WO2006124007A1 - Method and apparatus for in-line inspection and mapping liquid crystal cell gap - Google Patents

Method and apparatus for in-line inspection and mapping liquid crystal cell gap Download PDF

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Publication number
WO2006124007A1
WO2006124007A1 PCT/UA2005/000019 UA2005000019W WO2006124007A1 WO 2006124007 A1 WO2006124007 A1 WO 2006124007A1 UA 2005000019 W UA2005000019 W UA 2005000019W WO 2006124007 A1 WO2006124007 A1 WO 2006124007A1
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Prior art keywords
liquid crystal
crystal cell
cell
light
filled
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PCT/UA2005/000019
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French (fr)
Inventor
Sergiy Valyukh
Kent Skarp
Irina Valyukh
Oleksandr Slobodyanyuk
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Conoptix Ab
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Priority to PCT/UA2005/000019 priority Critical patent/WO2006124007A1/en
Publication of WO2006124007A1 publication Critical patent/WO2006124007A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/1306—Details
    • G02F1/1309—Repairing; Testing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/14—Measuring arrangements characterised by the use of optical techniques for measuring distance or clearance between spaced objects or spaced apertures

Definitions

  • the present invention relates to an inspecting method and an inspecting apparatus for a liquid crystal cell, and for inline mapping liquid crystal cell gap in a production line.
  • the closest prior art solution to the present invention related to an inspecting method is the method for inspecting liquid crystal cell disclosed in US Patent 6,657,218 that comprises the steps of: applying a plurality of color lights to a member provided with a gap, to produce color interference fringes/ obtaining respective intensities of the color lights in the images of the interference fringes taken by a color camera, at each of predetermined positions of each image, so as to compute and actual ratio among the obtained intensities of the color lights for each of the predetermined positions; and obtaining gap values of a plurality of points of the gap provided in the member, based on said actual ratio and theoretical ratios each of which is computed based on intensities of said color lights in an image corresponding to each of preliminarily set gap values.
  • the drawback of this method is that it cannot measure absolute values of cell gap of empty liquid crystal cells, accuracy of measuring absolute values of cell gap of filled liquid crystal cells is low. Moreover, the method of calculation of cell gap is based on image processing that makes the method cumbersome.
  • the US Patent 6,657,218 also discloses the closest prior art solution to the present invention related to apparatus for a liquid crystal cell gap measuring that characterized by comprising a light source, a color camera for picking up images of light interference fringes; a filter for transmitting a plurality of different color lights to the color camera; optical means for directing the light from the light source toward an object to be measured and directing reflected light thereof from the object to be measured toward the color camera; an image memory for storing images of the interference fringe formed by the light from the optical means and picked up by the color camera; computing means for obtaining respective intensities of the color lights in the image at each of predetermined positions of each image, so as to compute an actual ratio among the obtained intensities of the color lights for each of the predetermined positions; a reference data memory for storing theoretical ratios among intensities of said color lights in an image correspondingly to each of preliminarily set gap values; and gap value comparing/determining means for determining gap values of a plurality of points of gap provided in the object to be measured according to
  • an object of the present invention to provide an inspecting method and an inspecting apparatus for inspecting a liquid crystal cell, capable of automatically in-situ carrying out an inspection of the cell gap and its uniformity in a production line that is necessary for quality control of produced liquid crystal cells .
  • the method for in-line inspection and mapping of the cell gap of liquid crystal cells in a production line comprises following steps of:
  • a linear array of the beams is used for inspection of the liquid crystal cell in direction perpendicular to the direction of the cell motion and above mentioned steps (b) , (c) , and (d) are executed separately for light originated from each beam of the said array of the beams. Then a map of liquid crystal cell gap is built and a preset acceptance criterion is applied to the said map.
  • the method is applicable both to reflective and transmissive liquid crystal cell, so the light after its interaction with the liquid crystal cell is the light transmitted through the cell or the light reflected from the cell.
  • ⁇ is the angle of incidence of the collimated light beam on the liquid crystal cell
  • N,M are numbers of the detected minima and maxima of interference oscillations in the measured spectrum of the light interacted with the liquid crystal cell, respectively
  • ⁇ min , ⁇ max are the wavelengths of the said minima and maxima, respectively.
  • ⁇ is the angle of incidence of the collimated light beam on the liquid crystal cell
  • N,M are numbers of the detected minima and maxima of the interference oscillations in the measured spectrum of the light beam interacted with the filled liquid crystal cell, respectively
  • ⁇ min , ⁇ max are the wavelengths of the said minima and maxima , respectively
  • n is the average refractive index of the liquid crystal filled into the said liquid crystal cell.
  • I(y) is the measured spectrum containing the interference oscillations
  • v is the wave
  • a wave numbers in the measured spectrum containing interference oscillations, respectively, ⁇ is the angle of incidence of the collimated light beam on the liquid crystal cell, and the cell gap of a filled liquid crystal cell d f is calculated as an argument for that the following function
  • ⁇ is the average refractive index of the liquid crystal filled into the said liquid crystal cell
  • ⁇ is the angle of incidence of the collimated light beam on the liquid crystal cell.
  • the further embodiments of the method according to the invention applicable only to filled liquid crystal cell envisage use of polarized light to produce another kind of minima and maxima in spectra of the light interacted with the liquid crystal cell due to birefringence of the liquid crystal filled into cell.
  • the polarized collimated light illuminating tested areas of the filled liquid crystal cell is used and the light after its interaction with the moved liquid crystal cell and before measuring its spectra passes through a polarizing analyser.
  • the collimated light is linearly polarized light and polarizing analyser is a linear polarizer.
  • the liquid crystal cell gap is calculated from a spectral position of at least one minimum in the measured spectrum.
