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WO1999044041A1 - Apparatus and method for determining a characteristic parameter of print material - Google Patents

Apparatus and method for determining a characteristic parameter of print material Download PDF

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Publication number
WO1999044041A1
WO1999044041A1 PCT/GB1999/000555 GB9900555W WO9944041A1 WO 1999044041 A1 WO1999044041 A1 WO 1999044041A1 GB 9900555 W GB9900555 W GB 9900555W WO 9944041 A1 WO9944041 A1 WO 9944041A1
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WO
WIPO (PCT)
Prior art keywords
print material
characteristic parameter
fourier transform
print
light beam
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/GB1999/000555
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French (fr)
Inventor
Martin William Mccall
Antoine Yvon Messiou
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Imperial College of London
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Imperial College of London
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Filing date
Publication date
Application filed by Imperial College of London filed Critical Imperial College of London
Publication of WO1999044041A1 publication Critical patent/WO1999044041A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/47—Scattering, i.e. diffuse reflection
    • G01N21/4738—Diffuse reflection, e.g. also for testing fluids, fibrous materials
    • G—PHYSICS
    • G06—COMPUTING OR CALCULATING; COUNTING
    • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
    • G06V10/00—Arrangements for image or video recognition or understanding
    • G06V10/98—Detection or correction of errors, e.g. by rescanning the pattern or by human intervention; Evaluation of the quality of the acquired patterns
    • G06V10/993—Evaluation of the quality of the acquired pattern

