Device and method for detecting fluorescent and phosphorescent light

The apparatus has an illuminating device for illuminating sheet material with clocked excitation light. Both during the light phase of clocked excitation light and during the dark phase of clocked excitation light a sensor detects an intensity of the light emitted by the sheet material in each case. In an evaluation device an intensity of fluorescently emitted light and an intensity of phosphorescently emitted light are derived from the intensities detected in the light phase and in the dark phase of clocked excitation light. In order to ensure long preillumination with high intensity, the sensor preferably detects the intensities of emitted light within, and toward the end (in the transport direction) of, the area of the sheet material illuminated by the illuminating device. Additionally the illuminated area of the sheet material is selected to be so great that it is a multiple of the desired resolution.

This invention describes an apparatus and method for detecting
 fluorescently and phosphorescently emitted light from sheet material such
 as papers of value or bank notes.
 Such an apparatus is already known from U.S. Pat. No. 3,473,027. The
 apparatus described therein has an illuminating device for illuminating
 sheet material with ultraviolet excitation light. The sheet material is
 preferably illuminated continuously by ultraviolet excitation light. If
 required, clocked illumination of the sheet material is also possible. The
 light emitted by the sheet material is detected by a sensor. For this
 purpose the emitted light is imaged by a lens system onto a prism which
 then decomposes the emitted light into certain wave ranges. The individual
 wave ranges are each imaged by a further lens system onto a separate
 detector which then emits an electric signal proportional to the intensity
 of the wave range. To permit the sheet material to be detected along a
 track with a desired resolution, the sheet material is transported by a
 transport system along a transport direction past the illuminating device
 and sensor.
 A disadvantage of the known apparatus is that the light emitted by the
 sheet material cannot be divided into fluorescent and phosphorescent
 fractions.
 An apparatus and method for detecting fluorescently and phosphorescently
 emitted light from an identification mark on a parcel is known from U.S.
 Pat. No. 3,592,326. This print describes, in connection with a parcel
 singulating and orienting apparatus, an optical scanning means including
 an illuminating device wherein the parcels transported on conveyor belts
 are illuminated in clocked fashion during the transport motion by lamps
 focused on a scanning line. The light emitted by the parcel or
 identification mark is supplied via a rotating mirror assembly, whose
 rotary axis extends parallel to the transport direction and which is
 located exactly above said scanning line, via two prisms and associated
 filters to one of two sensors in each case. One sensor is in charge of
 detecting reflection and fluorescence when the illumination is switched
 on, and the other sensor for ascertaining phosphorescently emitted light
 when the illumination is switched off.
 The known apparatus firstly has an elaborate structure and secondly
 requires at least two sensors, involving corresponding adjustment,
 calibration and servicing effort. Due to the illumination and scanning
 oriented toward the scanning line, the excitation of the phosphorescently
 glowing identification mark is low so that little intensity is available
 for detecting phosphorescently emitted light and no exact, reproducible
 measurement is ensured.
 The invention is therefore based on the problem of providing a very exactly
 measuring apparatus and method for detecting fluorescent and
 phosphorescent light from sheet material wherein the light emitted by the
 sheet material can be divided into a fluorescent and a phosphorescent
 fraction with one common sensor.
 This problem is solved by the features in the characterizing parts of the
 main claim and the independent claim.
 According to the invention, the sensor detects one intensity of emitted
 light during the light phase of clocked excitation light and a further
 intensity of emitted light during the dark phase of clocked excitation
 light. In an evaluation device an intensity of fluorescently emitted light
 and an intensity of phosphorescently emitted light are derived from the
 intensities detected in the light phase and dark phase of clocked
 excitation light. The intensity of phosphorescently emitted light
 corresponds to the intensity of the dark phase, and the intensity of
 fluorescently emitted light is derived as the difference of intensity in
 the light phase and intensity in the dark phase. Furthermore, the sensor
 detects the intensities of emitted light within, and toward the end (in
 the transport direction) of, the area of the sheet material illuminated by
 the illuminating device. Additionally, the area of the sheet material
 illuminated by the illuminating device is selected to be so great that it
 is a multiple of the desired resolution.
 This permits the intensity of phosphorescently emitted light to be
 relatively great since it ensures long preillumination with high
 intensity.

FIG. 1a shows a schematic diagram of a preferred embodiment of the
 inventive apparatus. In lightproof housing 10 with transparent window 11
 there is illuminating device 20 and two sensors 30 and 40. Window 11
 transmits both the wave range of the excitation light and the wave range
 of the fluorescently and phosphorescently emitted light.
 Illuminating device 20 has lightproof housing 21 with filter 22 which does
 not transmit the wave range of the fluorescently and phosphorescently
 emitted light to be detected. In housing 21 there is excitation lamp 23
 which is clocked suitably via a control device not shown here. The light
 emitted by excitation lamp 23 contains at least the wave range necessary
 for exciting fluorescently and phosphorescently emitted light.
 As excitation lamp 23 one preferably uses a gas discharge lamp emitting at
 least UV light. In general one can also use as excitation lamp 23 a
 fluorescent lamp or gas discharge lamp without fluorescent substance. It
 is further possible to use gas discharge lamps emitting light due to a
 reaction of excited noble gases with halogen.