  • ⁇ min is the wavelength of the minimum in the measured spectrum of the light transmitted through the liquid crystal cell
  • ⁇ n is the birefringence of the liquid crystal filled into the cell
  • N is the order of interference.
  • ⁇ is the twist angle of the liquid crystal
  • ⁇ is the angle between the plane of vibration in the incident collimated light and the input director of the liquid crystal filled into the cell
  • ⁇ n is the birefringence of the liquid crystal filled into the cell
  • ⁇ min is the wavelength of the minimum in the measured spectrum of the light transmitted through the liquid crystal cell.
  • the transmission axis of the linear polarizing analyser is crossed with the plane of vibration of the incident collimated light, and the cell gap d f of a filled liquid crystal is calculated according to the formula
  • ⁇ mxn is the wavelength of the minimum in the measured spectrum of the light reflected from the liquid crystal cell
  • N is the order of interference
  • the transmission axis of the polarizing analyser is crossed with the plane of vibration in the beam of collimated light
  • ⁇ min is the wavelength of the minimum in the measured spectrum of the light reflected from the liquid crystal cell.
  • an apparatus for in-situ inspection of liquid crystal cells in a production line comprising: a wide-band light source; a projection optical system for illuminating with one or more collimated light beams from the said wide-band light source at one or more tested areas of a liquid crystal cell moved uniformly in a production line; a real-time spectrometer for measuring spectra of the light interacted with the said liquid crystal cell; means for collection of light interacted with the said tested area of the liquid crystal cell and its transportation to the optical entrance of the said spectrometer; a computer processing the measured spectra in real time and calculating the cell gap of the liquid crystal cell from wavelengths of extremes in the measured spectra, herein the computer applies the preset criteria of the liquid crystal cell gap uniformity and distinguishes the different cells in a stream of the production line from changes in the measured spectrum, and then generates control signals on a pass or failure of the inspected liquid crystal cells.
  • the projection optical system provides a linear array of the light beams to illuminate tested areas of the liquid crystal cell located along the line perpendicular to the direction of movement of the liquid crystal cell;
  • the real-time spectrometer is multichannel spectrometer, herein the number of its independent channels equals the said number of light beams; means for collection of the light interacted with the tested areas of the liquid crystal cell and its transportation to the entrances of the corresponding channels of the spectrometer;
  • the computer creates data array of the cell gap values versus spatial coordinates, applies preset criteria of quality and makes decision on a pass a failure.
  • a polarizer is placed at output of the projection optical system and an analyzer is placed before the entrance of the spectrometer, and, particularly, the said polarizer and analyzer are linear.
  • the said polarizer and analyzer are located on different sides of the inspected liquid crystal cell moved in the stream of the production line and in embodiment of the apparatus for reflective cells the said polarizer and analyzer are located on the same side of the inspected liquid crystal cell moved in the stream of the production line.
  • the apparatus additionally comprises an optical coupling unit based on beam splitter that is positioned between polarizer and analyzer and the liquid crystal cell to direct the incident linearly polarized light normally to the front liquid crystal cell surface and to direct light reflected back from the tested areas of the liquid crystal cell to the analyser.
  • an optical coupling unit based on beam splitter that is positioned between polarizer and analyzer and the liquid crystal cell to direct the incident linearly polarized light normally to the front liquid crystal cell surface and to direct light reflected back from the tested areas of the liquid crystal cell to the analyser.
  • FIG. 1 is a configuration diagram showing the optical arrangement for in-line mapping liquid crystal cells from interference oscillations according to the present invention (embodiment for reflective cells is shown) ;
  • Fig.2 is an explanatory view for showing cell gap measuring points on the surface of a liquid crystal cell to be mapped in a uniformly moved section of a production line.
  • Fig.3 shows typical interference oscillations in the spectrum of light interacted with a liquid crystal cell and measured by one of the channels in the system shown in Fig.l;
  • FIG.4 shows an example of light spectrum measured by one of the channels of the multi-channel spectrometer in the apparatus shown in Fig.l when a light crystal cell is not under the light beam produced by the light source 1;
  • FIG.5 is a flowchart of a spectrum processing operation of the liquid crystal cell inspecting apparatus shown in Fig.l according to the present invention;
  • FIG.6 shows the optical arrangement that can be used to measure the cell gap and mapping its uniformity of a filled transmissive liquid crystal cell according to the present invention
  • FIG.7 shows the optical arrangement that can be used to measure the cell gap and mapping its uniformity of a filled reflective liquid crystal cell according to the present invention
  • Fig.8 shows an example of light spectrum measured by one of the channels of the multi-channel spectrometer in the optical arrangement shown in Fig.6 or 7 when the crystal cell is under the light beam produced by the light source 11;
  • FIG. 9 is a flowchart of a spectrum processing operation of the liquid crystal cell inspecting apparatus shown in Fig.7 and Fig.8.
  • This invention is capable of in-situ measuring cell gap and its uniformity of reflective and transmissive filled and empty liquid crystal cells in a production line.
  • FIG. 1 shows an optical system for in-line mapping liquid crystal cells according to the present invention.
  • the optical system comprises a light source 11 that has a broad spectrum and equipped with a light projection set 12 for illumination a mapped liquid crystal cell 13b by a collimated light beam, a light guiding set 14 for projection of light beams transmitted through the liquid crystal cell or reflected from it on a real-time multichannel spectrometer 15 that has N channels, a computer 16 that treats the light spectrum, calculates cell gap, collects data for mapping uniformity of the liquid crystal cell and make decision about defectiveness of tested liquid crystal cells.
  • 13a is a liquid crystal cell previously mapped
  • 13c is a liquid crystal cell that will be mapped after the cell 13b.
  • Liquid crystal cells 13a, 13b, and 13c are uniformly moved by a moved section 17 of a product line.
