Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
  • H04N1/04—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa
  • H04N1/10—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using flat picture-bearing surfaces
  • H04N1/1013—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using flat picture-bearing surfaces with sub-scanning by translatory movement of at least a part of the main-scanning components
  • H04N1/1017—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using flat picture-bearing surfaces with sub-scanning by translatory movement of at least a part of the main-scanning components the main-scanning components remaining positionally invariant with respect to one another in the sub-scanning direction
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
  • G02B27/4233—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect having a diffractive element [DOE] contributing to a non-imaging application
  • G02B27/4238—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect having a diffractive element [DOE] contributing to a non-imaging application in optical recording or readout devices
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
  • H04N1/024—Details of scanning heads ; Means for illuminating the original
  • H04N1/028—Details of scanning heads ; Means for illuminating the original for picture information pick-up
  • H04N1/03—Details of scanning heads ; Means for illuminating the original for picture information pick-up with photodetectors arranged in a substantially linear array
  • H04N1/0301—Details of scanning heads ; Means for illuminating the original for picture information pick-up with photodetectors arranged in a substantially linear array using a bent optical path between the scanned line and the photodetector array, e.g. a folded optical path
  • H04N1/0303—Details of scanning heads ; Means for illuminating the original for picture information pick-up with photodetectors arranged in a substantially linear array using a bent optical path between the scanned line and the photodetector array, e.g. a folded optical path with the scanned line and the photodetector array lying in non-parallel planes
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N2201/00—Indexing scheme relating to scanning, transmission or reproduction of documents or the like, and to details thereof
  • H04N2201/024—Indexing scheme relating to scanning, transmission or reproduction of documents or the like, and to details thereof deleted
  • H04N2201/02452—Arrangements for mounting or supporting elements within a scanning head
  • H04N2201/02454—Element mounted or supported
  • H04N2201/02456—Scanning element, e.g. CCD array, photodetector

Definitions

• This invention relates to an optical device and to a method of imaging an object.
• a large object such as a rather extended, planar object is to be imaged onto a detector, such as a CCD (a charge-coupled device, as used in, for example, digital cameras), then given the limited field of view this requires a minimum building height.
• a detector such as a CCD (a charge-coupled device, as used in, for example, digital cameras)
• CCD charge-coupled device
• the building height of the setup shown in Figure 1 can be reduced by using a 45 degrees mirror, as illustrated in Figure 2.
• the light path is folded using a simple 45-degrees mirror. This is a known arrangement used in devices such as flat bed scanners and overhead projectors.
• the reduction in building height which is achieved is limited and results in a building height comparable to the object size, which in many applications is overly bulky.
• an optical device comprising a body, the body housing a detector, a diffraction grating and a lens for focusing light reflected by the diffraction grating onto the detector.
• an imaging method comprising receiving light from an object, reflecting the light with a diffraction grating, focusing the light with a lens, and detecting the resulting focused light at a detector.
• an optical device for imaging an object comprising a body, the body housing a detector, the detection plane substantially at right angles to the object, a variable focus lens for focusing light onto the detector, and means for controlling the focus of the lens.
• an imaging method for imaging an object comprising receiving light from the object, focusing the light with a lens, scanning the object by varying the focus of the lens and detecting the resulting focused light at a detector.
• a first solution is the use of planar imaging using a diffraction grating (preferably with a variable pitch) to compact the light path of an imaging system.
• the second solution is to use a variable focus lens to scan the object being imaged.
• a quasi-planar imaging system can be obtained with a large field of view.
• the body is provided with a window, the window for allowing light to pass from outside of the optical device to the diffraction grating. This allows the optical device to be a single self contained unit which can be used in a standalone situation, for example as a banknote checking device.
• the device further comprises a source of electromagnetic radiation for illuminating the object to be imaged by the detector.
• the electromagnetic radiation may be visible light or could be, for example, infra-red radiation.
• the latter is useful in some verification systems, where, for example, invisible infra-red sensitive patterns are present on banknotes as a security feature.
• the diffraction grating in the optical device, has a variable pitch.
• a variable pitch on the diffraction grating helps to correct the distortion that would occur, if the pitch of the diffraction grating is regular.
• the pitch of the diffraction grating is regular across its width, then the image reflected by the grating suffers distortion, and this is corrected by having a grating with a variable pitch.
• the sum of the exit and entrance angles of the diffraction grating is approximately ninety degrees.