Definitions

  • This invention relates to an optical technique to conveniently measure the characteristic parameters of print material consisting of an array array of dots typically (periodic but sometimes pseudo-random) that may be used in defining the quality of the print material.
  • Halftone print consists of small darkened shapes (dots), such as circles, being reproduced as a lattice onto the printing medium such that the grey level for a given area is coded by the dot separation or dot density (see Fig. 1). Dots vary in size and shape depending on print quality required, but are typically about lOO ⁇ m-m diameter. In colour printing, dot patterns representing each base colour are superposed to produce the final colour print.
  • an apparatus for determining at least one characteristic parameter of print material consisting of an array of dots comprising: a light source for providing a spatially coherent light beam to illuminate and be diffracted by said print material: detecting means arranged to receive the diffracted light beam and detect a Fourier transform distribution of said print material therein; processing means for processing data from said detecting means to calculate said at least one characteristic parameter from said Fourier transform distribution of said print material.
  • the optical Fourier transform is invariant with respect to transverse translations and, at least as far as the observed intensity is concerned, the position along the optical axis when illuminated with a collimated beam.
  • the use of an optical Fourier transform of the print material means that samples can be simply and rapidly inserted and changed.
  • the use of the Fourier transform results in intrinsic averaging of the image, observed features in the 2-D Fourier plane are automatically determined as averages over the entire 2-D object (spatial) plane.
  • the present invention also enables the parallel determination of print parameters.
  • Coherent illumination of an area encompassing several dots in the pattern creates a Fourier image from which average pattern features, such as transmissivity/reflectivity, dot period, mark-space ratio, contrast ratio, and edge density gradient are simultaneously accessible. They can then be conveniently converted to standard print assessment parameters, tone value, dot gain, fringe width, mid-tone spread, dot shape, screen ruling screen angle, etc.
  • analysis in the Fourier domain allows the more ready detection of large area beating or moire patterns that can occur due to interactions between the periodic nature of the dot pattern and periodic instabilities in the printing machinery.
  • Preferred embodiments of the apparatus may be adapted to determine the characteristic parameters of a print film transparency, a print plate or a printed sheet.
  • the light source is a laser.
  • a laser is a good source of coherent light.
  • the laser is a helium/neon laser or a diode laser, such lasers being readily and inexpensively available and producing light of an appropriate intensity and wavelength.
  • the apparatus comprises a lens arranged to focus the diffracted light beam onto the detecting means.
  • a lens arranged to focus the diffracted light beam onto the detecting means.
  • the apparatus further comprises recording means for recording the data from said detecting and calculating means.
  • recording means for recording the data from said detecting and calculating means.
  • the detecting means is a CCD camera, such a camera being an accurate and convenient means of detecting diffraction fringes.
  • one of the characteristic parameters to be determined is screen angle. This is a parameter that is quite directly accessible from the Fourier transform distribution of the print material, it being calculated from the orientation of the peaks of the Fourier transform with respect to a reference direction.
  • tone value This is calculated by integrating the intensity of the beam and comparing it with calibrated values.
  • fringe width or edge hardness is also calculated.
  • Fringe width may be defined as the average distance between 10% and 90% density contour lines on a dot, and measures the edge quality of dots.
  • the average optical density gradient (which relates closely to the fringe width) at the dot's edge may be inferred by comparing the observed heights of the maximum with that expected for a similar "hard-edged" pattern.
  • This parameter in particular should be averaged over many screen periods and is tedious to deduce from conventional microdensitometer measurements, but significantly affects the visual characteristics of final colour prints.
  • fringe width can be conveniently calculated by the present invention from the relative heights and positions of the Fourier transform peaks.
  • a further preferred characteristic parameter is screen ruling. This is determined by measuring the distance between the zeroth and first order peaks of the Fourier transform, and finding the reciprocal.
  • embodiments of the present invention can conveniently and simultaneously measure a variety of parameters that may be used in defining the print quality of print material consisting of a periodic array of dots.
  • a method for dete ⁇ mning at least one characteristic parameter of print material consisting of a periodic array of print dots comprising: illuminating said print material with a spatially coherent light beam generated by a light source, said print material acting to diffract said light beam; detecting a Fourier transform distribution of said print material from said diffracted beam; determining from said at least one characteristic parameter from said Fourier transform distribution of said print material.
  • Figure 1 illustrates half-tone dot representation of grey scale
  • Figure 2 illustrates the apparatus according to one embodiment of the invention
  • Figure 3 illustrates a rectangular lattice of square dots and their transmission profile
  • Figure 4 illustrates a line scan of the Fourier transform distribution of the lattice of square dots.
  • FIG 1 shows an example of the sort of dot print material of which the present invention can determine the quality.
  • interrogating light from a helium-neon laser 2 is passed through lenses 6 and diaphragm 7 to produce a broad spatially coherent beam 3.
  • This beam 3 is then used to illuminate an area of the print material 1 containing a plurality of dots and is diffracted by them.
  • the diffracted beam then passes through a lens 8 which acts to focus the diffracted beam that is a 2-D Fourier distribution of the dots and contains information about the spatial frequencies present in the original, onto a CCD camera 4.
  • the CCD camera 4 is attached to processing means 5 (a programmed computer) which records and processes data relating to the Fourier transform distribution, and calculates quality defining parameters from this data.
  • print material comprising a rectangular lattice of square dots is illustrated along with its transmission profile and a line scan of the Fourier pattern.
  • Other parameters such as mid-tone spread, dot gain and dot shape are either combinations of the above made on different samples/colour separations or can be deduced using similar principles.
  • edge hardness or edge optical density gradient which relates closely to fringe width, is defined by:
  • B - ( ⁇ x)e ⁇ x [ ⁇ I / - 1] - e ⁇ x ( ⁇ x)cos ⁇ a/L - (1 - e ⁇ x )cos ⁇ a/L
  • ⁇ x can be replaced by [(D ' - D.) / ⁇ x] .

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Quality & Reliability (AREA)
  • General Physics & Mathematics (AREA)
  • General Health & Medical Sciences (AREA)
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  • Analytical Chemistry (AREA)
  • Chemical & Material Sciences (AREA)
  • Immunology (AREA)
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  • Life Sciences & Earth Sciences (AREA)
  • Health & Medical Sciences (AREA)
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  • Length Measuring Devices By Optical Means (AREA)

Abstract

An apparatus and method for determining at least one characteristic parameter (quality defining characteristic) of print material (1) consisting of a periodic array of dots. The apparatus comprising a light source (2) for providing a spatially coherent light beam (3) to illuminate and be diffracted by the print material (1); detecting means (4) arranged to detect the Fourier transform distribution of the print material from the diffracted light beam and processing means (5) for processing data from the detecting means to calculate the characteristic parameters from the Fourier transform distribution of the print material.