  • Fig.2 is an explanatory view for showing cell gap measuring points on the surface of a liquid crystal cell 13 to be mapped in a uniformly moved section 17 of a production line.
  • An array of points on the surface of the liquid crystal 13 to be measured at a moment is denoted as 21a, 21b, and 21N.
  • Loci of measured places on the surface of the moved liquid crystal 13 are denoted as 21a, 21b, and 21N.
  • Fig.3 shows typical interference oscillations in the spectrum of light interacted with a liquid crystal cell and measured by one of the channels in the system shown in Fig.l;
  • Fig.4 shows an example of light spectrum measured by the apparatus shown in Fig.l when a light crystal cell is not under the light beam produced by the light source 1;
  • step S51 it is measured the light spectrum in the apparatus shown in Fig.l in step S51.
  • step S52 the measured spectrum is divided by the spectrum of the light source 11.
  • step S53 the computer program analyzes if the measured spectrum contains interference oscillations or not. If the measured spectrum contains the interference oscillations, that exit in the spectrum of the light interacted with the said liquid crystal cell then the process goes to step S54, in which the cell gap d e of empty liquid crystals is calculated either from the following formula
  • N r M are numbers of the detected minima and maxima in the measured spectrum of the light interacted with the liquid crystal cell, respectively,
  • a mia , ⁇ max are the wavelengths of the said minima and maxima, respectively, or as an argument for which the following function
  • a wave numbers in the measured spectrum containing interference oscillations, respectively, ⁇ is an angle of incidence of the collimated light beam on the liquid crystal cell.
  • the cell gap d f of filled liquid crystal cells is calculated either from the following formula
  • n is the average refractive index of the liquid crystal filled into the said liquid crystal cell
  • ⁇ is the angle of incidence of the collimated light beam on the liquid crystal cell.
  • a coordinate X of a measured point of the liquid crystal cell is calculated as
  • v is a speed of the uniformly moved section 17 of the production line
  • t is time since the first measuring point of the liquid crystal cell have observed
  • Y coordinate is defined from number of channel of the spectrometer 15.
  • the distribution of a liquid crystal cell with 50 centimeters length consist of 250 x N points for mapping.
  • the computer program saves results for cell gap distribution of the previous liquid crystal cell and makes decision about its defectiveness from comparison of the measured results with a preset acceptance criterion for possible value of the cell gap and accepted tolerance.
  • FIG. 6 shows the optical arrangement that can be used to mapping the cell gap and its uniformity of a transmissive liquid crystal cell.
  • the apparatus comprises a collimated white light source 11, a polarizer 62, and a filled liquid crystal cell 63b under testing, an analyzer 64, a real-time multi-channel spectrometer 15, and a computer 16 controlled the measuring process.
  • FIG. 7 shows the optical arrangement for reflective cell gap measurement.
  • the basic elements are the same as in the transmissive case of FIG. 6, except that a partial mirror 70 is used to redirect the incident linearly polarized light.
  • df is the cell gap
  • An is the birefringence of the liquid crystal in the cell
  • N is the order of the interference
  • ⁇ mn is the wavelength of the interference minimum.
  • the second case is when the angle between transmission axis of the analyzer 64 and input director of the liquid crystal cell is
  • ⁇ is the angle between the plane of vibration in the beam of collimated light and the input director of the liquid crystal filled the cell.
  • minimum reflectance is observed for a wavelength satisfied one of the conditions (7) or (9) and crossed the polarizer 62 and the analyzer 64.
  • FIG.8 is a graph of a typical measured spectral transmittance in the apparatus shown in Fig. 6 when orientations of the polarizer 62 and analyzer 64 satisfied to one of the conditions (6) or (8), or measured spectral reflectance in the apparatus shown in Fig. 7 when orientations of the polarizer 62 and analyzer 64 are crossed.
  • step S91 it is measured the light spectrum in the apparatus shown in Fig.7 if a liquid crystal cell is transmissive or in the apparatus shown in Fig.8 if a liquid crystal cell is reflective.
  • step S92 the measured spectrum is divided by the spectrum of the light source 11.
  • step S93 the computer program compares the measured spectrum with the spectrum when a liquid crystal cell is out of the light beam and makes decision if a liquid crystal cell is under the test or not. If the measured spectrum is a spectrum of light interacted with a liquid crystal cell, then the process goes to step S94, in which the cell gap is calculated from one of the formula
  • df is the solution of the equation (9) if the angle between the polarizer 62 and the analyzer 64 corresponds to the condition (8)
  • the cell gap is defined from equation (7) or equation (9) .
  • the choice of the equation (7) or (9) that is necessary to apply is made from the parameters of the liquid crystal cells known preliminary.
  • the obtained result of the cell gap is saved in an array of data.
  • X coordinate of a measured point, is calculated from Eq. (2) .
  • the computer program saves results for cell gap distribution of the previous liquid crystal cell and makes, decision about its defectiveness from comparison of the measured results with set criterion for possible deviations of the cell gap values.

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  • Nonlinear Science (AREA)
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Abstract

The invention has an object of in-line inspection and mapping of the cell gap of liquid crystal cells in a production line. The cell gap is determined from spectral positions of extremes in the spectrum of a light passed through a moved liquid crystal cell or reflected from it. Decision about defectiveness of the liquid crystal cell is made from preset acceptance criterion. All measurements are in real-time and in-situ.

Description

METHOD AND APPARATUS FOR IN-LINE INSPECTION AND MAPPING LIQUID CRYSTAL CELL GAP
Field of the Invention The present invention relates to an inspecting method and an inspecting apparatus for a liquid crystal cell, and for inline mapping liquid crystal cell gap in a production line.
Background of the Invention
In the process of manufacturing a liquid crystal cell, there arise cases where a gap between the two plates of the cell becomes not uniform or has wrong value.