• a quasi-planar optical system is achieved if the sum of the exit and entrance angles is ninety degrees.
• the lens comprises a low numerical aperture lens.
• a low numerical aperture (NA) lens reduces the occurrence of certain types of optical error that can occur in a system that uses a diffraction grating.
• the incidence of coma and astigmatism is reduced by the use of a low NA lens.
• the detector is aligned at an angle that is non-perpendicular to the optical axis of the lens.
• the tilting of the detector corrects a further error introduced by the diffraction grating, which is particularly apparent if the diffraction grating has a regular, rather than variable, pitch.
• the imaging by the diffraction grating will add optical strength (as a function of the entrance and exit angles), which results in an image that will be tilted with respect to a plane perpendicular to the optical axis of the lens.
• the tilting of the detector relative to the axis through the lens corrects this imaging error, without the need for any processing to correct this error.
• the optical device further comprises one or more optical components, such as a cylindrical lens, located between an object to be imaged and the detector, the optical components for changing magnification of the light in one (x or y) direction only.
• one or more optical components such as a cylindrical lens, located between an object to be imaged and the detector, the optical components for changing magnification of the light in one (x or y) direction only.
• a standard diffraction grating will have different magnification in the x and y directions (anamorphic magnification).
• the use of one or more optical components in the optical device will correct this difference in magnification.
• FIG. 1 is a schematic diagram of a prior art optical device
• Figure 2 is a schematic diagram of a different prior art optical device
• Figure 3 is a schematic diagram of a first embodiment of an optical device
• Figure 4 is a diagram showing the effect of anamorphic magnification on an object
• Figure 5 is a schematic diagram of the optical device of Figure 3, with the addition of cylindrical optics to correct anamorphic magnification
• Figure 6 is a diagram showing the effect of image distortion on an object due to imaging using a diffraction grating
• Figure 7 is a diagram similar to Figure 6, showing the correction of the image distortion
• Figure 8 is a diagram similar to Figure 7 showing the correction of the image distortion and the anamorphic magnification
• Figure 9 is a further schematic diagram of the optical device of Figure 3 showing the tilt of the image plane with respect to the optical lens of the imaging lens,
• Figure 10 is a graph showing the relationship between grating pitch, light wavelength, and exit angle from the grating for a specific geometrical arrangement of the optical device
• Figure 11 is a further schematic diagram of the optical device of Figure 3 (of which Figure 10 shows the variable grating pitch for this geometrical arrangement),
• Figure 12 is a schematic diagram of the diffraction grating of the optical device of Figure 3.
• Figure 13 is a schematic diagram illustrating the tilt of an image plane with respect to the optical axis of a lens
• Figure 14 is a series of top plan and side views of the optical device of Figure 3 illustrating reflection by the diffraction grating, in case of a variable pitch grating,
• Figure 15 is a schematic diagram illustrating four different image spot sizes produced by the optical device of Figure 3, for four respective aperture sizes
• Figure 16 is a schematic diagram of a second embodiment of the optical device
• Figure 17 is a schematic diagram showing an adaptation of the optical device of Figure 16.
• Figures 1 and 2 are mentioned above in the discussion of the prior art. Effectively they show two alternative solutions to the problem of imaging a relatively large planar object 1 via a lens 2 onto a detector 3.
• the object 1 is imaged directly onto the detector 3, which results in an optical device that is cumbersome and impractically large in many situations.
• Figure 2 shows an improved device, which uses a 45 degrees mirror 4 to fold the optical path to reduce the overall size of the device.
• the device of Figure 2 is still of a size that is unnecessarily bulky for many types of large planar objects that need to be imaged (such as banknotes and security passes).
• Figure 3 shows an optical device 10 which comprises a body 12.
• the body 12 houses a detector 14, a diffraction grating 16 and a lens 18 for focusing light reflected by the diffraction grating 16 onto the detector 14.
• the body 12 of the optical device 10 is provided with a window 20.
• the window 20 is for allowing light to pass from an object 22, outside of the optical device 10, to the diffraction grating 16.
• the optical device 10 is for capturing an image of an object 22, such as a banknote or security pass, that is relatively planar.
• a diffraction grating can be manufactured in a number of different ways, one common method being to score lines on a thin sheet of glass.
• the pitch of the diffraction grating is the spacing between the scored lines.
• the diffraction grating 16 used in the optical device 10 may have an irregular (variable) pitch, which is discussed in more detail below.