Description

APPARATUS AND METHOD FOR DETERMINING A CHARACTERISTIC PARAMETER OF PRINT MATERIAL
This invention relates to an optical technique to conveniently measure the characteristic parameters of print material consisting of an array array of dots typically (periodic but sometimes pseudo-random) that may be used in defining the quality of the print material.
An example of such print material is half tone dot print as used in offset lithographic printing. Halftone print consists of small darkened shapes (dots), such as circles, being reproduced as a lattice onto the printing medium such that the grey level for a given area is coded by the dot separation or dot density (see Fig. 1). Dots vary in size and shape depending on print quality required, but are typically about lOOμm-m diameter. In colour printing, dot patterns representing each base colour are superposed to produce the final colour print.
Careful monitoring of output is required to ensure and maintain quality. The International Standards Office document, "Graphic technology - process control for the manufacture of half tone colour separations, proof and production prints, ISO 12647-1/2 1996(E)", defines a number of parameters through which the print quality is quantified (e.g. tone value, dot gain, fringe width, mid-tone spread, dot shape, screen ruling, screen angle). At present if these parameters are measured at all, then this is done using techniques such as by eye or by using time consuming serialised microdensitometer scans which produce restuls that are difficult to interpret.
In accordance with one aspect of the present invention, there is provided an apparatus for determining at least one characteristic parameter of print material consisting of an array of dots, said apparatus comprising: a light source for providing a spatially coherent light beam to illuminate and be diffracted by said print material: detecting means arranged to receive the diffracted light beam and detect a Fourier transform distribution of said print material therein; processing means for processing data from said detecting means to calculate said at least one characteristic parameter from said Fourier transform distribution of said print material. Thus the present invention overcomes the disadvantages of the prior art by providing an optical technique which enables an unskilled operator to make a rapid, parallel measurement over a region of the print material of the desired parameters. In particular, the optical Fourier transform is invariant with respect to transverse translations and, at least as far as the observed intensity is concerned, the position along the optical axis when illuminated with a collimated beam. Thus the use of an optical Fourier transform of the print material means that samples can be simply and rapidly inserted and changed. Furthermore, the use of the Fourier transform results in intrinsic averaging of the image, observed features in the 2-D Fourier plane are automatically determined as averages over the entire 2-D object (spatial) plane. The present invention also enables the parallel determination of print parameters. Coherent illumination of an area encompassing several dots in the pattern creates a Fourier image from which average pattern features, such as transmissivity/reflectivity, dot period, mark-space ratio, contrast ratio, and edge density gradient are simultaneously accessible. They can then be conveniently converted to standard print assessment parameters, tone value, dot gain, fringe width, mid-tone spread, dot shape, screen ruling screen angle, etc. In addition, analysis in the Fourier domain allows the more ready detection of large area beating or moire patterns that can occur due to interactions between the periodic nature of the dot pattern and periodic instabilities in the printing machinery.
Preferred embodiments of the apparatus may be adapted to determine the characteristic parameters of a print film transparency, a print plate or a printed sheet.
Advantageously, the light source is a laser. A laser is a good source of coherent light. Preferably, the laser is a helium/neon laser or a diode laser, such lasers being readily and inexpensively available and producing light of an appropriate intensity and wavelength.
In a preferred embodiment of the present invention, the apparatus comprises a lens arranged to focus the diffracted light beam onto the detecting means. This enables the distance between the print material and the detecting means to be shortened, the required distance in this case being the focal length of the lens, rather than the far field of the diffraction pattern, as is the case without the lens. Such an arrangement results in a more compact apparatus.
In another embodiment of the present invention, the apparatus further comprises recording means for recording the data from said detecting and calculating means. Thus several samples can be quickly tested and the results analysed later.
Preferably, the detecting means is a CCD camera, such a camera being an accurate and convenient means of detecting diffraction fringes.