As a conventional well known inspecting method for inspecting a cell gap in a production line of this liquid crystal cell, there has been employed a method comprising the steps, for example, of: illuminating the cell to be inspected with a sodium lamp or the like; counting the number of Newton rings appearing on the cell; and deciding a level of uniformity based on the number of Newton rings, whereby to decide on a pass or a failure of the liquid crystal cell. According to this inspecting method, in order to decide a level of uniformity of the gap in the liquid crystal cell, the center part of Newton rings is pressed by a projected object. When the inside liquid crystal tends to spread out by this pressing, a decision is made that the cell surface is convex. When the inside liquid crystal tends to gather together, a decision is made that the cell surface is concave. This conventional inspecting method has not been convenient in that it is necessary to press the surface of the liquid crystal display cell with this projected object. The closest prior art solution to the present invention related to an inspecting method is the method for inspecting liquid crystal cell disclosed in US Patent 6,657,218 that comprises the steps of: applying a plurality of color lights to a member provided with a gap, to produce color interference fringes/ obtaining respective intensities of the color lights in the images of the interference fringes taken by a color camera, at each of predetermined positions of each image, so as to compute and actual ratio among the obtained intensities of the color lights for each of the predetermined positions; and obtaining gap values of a plurality of points of the gap provided in the member, based on said actual ratio and theoretical ratios each of which is computed based on intensities of said color lights in an image corresponding to each of preliminarily set gap values.
The drawback of this method is that it cannot measure absolute values of cell gap of empty liquid crystal cells, accuracy of measuring absolute values of cell gap of filled liquid crystal cells is low. Moreover, the method of calculation of cell gap is based on image processing that makes the method cumbersome.
The US Patent 6,657,218 also discloses the closest prior art solution to the present invention related to apparatus for a liquid crystal cell gap measuring that characterized by comprising a light source, a color camera for picking up images of light interference fringes; a filter for transmitting a plurality of different color lights to the color camera; optical means for directing the light from the light source toward an object to be measured and directing reflected light thereof from the object to be measured toward the color camera; an image memory for storing images of the interference fringe formed by the light from the optical means and picked up by the color camera; computing means for obtaining respective intensities of the color lights in the image at each of predetermined positions of each image, so as to compute an actual ratio among the obtained intensities of the color lights for each of the predetermined positions; a reference data memory for storing theoretical ratios among intensities of said color lights in an image correspondingly to each of preliminarily set gap values; and gap value comparing/determining means for determining gap values of a plurality of points of gap provided in the object to be measured according to the actual ratio and the theoretical ratio.
Some shortcomings of this apparatus are inherited from the above method and other arise from the use of color camera, particularly from non-uniformity of sensitivity of its elements that requires correction means for correcting the intensities of the each color, and from necessity to use time consuming procedure to compare images.
Summary of the Invention In the view of the above problems, it is an object of the present invention to provide an inspecting method and an inspecting apparatus for inspecting a liquid crystal cell, capable of automatically in-situ carrying out an inspection of the cell gap and its uniformity in a production line that is necessary for quality control of produced liquid crystal cells .
To this end the method for in-line inspection and mapping of the cell gap of liquid crystal cells in a production line comprises following steps of:
(a) illumination of a liquid crystal cell moved in a production line with one or more beams of a collimated light from a broad band light source,
(b) real-time measuring spectra of the light after its interaction with the said moved liquid crystal cell,
(c) analysis of the measured spectra for the occurrence of interference oscillations, (d) calculating the liquid crystal cell gap from the measured spectra that contain the said interference oscillations,
(e) applying a preset acceptance criterion for measured cell gap uniformity of the said liquid crystal cell,
(f) making a decision if the tested liquid crystal cell should pass or be rejected.
In order to inspect simultaneously all width of the tested liquid crystal cell and to increase productivity and reliability of the method a linear array of the beams is used for inspection of the liquid crystal cell in direction perpendicular to the direction of the cell motion and above mentioned steps (b) , (c) , and (d) are executed separately for light originated from each beam of the said array of the beams. Then a map of liquid crystal cell gap is built and a preset acceptance criterion is applied to the said map.
The method is applicable both to reflective and transmissive liquid crystal cell, so the light after its interaction with the liquid crystal cell is the light transmitted through the cell or the light reflected from the cell.
For inspection of an empty liquid crystal cell gap with method according to the invention the following formula is used to calculate an empty liquid crystal cell gap
Figure imgf000005_0001
where α is the angle of incidence of the collimated light beam on the liquid crystal cell; N,M are numbers of the detected minima and maxima of interference oscillations in the measured spectrum of the light interacted with the liquid crystal cell, respectively; λmin, λmax are the wavelengths of the said minima and maxima, respectively.
For inspection of a filled liquid crystal cell with the method according to the invention and calculation of the cell gap the following formula is used
Figure imgf000006_0001
where α is the angle of incidence of the collimated light beam on the liquid crystal cell, N,M are numbers of the detected minima and maxima of the interference oscillations in the measured spectrum of the light beam interacted with the filled liquid crystal cell, respectively, λmin, λmax are the wavelengths of the said minima and maxima , respectively, and n is the average refractive index of the liquid crystal filled into the said liquid crystal cell. In other embodiment of the method the cell gap of an empty liquid crystal cell de is calculated as an argument for that the function
Figure imgf000006_0002
reaches its maximum, where I(y) is the measured spectrum containing the interference oscillations, v is the wave
1 ~ number, v=— ; v min and vmax are the minimal and the maximal
A wave numbers in the measured spectrum containing interference oscillations, respectively, α is the angle of incidence of the collimated light beam on the liquid crystal cell, and the cell gap of a filled liquid crystal cell df is calculated as an argument for that the following function
Figure imgf000006_0003
where ή is the average refractive index of the liquid crystal filled into the said liquid crystal cell, α is the angle of incidence of the collimated light beam on the liquid crystal cell. The further embodiments of the method according to the invention applicable only to filled liquid crystal cell envisage use of polarized light to produce another kind of minima and maxima in spectra of the light interacted with the liquid crystal cell due to birefringence of the liquid crystal filled into cell. To this end in the method the polarized collimated light illuminating tested areas of the filled liquid crystal cell is used and the light after its interaction with the moved liquid crystal cell and before measuring its spectra passes through a polarizing analyser. In one of the embodiment the collimated light is linearly polarized light and polarizing analyser is a linear polarizer. In this embodiment the liquid crystal cell gap is calculated from a spectral position of at least one minimum in the measured spectrum.