• the optical system can be folded by 90 degrees, whereas the field in the direction perpendicular to the grating lines is decreased considerably (anamorphic magnification).
• the diffraction grating 16 does not reflect light in the same manner as a mirror.
• the angles of incidence and reflection are equal, whereas for a diffraction grating this is not the case.
• the structure of the optical device 10 allows the object 22 to be imaged accurately at the detector 14, without the need for a large building height.
• a drawback of the optical device 10 of Figure 3 is the angular dispersion of the diffraction grating 16, which limits the operation of the optical system 10 to a single wavelength.
• the device 10 can be used, for example, in an infrared security imaging system, validating the IR-watermarks on paper money.
• an infrared security imaging system validating the IR-watermarks on paper money.
• only one colour is used (illuminated by an infrared LED) and imaging quality is not an issue.
• low building height is essential when used, for example, at supermarket cash registers, making the proposed optical system highly suitable.
• a diffraction grating 16 in the optical device 10 results in an anamorphic magnification of the object 22, effectively reducing the field of view of the optical system in one direction.
• the anamorphic magnification is determined by the grating angle ⁇ .
• Figure 4 illustrates the effect of anamorphic magnification, in which a square object 22 would be imaged at the detector 14 as a rectangular image 23.
• the diffraction grating 16, when reflecting light from the object 22, will give different magnification in the x and y directions (the y direction being perpendicular to the grooves of the diffraction grating).
• Figure 5 shows one way that this can be corrected, which is to include in the light path from the grating 16 to the detector 14 cylindrical optics 24 to compress the image 23 in the x direction.
• the optics 24 have to be matched to the grating 16, and when correctly matched, the image 23 received by the detector 14 will have the same x/y ratio as the original object 22.
• the optics 24 can be used in either of two ways, stretching the image in one direction, or compressing it in the other direction.
• the diffraction grating 16 in the preferred embodiment, has a variable pitch.
• the object 22 would be distorted, as perceived by the detector 14.
• Inherent to grating imaging is a distortion of the image perpendicular to the groove direction.
• Figure 7 shows how the image 23 would look when the preferred embodiment of the optical device 10 is used, in which the diffraction grating 16 in the device 10 has a variable pitch.
• the image 23 received by the detector 14 is no longer distorted and although (if no cylindrical optics 24 are present) anamorphic magnification will occur, the different magnifications in the x and y directions will be regular, that is, without any image distortion.
• the tuning of the pitch of the diffraction grating 16 is discussed in more detail below, with reference to Figure 12.
• Figure 8 shows the image received by the detector 14 when the cylindrical optics is present in the device 10 and when the diffraction grating 16 has a variable pitch.
• the image 23 is now a scale representation of the original object 22, with the correction of the optical errors that are introduced by the use of a diffraction grating.
• the x/y ratios of the object 22 and image 23 are the same, and no distortion occurs across the image 23, as received by the detector 14.
• a further error that the diffraction grating 16 introduces is the apparent tilting of the object 22, once it has been reflected. This error is particularly apparent if the diffraction grating 16 has a regular pitch. Inherent to grating imaging is this defocus, which is dependent on the entrance angle at the grating surface. This error is overcome by similarly tilting the detector 14 an equivalent amount.
• Figure 9 shows the tilting of the detector 14 to match the tilt of the image 23.
• Figure 13 shows, in more detail, the tilt introduced by a diffraction grating 16 with regular pitch.
• Three separate rays of light from the object 22 (marked with a triangle, a square and an upside down triangle) are reflected by the grating 16 and focussed by the lens 18.
• the image of the object 22 is tilted with respect to the optical axis of the lens 18.
• the detector in order to correct the tilt would be positioned on the line 100.
• Figure 14 shows the effect that a diffraction grating 16 with a variable pitch has on the reflections of the light from the object 22.
• three separate rays of light from the object 22 (marked with a triangle, a square and an upside down triangle) are reflected by the grating 16 and focussed by the lens 18. These three rays are illustrated respectively in the left hand top and side views, centre top and side views and right hand top and side views.
• the pitch of the grating 16 at the particular points of the reflection of these rays is 1.3 lines/ ⁇ m, .56 lines/ ⁇ m and .04 lines/ ⁇ m respectively.
• the line 110 the line joining the in focus points of the reflected rays is curved, and some optical adjustment is required to the reflected rays. This can be provided by field flatteners placed between the grating 16 and the detector to effectively flatten the line of focus 110 into a plane to be received by the detector.