In a preferred embodiment of the present invention, one of the characteristic parameters to be determined is screen angle. This is a parameter that is quite directly accessible from the Fourier transform distribution of the print material, it being calculated from the orientation of the peaks of the Fourier transform with respect to a reference direction.
Another preferred characteristic parameter that may be determined is tone value. This is calculated by integrating the intensity of the beam and comparing it with calibrated values.
Advantageously, fringe width or edge hardness is also calculated. Fringe width may be defined as the average distance between 10% and 90% density contour lines on a dot, and measures the edge quality of dots. The average optical density gradient (which relates closely to the fringe width) at the dot's edge may be inferred by comparing the observed heights of the maximum with that expected for a similar "hard-edged" pattern. This parameter in particular should be averaged over many screen periods and is tedious to deduce from conventional microdensitometer measurements, but significantly affects the visual characteristics of final colour prints. In contrast, fringe width can be conveniently calculated by the present invention from the relative heights and positions of the Fourier transform peaks. A further preferred characteristic parameter is screen ruling. This is determined by measuring the distance between the zeroth and first order peaks of the Fourier transform, and finding the reciprocal.
Thus by using optical techniques to provide the Fourier transform distribution of the print material, embodiments of the present invention can conveniently and simultaneously measure a variety of parameters that may be used in defining the print quality of print material consisting of a periodic array of dots.
In accordance with another aspect of the present invention, there is provided a method for deteπmning at least one characteristic parameter of print material consisting of a periodic array of print dots, said method comprising: illuminating said print material with a spatially coherent light beam generated by a light source, said print material acting to diffract said light beam; detecting a Fourier transform distribution of said print material from said diffracted beam; determining from said at least one characteristic parameter from said Fourier transform distribution of said print material.
An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:
Figure 1 illustrates half-tone dot representation of grey scale;
Figure 2 illustrates the apparatus according to one embodiment of the invention; Figure 3 illustrates a rectangular lattice of square dots and their transmission profile; and
Figure 4 illustrates a line scan of the Fourier transform distribution of the lattice of square dots.
Figure 1 shows an example of the sort of dot print material of which the present invention can determine the quality. With reference to Figure 2, interrogating light from a helium-neon laser 2 is passed through lenses 6 and diaphragm 7 to produce a broad spatially coherent beam 3. This beam 3 is then used to illuminate an area of the print material 1 containing a plurality of dots and is diffracted by them. The diffracted beam then passes through a lens 8 which acts to focus the diffracted beam that is a 2-D Fourier distribution of the dots and contains information about the spatial frequencies present in the original, onto a CCD camera 4. The CCD camera 4 is attached to processing means 5 (a programmed computer) which records and processes data relating to the Fourier transform distribution, and calculates quality defining parameters from this data.
With reference to Figures 3 and 4, an example of a print material comprising a rectangular lattice of square dots is illustrated along with its transmission profile and a line scan of the Fourier pattern. Many print parameters (screen ruling, screen angle, tone value) can be inferred directly from the data, e.g. screen ruling = 1/L. Other parameters such as mid-tone spread, dot gain and dot shape are either combinations of the above made on different samples/colour separations or can be deduced using similar principles.
Another parameter, edge hardness or edge optical density gradient which relates closely to fringe width, is defined by:
β = [(D '- D ) / δx] . lnVlO
This parameter significantly affects the visual characteristics of final colour prints and can be found, for the lattice of Figure 3, by solving the following cubic equation:
Aβ3 + Bβ2 + Cβ + D = 0
where:
A == r [^(eβpδo*x - l)a/2 + L/2][ΔI/L - 1] + [(epox - 1) sinπa/L] / [2π/L] B = - (βδx)eβδx[ΔI / - 1] - eβδx(βδx)cosπa/L - (1 - eβδx)cosπa/L
C = (2π/L)2[(eβδx - l)a/2 +L/2][ ΔM„ - 1] + (2π/L) βδx eβδxsinπa/L
D = [(2π/L)2(eβδx - 1) - eβδx(βδx)] [Mil- 1]
wherein: βδx can be replaced by [(D '- D.) / δx] . In VlO and (ΔI/ - 1) by (-Ii/L ) with Ii, equal to 1st order and L equal to zeroth order intensity
It will be apparent, of course, that the present invention has been described above by way of example only and that modifications may be made within the scope of the appended claims.