In one of further embodiments of the method applicable to transmissive cells the angle between the plane of vibration in the incident collimated light and the input director of the liquid crystal filled into the cell is chosen as
Figure imgf000007_0001
where φ is the twist angle of the liquid crystal,
and the cell gap of a filled liquid crystal cell df±s calculated according to the formula
1
Figure imgf000007_0002
where λmin is the wavelength of the minimum in the measured spectrum of the light transmitted through the liquid crystal cell, Δn is the birefringence of the liquid crystal filled into the cell, N is the order of interference.
In other embodiments also applicable to transmissive cells the angle between the transmission axis of the polarizing analyser and the input director of the liquid crystal cell is
a =φ- β±—
where φ is the twist angle of the liquid crystal, β is the angle between the plane of vibration in the incident collimated light and the input director of the liquid crystal filled into the cell, and
the cell gap of a filled liquid crystal cell df is calculated as the solution of the equation
Figure imgf000008_0001
where Δn is the birefringence of the liquid crystal filled into the cell, λmin is the wavelength of the minimum in the measured spectrum of the light transmitted through the liquid crystal cell.
In further embodiment applicable for reflective cells the transmission axis of the linear polarizing analyser is crossed with the plane of vibration of the incident collimated light, and the cell gap df of a filled liquid crystal is calculated according to the formula
Figure imgf000009_0001
where λmxn is the wavelength of the minimum in the measured spectrum of the light reflected from the liquid crystal cell, N is the order of interference.
In other embodiment applicable for reflective cells the transmission axis of the polarizing analyser is crossed with the plane of vibration in the beam of collimated light, and
a filled liquid crystal cell gap df is calculated as the solution of the equation
= tanlβ
Figure imgf000009_0002
where λmin is the wavelength of the minimum in the measured spectrum of the light reflected from the liquid crystal cell.
For implementation of the method according to the invention here is disclosed an apparatus for in-situ inspection of liquid crystal cells in a production line, comprising: a wide-band light source; a projection optical system for illuminating with one or more collimated light beams from the said wide-band light source at one or more tested areas of a liquid crystal cell moved uniformly in a production line; a real-time spectrometer for measuring spectra of the light interacted with the said liquid crystal cell; means for collection of light interacted with the said tested area of the liquid crystal cell and its transportation to the optical entrance of the said spectrometer; a computer processing the measured spectra in real time and calculating the cell gap of the liquid crystal cell from wavelengths of extremes in the measured spectra, herein the computer applies the preset criteria of the liquid crystal cell gap uniformity and distinguishes the different cells in a stream of the production line from changes in the measured spectrum, and then generates control signals on a pass or failure of the inspected liquid crystal cells.
In one of the embodiments of the apparatus the projection optical system provides a linear array of the light beams to illuminate tested areas of the liquid crystal cell located along the line perpendicular to the direction of movement of the liquid crystal cell; the real-time spectrometer is multichannel spectrometer, herein the number of its independent channels equals the said number of light beams; means for collection of the light interacted with the tested areas of the liquid crystal cell and its transportation to the entrances of the corresponding channels of the spectrometer; the computer creates data array of the cell gap values versus spatial coordinates, applies preset criteria of quality and makes decision on a pass a failure.
In other embodiment of the apparatus for inspection of filled liquid crystal cells with use of polarized light a polarizer is placed at output of the projection optical system and an analyzer is placed before the entrance of the spectrometer, and, particularly, the said polarizer and analyzer are linear.
In further embodiment of the apparatus for transmissive cells the said polarizer and analyzer are located on different sides of the inspected liquid crystal cell moved in the stream of the production line and in embodiment of the apparatus for reflective cells the said polarizer and analyzer are located on the same side of the inspected liquid crystal cell moved in the stream of the production line.
In the last embodiment for reflective cells the apparatus additionally comprises an optical coupling unit based on beam splitter that is positioned between polarizer and analyzer and the liquid crystal cell to direct the incident linearly polarized light normally to the front liquid crystal cell surface and to direct light reflected back from the tested areas of the liquid crystal cell to the analyser.
Brief Description of the Drawings
FIG. 1 is a configuration diagram showing the optical arrangement for in-line mapping liquid crystal cells from interference oscillations according to the present invention (embodiment for reflective cells is shown) ;
Fig.2 is an explanatory view for showing cell gap measuring points on the surface of a liquid crystal cell to be mapped in a uniformly moved section of a production line.
Fig.3 shows typical interference oscillations in the spectrum of light interacted with a liquid crystal cell and measured by one of the channels in the system shown in Fig.l;
Fig.4 shows an example of light spectrum measured by one of the channels of the multi-channel spectrometer in the apparatus shown in Fig.l when a light crystal cell is not under the light beam produced by the light source 1; FIG.5 is a flowchart of a spectrum processing operation of the liquid crystal cell inspecting apparatus shown in Fig.l according to the present invention;
FIG.6 shows the optical arrangement that can be used to measure the cell gap and mapping its uniformity of a filled transmissive liquid crystal cell according to the present invention;
FIG.7 shows the optical arrangement that can be used to measure the cell gap and mapping its uniformity of a filled reflective liquid crystal cell according to the present invention;
Fig.8 shows an example of light spectrum measured by one of the channels of the multi-channel spectrometer in the optical arrangement shown in Fig.6 or 7 when the crystal cell is under the light beam produced by the light source 11;
FIG. 9 is a flowchart of a spectrum processing operation of the liquid crystal cell inspecting apparatus shown in Fig.7 and Fig.8.
