprises a blocking formation for reducing, at a predetermined angle, the emissivity of the device for luminescent radiation of one of said first and second wavelengths relative to the emissivity of the device for luminescent radiation at the other of said first and second wavelengths.

60. A device as claimed in claim 57, wherein said optically variable structure comprises a series of side-by-side ridge formations each having first and second sides, wherein a side of each ridge formation comprises one of said first and second portions.

61. A device as claimed in claim 1, wherein said luminescent means comprises one or more first regions of a luminescent material which produces luminescent radiation of said first wavelength, one or more regions of a luminescent material which produces luminescent radiation of said second wavelength, and wherein said optically variable structure comprises one or more first areas and one or more second areas, wherein said one or more first areas has a higher reflectivity than said one or more second areas for radiation of said first wavelength, and said one or more first areas are arranged to provide a background for said one or more first regions at said first angle of emission and said one or more second areas are arranged to provide a background for said one or more first regions at said second angle of emission, and wherein the background area for said one or more first regions provided by said one or more first areas relative to the background area for said one or more first regions provided by said one or more second areas changes with changes in said emission angle from said first angle to said second angle.

62. A device as claimed in claim 61, wherein said one or more second areas are arranged to provide a background for said one or more second regions at said first angle of emission and said one or more first areas are arranged to provide a background for said one or more second regions at said second angle of emission, and the background area for said one or more second regions provided by said one or more second areas relative to the background area provided by said one or more first areas changes with changes in said emission angle from said first angle to said second angle.

63. A device as claimed in claim 61, wherein said one or more first areas and said one or more second areas are positioned below and spaced from said one or more first regions and said one or more second regions.

64. A device as claimed in claim 61, comprising a first array of alternating first and second regions and a second array of alternating first and second areas wherein said second array is positioned below and spaced from said first array.

65. A device as claimed in claim 61, wherein the device extends along a plane, and at least one of (1) said one or more first region is registered with said one or more first area in a direction orthogonal to said plane and (2) said one or more second regions are registered with said one or more second areas in a direction orthogonal to said plane.

66. A device as claimed in claim 1, wherein said optically variable structure is positioned to provide a background to said luminescent material, and wherein at said first emission angle, said optically variable structure reflects luminescent radiation at said first wavelength, and wherein the reflectivity of said optically variable structure for luminescent radiation of said first wavelength changes with a change in emission angle from said first angle.

67. A device as claimed in claim 66, wherein the reflectivity of said optically variable structure for luminescent radiation of said first wavelength decreases with said change.

68. A device as claimed in claim 66, wherein the reflectivity of said optically variable structure for luminescent radiation of said first wavelength at said second angle is less than the reflectivity of said optically variable structure for said luminescent radiation of said first wavelength at said first angle.

69. A device as claimed in claim 66, wherein, at said second emission angle, said optically variable structure reflects luminescent radiation of said second wavelength, and wherein the reflectivity of said optically variable structure for luminescent radiation of said second wavelength changes with a change in emission angle away from said second angle.

70. A device as claimed in claim 69, wherein the reflectivity of the optically variable structure for said second wavelength decreases with said change.

71. A device as claimed in claim 69, wherein the reflectivity of the optically variable structure for said luminescent radiation of said second wavelength at said first angle of emission is less than the reflectivity of said optically variable structure for said luminescent radiation of said second wavelength at said second emission angle.

72. A device as claimed in claim 1, wherein luminescent means comprises a material which luminesces at said first wavelength and a material which luminesces at said second wavelength, and said luminescent material which luminesces at said first wavelength is in the same layer as the luminescent material which luminesces at the second wavelength or said luminescent material which luminesces at said first wavelength is in a different layer to the luminescent material which luminesces at said second wavelength.

73. A device as claimed in claim 72, wherein the luminescent material which luminesces at said first wavelength has a boundary defining a first predetermined shape.

74. A device as claimed in claim 73, wherein the luminescent material which luminesces at said second wavelength has a boundary defining a second predetermined shape.

75. A device as claimed in claim 74, wherein at least one of the form and size of said second predetermined shape is different to that of said first predetermined shape.

76. A device as claimed in claim 75, wherein said first and second predetermined shapes at least partially overlap one another.

77. A device as claimed in claim 76, wherein at least one of (1) in the overlapping region, the luminescent materials which luminesce at said first and second wavelengths comprise respective different layers and (2) in the overlapping region, the first and second materials are in the same layer, and in a non-overlapping region, the material which luminesces at the first or second wavelength produces substantially the same luminescent emission per unit area as the material which luminesces at the respective first or second wavelength in the overlapping region.

78. A device as claimed in claim 76, wherein the optically variable structure overlaps at least part of the overlapping region of said luminescent materials.

79. A device as claimed in claim 78, wherein at least one of (1) a non-overlapping part of the luminescent material which luminesces at one of the first and second wavelengths extends beyond a boundary of the optically variable structure, (2) an overlapping part of the luminescent materials which luminesce at said first and second wavelengths extend beyond a boundary of the optically variable structure.