 Sensors 30 and 40 are of substantially analogous construction. They
 preferably have detector array 31, 41 which convert light emitted by the
 sheet material into an electric signal proportional to the intensity of
 emitted light. As detector array 31, 41 one can use for example photodiode
 arrays or CCD arrays. If only one track on the sheet material is to be
 detected for example, detector array 3, 41 can also be replaced by a
 single detector. Detector array 31, 41 is preferably selected so that
 light emitted over the total width of the sheet material can be detected
 in contiguous tracks.
 Further, sensors 30, 40 each have optical system 33, 43 for imaging an area
 of the sheet material which is preferably smaller than the desired
 resolution onto a detector of detector array 31, 41. As optical system 33,
 43 one can use lens systems for example. However one preferably uses
 optical systems 33, 43 having at least one imaging unit of photoconductive
 material. The advantage of an imaging unit of photoconductive material is
 that it is of much more compact construction than lens systems.
 Further, filter 32, 42 can be provided in optical axis 34, 44 of sensor 30,
 40. Suitable choice of the wave ranges of filters 32, 42 will be dealt
 with below.
 In order to ensure a compact structure of the apparatus, optical axes 34,
 44 of sensors 30, 40 are rotated by angle .alpha. to a perpendicular to
 transport direction V. Undesirable reflections on window 11 are prevented
 by transparent window 11 being dereflected at least for light incident at
 angle .alpha.. Additionally, filter 22 consists of two legs each disposed
 at fixed angle .beta. to a perpendicular to the transport direction. Angle
 .beta. results as .beta.=90.degree.-.alpha..
 Sheet material 50 is transported past illuminating device 20 and sensors 30
 and 40 in a transport direction marked by an arrow and at given transport
 speed V by a transport system not shown here.
 FIG. 1b shows the intensity of excitation light produced by the
 illuminating device in units relative to the spatial extent in the
 transport direction. In area B illuminated by the illuminating device the
 intensity of excitation light first rises to a maximum, then dropping
 again at the other end of the area. Sensors 30, 40 are disposed
 symmetrically to the maximum intensity of excitation light and detect the
 intensities of emitted light within illuminated area B. In the shown
 embodiment, sensors 30 and 40 detect the intensity of emitted light where
 the intensity of excitation light has dropped to half.
 To permit the intensity detected by one of sensors 30, 40 to be assigned to
 a certain place in the transport direction on the sheet material, clock T
 is produced whose frequency results as the quotient of transport speed V
 of the transport system and desired local resolution A in the transport
 direction. It holds that T=V/A. For example for a transport speed of V=10
 m/s and desired resolution A of 2 mm one obtains clock frequency T=5 kHz.
 The clock preferably has a logical 1 for half a pulse duration P=1/T and a
 logical 0 for the other half of the pulse duration.
 FIGS. 1c and 1d show bank note 50 with clock T. The above definition of the
 clock frequency of clock T ensures that the logical 1 or logical 0 of
 clock T is each linked with a certain place on bank note 50 independently
 of transport speed V. Desired resolution A contains a period of clock T in
 each case
 For detecting fluorescently and phosphorescently emitted light from sheet
 material 50 one first illuminates it with clocked excitation light from
 illuminating device 20. The light emitted by sheet material 50 is detected
 by sensor 30 within illuminated area B toward the end (in the transport
 direction) of the illuminated area, preferably behind the maximum
 intensity of excitation light.
 Since illuminated area B is much greater than desired resolution A, each
 area of resolution A is illuminated by excitation light from illuminating
 device 20 over several periods of clock T during transport of sheet
 material 50. Since the intensity of emitted light is detected by sensor 30
 only toward the end (in the transport direction) of the illuminated area,
 preferably behind the maximum intensity of excitation light, it is ensured
 that each area A of sheet material 50 has relatively long preillumination
 with high intensity before the emitted light is detected by sensor 30.
 Long preillumination with high intensity causes initial intensity I.sub.O
 of a phosphorescently emitting substance to be relatively high. Since the
 intensity of emitted light from phosphorescent substances depends on
 initial intensity I.sub.O and drops exponentially with time, high initial
 intensity I.sub.O is necessary for exact measurement. The intensity of
 emitted light of a phosphorescent substance as a function of time meets
 the equation I (t)=I.sub.O /(1+(t/.tau.).sub..alpha.). Decay time .tau. up
 to half the intensity and value .alpha. are properties of the
 phosphorescently emitting substance.
 The time histories in the detection of emitted light are shown in FIG. 2.
 Clocks T.sub.1 to T.sub.3 are clocks at different transport speeds V and
 are determined by the above equation. The light phase and dark phase of
 clocked excitation light are produced with clock L. In the light phase
 excitation lamp 23 is clocked with certain, freely selectable clock L
 which has a higher frequency than clock T. At the beginning of a logical 1
 of clock T clock L sends a certain number of logical 1s to the control
 unit of excitation lamp 23. At each logical 1 of clock L excitation lamp
 23 produces a light pulse. In the light phase one thus has an excitation
 light having a certain number of light pulses emitted at the beginning of
 clock T. For the rest of clock T clock L provides a logical 0 and no
 excitation light is emitted by excitation lamp 23.