Detailed Description of the Preferred Embodiment
This invention is capable of in-situ measuring cell gap and its uniformity of reflective and transmissive filled and empty liquid crystal cells in a production line.
Embodiments of an inspecting apparatus for inspecting a liquid crystal cell according to the present invention will be explained below with reference to the drawings. A configuration diagram in FIG. 1 shows an optical system for in-line mapping liquid crystal cells according to the present invention. The optical system comprises a light source 11 that has a broad spectrum and equipped with a light projection set 12 for illumination a mapped liquid crystal cell 13b by a collimated light beam, a light guiding set 14 for projection of light beams transmitted through the liquid crystal cell or reflected from it on a real-time multichannel spectrometer 15 that has N channels, a computer 16 that treats the light spectrum, calculates cell gap, collects data for mapping uniformity of the liquid crystal cell and make decision about defectiveness of tested liquid crystal cells. 13a is a liquid crystal cell previously mapped; 13c is a liquid crystal cell that will be mapped after the cell 13b. Liquid crystal cells 13a, 13b, and 13c are uniformly moved by a moved section 17 of a product line.
Fig.2 is an explanatory view for showing cell gap measuring points on the surface of a liquid crystal cell 13 to be mapped in a uniformly moved section 17 of a production line. An array of points on the surface of the liquid crystal 13 to be measured at a moment is denoted as 21a, 21b, and 21N. Loci of measured places on the surface of the moved liquid crystal 13 are denoted as 21a, 21b, and 21N.
Fig.3 shows typical interference oscillations in the spectrum of light interacted with a liquid crystal cell and measured by one of the channels in the system shown in Fig.l;
Fig.4 shows an example of light spectrum measured by the apparatus shown in Fig.l when a light crystal cell is not under the light beam produced by the light source 1;
The measuring of cell gap and inspection of uniformity of the cell gaps in the liquid crystal cells 13a, 13b, 13c by the computer program controlled the measuring process will be explained next with references to the flowchart shown in Fig5,
At first, it is measured the light spectrum in the apparatus shown in Fig.l in step S51. Next, the process goes to step S52, in which the measured spectrum is divided by the spectrum of the light source 11. Next, the process proceeds to step S53, in which the computer program analyzes if the measured spectrum contains interference oscillations or not. If the measured spectrum contains the interference oscillations, that exit in the spectrum of the light interacted with the said liquid crystal cell then the process goes to step S54, in which the cell gap de of empty liquid crystals is calculated either from the following formula
Figure imgf000014_0001
where α is an angle of incidence of the collimated light beam on the liquid crystal cell, NrM are numbers of the detected minima and maxima in the measured spectrum of the light interacted with the liquid crystal cell, respectively, Amia, λmax are the wavelengths of the said minima and maxima, respectively, or as an argument for which the following function
Figure imgf000014_0002
reaches its maximum, where /(v) is the measured spectrum containing the interference oscillations, v is the wave
number, v=— ; v mχn and vmax are the minimal and the maximal
A wave numbers in the measured spectrum containing interference oscillations, respectively, α is an angle of incidence of the collimated light beam on the liquid crystal cell.
The cell gap df of filled liquid crystal cells is calculated either from the following formula
Figure imgf000015_0001
or as an argument for that the following function
Figure imgf000015_0002
reaches its maximum, where /(P) is the measured spectrum containing the interference oscillations, v is the wave
number, v=— ; v mln and vmax are the minimal and the maximal
A wave numbers in the measured spectrum containing interference oscillations, respectively, and n is the average refractive index of the liquid crystal filled into the said liquid crystal cell, α is the angle of incidence of the collimated light beam on the liquid crystal cell.
A coordinate X of a measured point of the liquid crystal cell is calculated as
x=vt, (5)
where v is a speed of the uniformly moved section 17 of the production line, t is time since the first measuring point of the liquid crystal cell have observed, the axis X orientated parallel to the direction of the movement. Y coordinate is defined from number of channel of the spectrometer 15.
For example, if average measuring time is 0,1 second, the speed of the moving section 17 of the production line is 2 centimeters per second, then the distribution of a liquid crystal cell with 50 centimeters length consist of 250 x N points for mapping.
If the measured spectra do not contain the interference oscillations that are typical for liquid crystal cells (for example as shown in Fig.4), this means that the liquid crystal cell is not under the testing and the next point with oscillations will be the first point of the next liquid crystal cell. In this case the process goes to step S55. If the previous measured points had oscillations then the computer program saves results for cell gap distribution of the previous liquid crystal cell and makes decision about its defectiveness from comparison of the measured results with a preset acceptance criterion for possible value of the cell gap and accepted tolerance.
FIG. 6 shows the optical arrangement that can be used to mapping the cell gap and its uniformity of a transmissive liquid crystal cell. The apparatus comprises a collimated white light source 11, a polarizer 62, and a filled liquid crystal cell 63b under testing, an analyzer 64, a real-time multi-channel spectrometer 15, and a computer 16 controlled the measuring process.
FIG. 7 shows the optical arrangement for reflective cell gap measurement. The basic elements are the same as in the transmissive case of FIG. 6, except that a partial mirror 70 is used to redirect the incident linearly polarized light.