• the diffraction grating 16 in the optical device 10 has a variable pitch.
• this variable pitch needs to be tuned to the physical structure of the optical device 10, in particular, the angle of the diffraction grating 16 with respect to the lens 18, the size of the object 22 that is being imaged, and the distances from the object 22 to the grating 16 and from the grating 16 to the imaging lens 18.
• Figure 10 shows the relationship between p/ ⁇ and ⁇ (p/ ⁇ being a dimensionless parameter), where p is the pitch of the diffraction grating, ⁇ is the wavelength of the light being imaged by the detector 14 and ⁇ , measured in radians, is the field angle.
• p is the pitch of the diffraction grating
• ⁇ is the wavelength of the light being imaged by the detector 14
• ⁇ measured in radians
• Each angle ⁇ corresponds, in the graph, to a specific position P ⁇ along the grating surface. See also Figure 12.
• the wavelength ⁇ will be known, and the pitch at position P ⁇ can be calculated as a function of the angle ⁇ , where ⁇ is ranging from - ⁇ F ov to + ⁇ F ov, where ⁇ F ov is the field of view at the objective lens.
• Figure 11 illustrates one example of the physical structure of an optical device 10 for imaging an object 22 of an approximate planar size of 60mm.
• the diffraction grating 16 would be advantageously placed at an angle of 25 deg to the horizontal, and the object 22 can be imaged with a relatively small building height for the optical device 10.
• the graph of Figure 10 shows the variable pitch for the physical structure of the optical device 10 of Figure 11.
• D is the distance along the optical axis from objective lens to grating
• C is the distance along the optical axis from objective lens to the centre of the object
• L is the length of the object
• ⁇ is the angle of the incoming chief ray with respect to the optical axis
• ⁇ is the angle of the grating with respect to the optical axis
• h is the (vertical) distance from object to the optical axis (negative number).
• FIG. 16 shows a second embodiment of the optical device 10, in which the planar imaging is achieved using a large depth of field.
• a variable focus lens system such as an electrowetting lens for realising the large depth of focus.
• the principle of the optical device 10 of Figure 17 is that at a certain focus position of the lens system the image on the detector 14 is only sharp (i.e. in focus) for objects in the object plane of the optical system.
• the optical device 10 includes means 42 for controlling the focal depth of the variable focus lens 40.
• the means 42 comprises a voltage source and electrodes to generate an electric field over the electrowetting lens 40.
• This lens 40 will vary its focus in dependence on the strength of the field across it.
• the optical device 10 of Figure 17 works by modulating the focus setting of the variable focus system, and recording the corresponding (in-focus) image column data. Image reconstruction of the column data finally leads to the complete picture, being sharp (in-focus) over a wide depth of field.
• the means 42 and the lens 40 carry out a scan of the object 22 to obtain in- focus image data for the entire object 22.
• variable focus setting V VF For a certain variable focus setting V VF some part of the image (in this imaging geometry a line) is imaged sharp (i.e. in focus) onto the detector 14.
• This image data (a column of the object 22) is stored in memory: lm[X V F,Y], where X VF corresponds to the column number of the image that is being stored, and Y is an integer running from 1 to the total number of rows comprising the image.
• X VF corresponds to the column number of the image that is being stored
• Y is an integer running from 1 to the total number of rows comprising the image.
• FIG. 16 Another imaging method in order to reduce building height is shown in Figure 16.
• the planar object 22 to be imaged onto the detector 14 is imaged at grazing angles. That is to say: the angle between object surface normal and optical axis of the sensor/objective optical system is nearly 90 degrees.
• a consequence of this grazing imaging is the need of a large depth of focus, which in general is not possible using standard camera lenses. It is necessary reduce the aperture (stop) of the lens system such that enough depth of field is acquired, but this also reduces the optical throughput of the system.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Lenses (AREA)
• Structure And Mechanism Of Cameras (AREA)
• Studio Devices (AREA)
• Photometry And Measurement Of Optical Pulse Characteristics (AREA)
• Endoscopes (AREA)

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• 2006
  • 2006-03-08 EP EP06711042A patent/EP1861995A2/de not_active Withdrawn
  • 2006-03-08 WO PCT/IB2006/050716 patent/WO2006095314A2/en not_active Application Discontinuation
  • 2006-03-08 JP JP2008500325A patent/JP2008538010A/ja not_active Withdrawn

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