80. A device as claimed in claim 1, including at least one of (1) luminescent material which extends or is positioned beyond a boundary of said optically variable structure, and (2) a material exhibiting one or more predetermined colour(s) positioned beyond said boundary.

81. A device as claimed in claim 80, wherein at least one of (1) said luminescent material that extends or is positioned beyond said boundary produces, when stimulated, luminescent emission of one or both of said first and second wavelength(s) or a different wavelength to said first and second wavelengths, (2) the emissivity of the security device for luminescent emission from the luminescent material that extends or is positioned beyond said boundary is substantially independent of emission angle or has a different angular dependence than luminescent emission from the luminescent material within the boundary of the optically variable structure, and (3) a predetermined colour positioned beyond said boundary substantially matches the colour of luminescent emission of said first or second wavelengths or matches a colour of luminescence emitted from an area of said optically variable structure.

82. A device as claimed in claim 1, wherein at least a portion of the optically variable structure is transmissive to visible light.

83. A device as claimed in claim 82, wherein said portion is adapted to control transmission of visible light therethrough and its direction based on the wavelength of said light.

84. A device as claimed in claim 83, wherein said optically variable structure comprises first and second opposed interfaces, and at a first predetermined angle of emission from one of said interfaces, permits emission from said interface of visible light at a first wavelength, and, at a second predetermined angle of emission from said interface, different from said first predetermined angle permits emission of visible light at a second wavelength different to said first wavelength, and the transmissivity of said optically variable structure for visible light of said first wavelength relative to the transmissivity of said optically variable structure for visible light of said second wavelength changes with a change in emission angle from said first predetermined angle to said second predetermined angle.

85. A device as claimed in claim 1, wherein said optically variable structure is adapted to cause the relative emissivity of the device for luminescent radiation of said first and second wavelengths to change with a change in the direction or angle of incidence of stimulating radiation on said device for stimulating said luminescent means.

113. A bank note comprising a substrate and a device as claimed in claim 1.

118. A method as claimed in claim 116, wherein the security device comprises a device of claim 1.

86. A device comprising a luminescent material, which, when stimulated, emits luminescent radiation, and an optically variable structure, which controls the intensity of emission of said luminescent radiation from said device with angle of emission of said luminescent radiation from said device, based on the wavelength of said luminescent radiation.

87. A device as claimed in claim 86, wherein said luminescent radiation includes luminescent radiation of a first wavelength, and the optically variable structure permits emission of luminescent radiation of said first wavelength from said device at a first emission angle and changes the intensity of said luminescent radiation of said first wavelength with a change in emission angle from said first angle.

88. A device as claimed in claim 87, wherein the intensity of luminescent radiation of said first wavelength emitted from said device decreases with at least one of (1) a change in a first direction of emission angle away from said first angle and (2) a change in a second direction of emission angle away from said first angle, wherein said second direction of change is opposite to said first direction of change.

89. A device as claimed in claim 86, wherein said luminescent radiation includes luminescent radiation of a second wavelength, different to said first wavelength, and the optically variable structure permits emission of luminescent radiation of said second wavelength from said device at a second angle, different from said first angle, and changes the intensity of said luminescent radiation of said second wavelength with a change in emission angle from said second angle.

90. A device as claimed in claim 89, wherein the intensity of luminescent radiation of said second wavelength emitted from said device decreases with at least one of (1) a change in a first direction of emission angle away from said second angle and (2) a change in a second direction away from said second angle, wherein the direction of said second change is opposite to the direction of said first change.

91. A device as claimed in claim 86, comprising an interface for emitting said luminescent radiation from said device, wherein said optically variable structure is disposed between said interface and said luminescent material and said luminescent radiation is transmitted through said optically variable structure.

92. A device as claimed in claim 91, wherein the transmissivity of said optically variable structure for luminescent radiation of a predetermined wavelength varies with the angle at which said luminescent radiation is transmitted through said optically variable structure.

93. A device as claimed in claim 86, comprising an interface for emitting luminescent radiation from said device, and wherein said luminescent material is disposed between said optically variable structure and said interface.

94. A device as claimed in claim 93, wherein the reflectivity of said optically variable structure for luminescent radiation of a predetermined wavelength varies with the angle of reflection of luminescent radiation from said optically variable structure.

95. A device as claimed in claim 94, wherein the intensity of luminescent radiation of a first wavelength reflected by said optically variable structure decreases with a change of reflection or emission angle away from a first angle.

96. A device as claimed in claim 95, wherein the intensity of luminescent radiation of a second wavelength, different to said first wavelength and reflected from said optically variable structure decreases with a change in reflection or emission angle from a second angle different to said first angle.

97. A device as claimed in claim 86, wherein one or both of said first and second wavelengths are in the visible spectrum.

98. A device as claimed in claim 86, wherein said luminescent material is formed as a layer.

99. A device as claimed in claim 86, wherein said optically variable structure comprises an optical interference structure.

100. A device as claimed in claim 99, wherein said optical interference structure comprises three or more layers of material formed as a stack.

101. A device as claimed in claim 86, wherein at least a portion of said luminescent material is within said optically variable structure.