 Intensity R of emitted light is thus approximately constant during the
 light phase and contains all wave ranges of the emitted light. Filter 32
 is preferably provided in optical axis 34 of sensor 30 for transmitting
 only the wave range of fluorescently and phosphorescently emitted light.
 In the dark phase beginning after the last light pulse of excitation light,
 only the intensity of phosphorescently emitted light is still present,
 dropping in accordance with the abovementioned power law depending on the
 selected substance.
 Clock D controls the time of detection of emitted light by sensor 30. Clock
 D contains two areas with a logical 1. The first area controls detection
 of emitted light in the area of the light phase and the second area
 controls detection in the area of the dark phase. The time interval
 between the first area and the second area of clock D is selected to be
 constant. The time interval from the beginning of the first area of clock
 T to the beginning of clock D is also constant. The time areas of clock D
 and their position in the light or dark phase can fundamentally be
 selected at will. However the position and width of the first area of
 clock D are preferably selected so that the intensity of emitted light is
 measured in the light phase of a clock during the last light pulse. The
 position of the second area of clock D is laid so that the intensity of
 emitted light is measured in the dark phase after a constant time period
 after the last light pulse. The constant time period is selected so that
 detection of the intensity of emitted light in the dark phase takes place
 within shortest possible clock T.
 Since clock T depends on transport speed V of the sheet material, as
 described above, it varies with a variation of transport speed V. Since
 the above-described method for detecting the intensity of emitted light in
 the light or dark phase depends only on the beginning of clock T, a
 slow-down of clock T, i.e. a slow-down of transport speed V, can be
 tolerated within certain limits. Since detection of emitted light in the
 dark phase is measured after a constant time period after the last light
 pulse, the reproducibility of the intensity of emitted light in the dark
 phase is also ensured despite the exponential drop in intensity of
 phosphorescently emitted light.
 An intensity of fluorescently emitted light and an intensity of
 phosphorescently emitted light are derived from the intensities detected
 in the light phase and in the dark phase of clocked excitation light in
 each case. The intensity of phosphorescently emitted light can correspond
 to the intensity in the dark phase for example. The intensity of
 fluorescently emitted light can be derived as the difference of intensity
 in the light phase and intensity in the dark phase. It is of course also
 possible for the expert to use other arithmetical operations to derive the
 intensity of fluorescently or phosphorescently emitted light here.
 Using second sensor 40 one can detect the light emitted by the sheet
 material in several different wave ranges. For this purpose, filter 42 is
 provided in sensor 40 in optical axis 44 for transmitting only a subrange
 of the wave range of fluorescently and phosphorescently emitted light.
 Since sensors 30, 40 are disposed symmetrically to the maximum intensity
 of illuminating device 20, sensor 40 detects the intensity of emitted
 light in the transport direction at the beginning of the illuminated area,
 preferably before the maximum intensity of excitation light. It follows
 that only negligibly small preillumination of the phosphorescent substance
 has taken place during detection of emitted light by sensor 40. Emitted
 light detected by sensor 40 in the dark phase can thus be substantially
 only undesirable stray light, so that the intensity of light detected in
 the dark phase by sensor 40 can be used for example to standardize all
 other measured intensities. Emitted light detected by sensor 40 during the
 light phase thus contains fluorescently emitted light which is restricted
 by filter 42 to a certain wave range.
 During the light phase of excitation light, a total intensity of
 fluorescently emitted light can thus be derived from sensor 30 and an
 intensity of a certain wave range of fluorescently emitted light from
 sensor 40. By forming a difference of the detected total intensity of
 sensor 30 and the detected intensity of sensor 40 for example, one can
 also derive an intensity of fluorescently emitted light in the wave range
 complementary to the wave range of sensor 40.
 During the dark phase, sensor 30 detects the intensity of phosphorescently
 emitted light. Via clock T the derived intensities can be assigned to a
 place with desired resolution A on bank note 50.
 As a result of the method one obtains, as shown in FIG. 3a, an intensity
 pattern of emitted light resolved according to wave ranges for each sensor
 30, 40 along each track over the total length of the sheet material. In
 the light phase sensor 30 detects intensity pattern I.sub.F containing the
 total wave range of emitted light. In the light phase sensor 40 detects
 intensity pattern I.sub.R containing only the red wave range of emitted
 light here for example. Intensity pattern I.sub.G of yellow-green emitted
 light results as the difference of intensity pattern I.sub.F and intensity
 pattern I.sub.R. Further, one obtains intensity pattern I.sub.P for light
 emitted in the dark phase, which is shown in FIG. 3b. One then derives
 from the intensity patterns the intensities for phosphorescent light and
 fluorescent light in different wave ranges, as explained above.
 As described above, by a suitable choice of tracks one can detect the light
 emitted fluorescently and phosphorescently by the total sheet material
 with a desired resolution.