The procedure of measurement and the specific method of treating the data to obtain the desired results are described below. In the apparatus of the embodiment of FIG. 6 (the transmissive liquid crystal cell embodiment) , linearly polarized light from the collimated light source 11 and the polarizer 62 is incident on the liquid crystal cell 63b and the resultant output light is then incident on the analyzer 64. The polarizer 62 and the analyzer 64 are orientated in such a way that spectrum of measured light contains an extreme/ wavelength of which depends on cell gap value. There are two cases when spectrum of the transmitted light has a minimum. The first one is when the angle between transmission axes of the polarizer 62 and the analyzer 64 is
r = <p±^f (6)
where φ is the twist angle of the liquid crystal. The minimum in spectral transmittance will be observed for wavelengths satisfied the following condition
Figure imgf000017_0001
where df is the cell gap, An is the birefringence of the liquid crystal in the cell, N is the order of the interference, and λmn is the wavelength of the interference minimum.
The second case is when the angle between transmission axis of the analyzer 64 and input director of the liquid crystal cell is
Figure imgf000017_0002
where, β is the angle between the plane of vibration in the beam of collimated light and the input director of the liquid crystal filled the cell. The minimum in spectral transmittance will be observed for wavelengths satisfied the following condition
Figure imgf000018_0001
In the apparatus of the embodiment of FIG. 7 (the reflective liquid crystal cell embodiment) , minimum reflectance is observed for a wavelength satisfied one of the conditions (7) or (9) and crossed the polarizer 62 and the analyzer 64.
FIG.8 is a graph of a typical measured spectral transmittance in the apparatus shown in Fig. 6 when orientations of the polarizer 62 and analyzer 64 satisfied to one of the conditions (6) or (8), or measured spectral reflectance in the apparatus shown in Fig. 7 when orientations of the polarizer 62 and analyzer 64 are crossed.
The measuring of cell gap and mapping of uniformity of the cell gaps in the liquid crystal cells 63a, 63b, 63c by the computer program controlled the measuring process will be explained next with references to the flowchart shown in Fig.9.
At first, in step S91, it is measured the light spectrum in the apparatus shown in Fig.7 if a liquid crystal cell is transmissive or in the apparatus shown in Fig.8 if a liquid crystal cell is reflective. Next, the process goes to step S92, in which the measured spectrum is divided by the spectrum of the light source 11. Next, the process proceeds to step S93, in which the computer program compares the measured spectrum with the spectrum when a liquid crystal cell is out of the light beam and makes decision if a liquid crystal cell is under the test or not. If the measured spectrum is a spectrum of light interacted with a liquid crystal cell, then the process goes to step S94, in which the cell gap is calculated from one of the formula
Figure imgf000019_0001
if the liquid crystal cell is measured in apparatus shown in Fig.7 and the angle between the polarizer 62 and the analyzer 64 corresponds to the condition (6)
or
df is the solution of the equation (9) if the angle between the polarizer 62 and the analyzer 64 corresponds to the condition (8)
If the liquid crystal cell is measured in the apparatus shown in Fig.8 and polarizer 62 and the analyzer 64 are crossed the cell gap is defined from equation (7) or equation (9) . The choice of the equation (7) or (9) that is necessary to apply is made from the parameters of the liquid crystal cells known preliminary. The obtained result of the cell gap is saved in an array of data. X coordinate of a measured point, is calculated from Eq. (2) .
If the measured spectra different from spectra of a liquid crystal cell (for example as shown in Fig.8) and correspond to spectra measured when a liquid crystal is not under the test (for example as shown in Fig.4), this means that a liquid crystal cell is not under the illuminated light beam and the next line of points with oscillations will be the first array of points of next liquid crystal cell. The process goes to step S96. If the previous array of points had oscillations then the computer program saves results for cell gap distribution of the previous liquid crystal cell and makes, decision about its defectiveness from comparison of the measured results with set criterion for possible deviations of the cell gap values.
Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected therein by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.

Claims

Claims
1. A method for in-line inspection and mapping of the cell gap of liquid crystal cells in a production line comprising steps of:
(a) illumination of a liquid crystal cell moved in a production line with one or more beams of a collimated light from a broad band light source,
(b) real-time measuring spectra of the light after its interaction with the said moved liquid crystal cell,
(c) analysis of the measured spectra for the occurrence of interference oscillations,
(d) calculating the liquid crystal cell gap from the measured spectra that contain the said interference oscillations,
(e) applying a preset acceptance criterion for measured cell gap uniformity of the said liquid crystal cell,
(f) making a decision if the tested liquid crystal cell should pass or be rejected.
2. The method of claim 1, wherein
at the step (a) a linear array of the beams is used for inspection of the liquid crystal cell in direction perpendicular to the direction of the cell motion;
the steps (b) , (c) , and (d) are executed separately for light originated from each beam of the said array of the beams;
a map of liquid crystal cell gap is built; the step (e) is applied to this map.
3. The method of any of claims 1,2 wherein the light after its interaction with the liquid crystal cell is the light transmitted through the cell.
4. The method of any of claims 1, 2 wherein the light after its interaction with the liquid crystal cell is the light reflected from the cell.
5. The method of any of claims 1-4, wherein at the step (d) to calculate the cell gap of an empty liquid crystal cell the following formula is used
Figure imgf000022_0001
where α is the angle of incidence of the collimated light beam on the liquid crystal cell; N, M are numbers of the detected minima and maxima of the interference oscillations in the measured spectrum of the light interacted with the liquid crystal cell, respectively; λmin, λmax are the wavelengths of the said minima and maxima, respectively.
6. The method of any of claims 1-4, wherein at the step (d) to calculate the cell gap of a filled liquid crystal cell the following formula is used
Figure imgf000022_0002
, )
where α is the angle of incidence of the collimated light beam on the liquid crystal cell, N, M are numbers of the detected minima and maxima of the interference oscillations in the measured spectrum of the light beam interacted with the filled liquid crystal cell, respectively, λmin, λmax are the wavelengths of the said minima and maxima, respectively, and ή is the average refractive index of the liquid crystal filled into the said liquid crystal cell.