102. A device as claimed in claim 86, wherein said optically variable structure comprises any one or more of a liquid crystal structure, a micro electrical mechanical system, a diffraction structure and a holographic structure.

103. A security device including an optically variable structure comprising first and second opposed electromagnetic radiation transmissive interfaces,

an electromagnetic radiation transmissive medium between said first and second interfaces, and control means for controlling the intensity of radiation emitted from one of said first and second interfaces that is passed through the other of said first and second interfaces and said medium and the emission angle of said electromagnetic radiation from said security device, based on the wavelength of said electromagnetic radiation.

104. A security device as claimed in claim 103, wherein said control means permits transmission of radiation of first and second wavelengths from said other interface to said one interface, said second wavelength being different from said first wavelength and permits emission of said radiation of said first wavelength from said security device at a first angle of emission and permits emission of radiation of said second wavelength from said security device at a second angle of emission, different from said first angle, and said control means causes the emissivity of said security device for said radiation of said first or second wavelengths to change relative to the emissivity of said security device for radiation of the other of said first or second wavelengths with a change in emission angle from said first emission angle to said second emission angle.

105. A security device as claimed in claim 104, wherein said control means is adapted to cause at least one of (1) the emissivity of said security device for radiation of said first wavelength at said first angle to be greater than the emissivity of said security device for radiation of said first wavelength at said second angle and (2) the emissivity of said security device for radiation of said second wavelength at said second angle to be greater than the emissivity of said security device for radiation of said second wavelength at said first angle.

106. A security device as claimed in claim 103, wherein one or both of said first and second wavelengths are in the visible spectrum.

107. A security device as claimed in claim 106, wherein both of said first and second wavelengths are in the visible spectrum, said control means causes a first colour, including said first wavelength to be emitted from said security device at a first emission angle and a second colour different from said first colour and including said second wavelength to be emitted from said security device at a second emission angle different from said first emission angle.

108. A security device as claimed in claim 104, wherein one or both of said first and second wavelengths are outside the visible spectrum.

109. A security device as claimed in claim 103, wherein said first and second opposed interfaces are substantially parallel to one another.

110. A security device as claimed in claim 103, wherein said control means comprises structure of said radiation transmissive medium.

111. A security device as claimed in claim 110, wherein said structure comprises an optical interference structure.

112. A security device as claimed in claim 111, wherein said optical interference structure comprises three or more layers of material.

122. A method as claimed in claim 119, wherein said security device comprises a device of claim 103.

114. A device comprising:

luminescent means, which, when stimulated, produces luminescent radiation of first and second wavelengths, the second wavelength being different to the first wavelength, said luminescent means including a luminescent material; and

an optically variable structure for controlling emission of luminescent radiation of at least one of the first and second wavelengths from the device, the device being arranged to permit, from within an area of the optically variable structure, emission of luminescent radiation of the first and second wavelength from the device, wherein the optically variable structure is arranged to cause, from said area, the relative emissivity of the device for luminescent radiation of the first and second wavelengths to change with at least one or both of (1) a change in emission angle of luminescent radiation from said device and (2) a change in the direction or angle of incidence of stimulating radiation on said device for stimulating said luminescent means.

115. A method of authenticating a security device which includes a luminescent material, which, when stimulated, produces luminescent radiation of first and second wavelengths, the second wavelength being different to the first wavelength, the method comprising the steps of:

exposing the security device to stimulating radiation for stimulating the luminescent material to produce luminescent radiation of the first and second wavelengths, and observing or detecting in the luminescent radiation from the security device a change in intensity of luminescent radiation of the first or second wavelengths relative to the intensity of the luminescent radiation of the other of the first or second wavelengths with at least one or both of (1) a change in emission angle from the security device and (2) a change in direction or angle of incidence of stimulating radiation on the security device for stimulating said luminescent material.

116. A method of authenticating a security device which includes a luminescent material, the method comprising the steps of exposing said security device to stimulating radiation for stimulating said luminescent material to produce luminescent radiation and observing a colour shift in the luminescent radiation from said security device with at least one of (1) a change in emission angle from said security device and (2) a change in the direction or angle of incident stimulating radiation on said security device.

117. A method as claimed in claim 116, comprising observing a first dominant colour at a first angle of emission and a second dominant colour different from said first dominant colour at a second angle of emission different from said first angle.

123. A method as claimed in claim 116, further comprising at least one of (1) comparing the observed or detected change or colour shift with a predetermined change or colour shift and (2) comparing the observed or detected colour with a predetermined colour.

119. A method of authenticating a security device, the security device having first and second opposed electromagnetic radiation transmissive interfaces and an electromagnetic radiation transmissive medium between said first and second interfaces, the method comprising the steps of exposing said first interface to electromagnetic radiation and observing or detecting from the second interface a change in relative intensities of first and second wavelengths of radiation emitted from the security device with a change in emission angle from said security device.

120. A method as claimed in claim 119, wherein the step of observing comprises observing a colour shift in radiation emitted from said security device with said change in emission angle.

121. A method as claimed in claim 120, wherein said step of observing comprises observing a first dominant colour at a first emission angle and a second dominant colour different from said first dominant colour at a second emission angle, different from said first emission angle.