Claims

1. Apparatus for determining at least characteristic parameter of print material consisting of an array of dots, said apparatus comprising:
5 a light source for providing a spatially coherent light beam to illuminate and be diffracted by said print material; detecting means arranged to receive the diffracted light beam and detect a Fourier transform distribution of said print material therein; processing means for processing data from said detecting means to calculate said at l o least one characteristic parameter from said Fourier transform distribution of said print material.
2. Apparatus according to claim 1, wherein said print material is a print film transparency which is back illuminated by said light source.
15
3. Apparatus according to claim 1, wherein said print material is a print plate which reflects said light beam.
4 Apparatus according to claim 1, wherein said print material is a printed sheet
20 which reflects said light beam.
5. Apparatus according to any of the preceding claims, wherein said light source is a laser.
25 6. Apparatus according to claim 5, wherein said laser is a helium/neon laser or a diode laser.
7. Apparatus according to any of the preceding claims, wherein said apparatus comprises a lens arranged to focus the diffracted light beam onto said detecting 30 means.
8. Apparatus according to any of the preceding claims, wherein said apparatus comprises recording means for recording the data from said detecting and calculating means.
9. Apparatus according to any of the preceding claims, wherein said detecting means is a CCD camera.
10. Apparatus according to any of the preceding claims, wherein said at least one characteristic parameter is screen angle.
11. Apparatus according to claim 10, wherein said processing means is adapted to calculate said screen angle from the orientation of the peaks of the Fourier transform with respect to a reference direction.
12. Apparatus according to any of the preceding claims, wherein said at least one characteristic parameter is tone value.
13 Apparatus according to claim 12, wherein said processing means is adapted to calculate said tone value by integrating the intensity of the beam and comparing it with calibrated values.
14. Apparatus according to any of the preceding claims, wherein said at least one characteristic parameter is fringe width.
15. Apparatus according to claim 14, wherein said processing means is adapted to calculate said fringe width from the relative heights and positions of the Fourier transform peaks.
16. Apparatus according to any of the preceding claims, wherein said at least one characteristic parameter is screen ruling.
17. Apparatus according to claim 16, wherein said processing means is adapted to calculate said screen ruling by measuring the distance between the zeroth and first order peaks of the Fourier transform, and finding the reciprocal.
18. A method for determining at least one characteristic parameter of print material consisting of a periodic array of print dots, said method comprising: illuminating said print material with a spatially coherent light beam generated by a light source, said print material acting to diffract said light beam; detecting a Fourier transform distribution of said print material from said diffracted beam; determining said at least one characteristic parameter from said Fourier transform distribution of said print material.
19. A method according to claim 18, wherein said at least one characteristic parameter is screen ruling and said determining step includes measuring the distance between the zeroth and first order peaks of said Fourier transform, and calculating the reciprocal value.
20. A method according to claims 18 or 19, wherein said at least one characteristic parameter is screen angle and said determining step includes measuring the orientation of the peaks of said Fourier transform with respect to a reference direction.
21. A jnethod according to any of claims 18 to 20, wherein said at least one characteristic parameter is tone value and said determining step includes integrating the intensity of said diffracted beam and comparing it with calibrated values.
22. A method according to any of claims 18 to 21, wherein said at least one characteristic parameter is fringe width or edge hardness and said deteπnining step includes measuring the relative heights and positions of the Fourier transform peaks.
PCT/GB1999/000555 1998-02-27 1999-02-23 Apparatus and method for determining a characteristic parameter of print material Ceased WO1999044041A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9804314.4 1998-02-27
GBGB9804314.4A GB9804314D0 (en) 1998-02-27 1998-02-27 Apparatus and method for determining a characteristic parameter of print material

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WO1999044041A1 true WO1999044041A1 (en) 1999-09-02

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1326881A (en) * 1969-08-26 1973-08-15 Sira Institute Method of and apparatus for analysing two dimensional patterns
US5241369A (en) * 1990-10-01 1993-08-31 Mcneil John R Two-dimensional optical scatterometer apparatus and process
JPH08247952A (en) * 1995-03-07 1996-09-27 Mitsubishi Paper Mills Ltd Image recording material image quality evaluation device

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1326881A (en) * 1969-08-26 1973-08-15 Sira Institute Method of and apparatus for analysing two dimensional patterns
US5241369A (en) * 1990-10-01 1993-08-31 Mcneil John R Two-dimensional optical scatterometer apparatus and process
JPH08247952A (en) * 1995-03-07 1996-09-27 Mitsubishi Paper Mills Ltd Image recording material image quality evaluation device

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 097, no. 001 31 January 1997 (1997-01-31) *

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