7. The method of any of claims 1-4, wherein at the step (d) the cell gap de of an empty liquid crystal cell is calculated as an argument for that the function
Figure imgf000023_0001
reaches its maximum, where I(v) is the measured spectrum containing the interference oscillations, v is the wave
number, v= — ; v mχΩ and v' maχ are the minimal and the maximal λ wave numbers in the measured spectrum containing interference oscillations, respectively, α is an angle of incidence of the collimated light beam on the liquid crystal cell.
8. The method of any of claims 1-4, wherein at the step (d) the cell gap of a filled liquid crystal cell df is calculated as an argument for that the following function
Figure imgf000023_0002
reaches its maximum, where I(y) is the measured spectrum containing the interference oscillations, v is the wave
number, v= — / v mχn and vmax are the minimal and the maximal λ wave numbers in the measured spectrum containing interference oscillations, respectively, and n is the average refractive index of the liquid crystal filled into the said liquid crystal cell, α is the angle of incidence of the collimated light beam on the liquid crystal cell.
9. The method of any of claims 1,2, wherein the collimated light is polarized.
10. The method of claim 9, wherein the collimated light is polarized linearly.
11. The method of claim 9, wherein the light after its interaction with the moved liquid crystal cell and before measuring its spectra passes through a polarizing analyser.
12. The method of claim 11, wherein the polarizing analyser is a linear polarizer.
13. The method of claim 12, wherein at the step (d) the liquid crystal cell gap is calculated from a spectral position of at least one minimum in the measured spectrum.
14. The method of claims 3 and 13, wherein the angle between the plane of vibration in the incident collimated light and the input director of the liquid crystal filled into the cell is chosen as
where φ is the twist angle of the liquid crystal,
and at the step (d) the cell gap of a filled liquid crystal cell dfis calculated according to the formula
Figure imgf000024_0001
where λmin is the wavelength of the minimum in the measured spectrum of the light transmitted through the liquid crystal cell, Δn is the birefringence of the liquid crystal filled into the cell, N is the order of interference.
15. The method of claims 3 and 13, wherein
the angle between the transmission axis of the polarizing analyser and the input director of the liquid crystal cell is
a-φ-β±—
where φ is the twist angle of the liquid crystal, β is the angle between the plane of vibration in the incident collimated light and the input director of the liquid crystal filled into the cell,
and
at the step (d) the cell gap of a filled liquid crystal cell df is calculated as the solution of the equation
tanlβ
Figure imgf000025_0001
where Δn is the birefringence of the liquid crystal filled into the cell, λmin is the wavelength of the minimum in the measured spectrum of the light transmitted through the liquid crystal cell.
16. The method of claims 4 and 13, wherein the transmission axis of the polarizing analyser is crossed with the plane of vibration of the incident collimated light, and
at the step (d) the cell gap df of a filled liquid crystal is calculated according to the formula
Figure imgf000026_0001
where λmin is the wavelength of the minimum in the measured spectrum of the light reflected from the liquid crystal cell, N is the order of interference.
17. The method of claims 4 and 13, wherein the transmission axis of the polarizing analyser is crossed with the plane of vibration in the beam of collimated light,
and
at the step (d) the cell gap dfot a filled liquid crystal cell is calculated as the solution of the equation
Figure imgf000026_0002
where' λmin is the wavelength of the minimum in the measured spectrum of the light reflected from the liquid crystal cell
18. An apparatus for in-situ inspection of liquid crystal cells in a production line, comprising:
a wide-band light source; a projection optical system for illuminating with one or more collimated light beams from the said wide-band light source at one or more tested areas of a liquid crystal cell moved uniformly in a production line;
a real-time spectrometer for measuring spectra of the light interacted with the said liquid crystal cell/
means for collection of light interacted with the said tested area of the liquid crystal cell and its transportation to the optical entrance of the said spectrometer;
a computer processing the measured spectra in real time and calculating the cell gap of the liquid crystal cell from wavelengths of extremes in the measured spectra, herein the computer applies the preset criteria of the liquid crystal cell gap uniformity and distinguishes the different cells in a stream of the production line from changes in the measured spectrum, and then generates control signals on a pass or failure of the inspected liquid crystal cells.
19. The apparatus of claim 18, wherein
the projection optical system provides a linear array of the light beams to illuminate tested areas of the liquid crystal cell located along the line perpendicular to the direction of movement of the liquid crystal cell;
the real-time spectrometer is multi-channel spectrometer, herein the number of its independent channels equals the said number of light beams;
means for collection of the light interacted with the tested areas of the liquid crystal cell and its transportation to the entrances of the corresponding channels of the spectrometer; the computer creates data array of the cell gap values versus spatial coordinates, applies preset criteria of quality and makes decision on a pass a failure.
20. The apparatus of claims 18,19, wherein
a polarizer is placed at output of the projection optical system;
an analyzer is placed before the entrance of the spectrometer.
21. The apparatus of claim 20, wherein the said polarizer and analyzer are linear.
22. The apparatus of claim 21, wherein the said polarizer and analyzer are located on different sides of the inspected liquid crystal cell moved in the stream of the production line.
23. The apparatus of claim 21, wherein the said polarizer and analyzer are located on the same side of the inspected liquid crystal cell moved in the stream of the production line.
24. The apparatus of claim 23, wherein an optical coupling unit comprising beam splitter is positioned between polarizer and analyzer and the liquid crystal cell to direct the incident linearly polarized light normally to the front liquid crystal cell surface and to direct light reflected back from the tested areas of the liquid crystal cell to the analyser.
PCT/UA2005/000019 2005-05-20 2005-05-20 Method and apparatus for in-line inspection and mapping liquid crystal cell gap WO2006124007A1 (en)

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