Testing method

Abstract

To provide an inspection method that is a reflection type inspection method and can easily determine whether there are defects on a circular polarizing plate.SOLUTION: An inspection method determines whether there are defects on a film-shaped inspection target 10 that includes a circular polarizing plate 1 in which a polarizing film 11 and a retardation film 14 are laminated, and a release film 16a which is laminated on the retardation film 14 side of the circular polarizing plate 1 and formed of a polyethylene terephthalate resin. A light source 4, a band-pass filter 2 which allows light having a predetermined wavelength to pass through, a first polarizing section 3A, an inspection target 10, and a second polarizing section 3B are disposed, and an angle θ of incidence of light onto the inspection target 10 is changed so as to minimize effects of the phase difference of the release film 16a. The method determines whether there are defects on the circular polarizing plate 1 by observing light reflected by the inspection target 10 from the second polarizing section 3B side.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an inspection method.

Background Art

[0002] A polarizing plate used in a liquid crystal display device, an organic EL display device, etc. is generally configured such that two polarizers are sandwiched between two protective films. In order to attach the polarizing plate to the display device, an adhesive layer is laminated on one of the protective films, and a release film is further laminated on the adhesive layer. In many cases, a release film (surface protective film) for protecting the surface is also laminated on the other protective film. The polarizing plate is circulated and transported in a state where the release film is laminated, and the release film is peeled off when it is laminated on the display device in the manufacturing process of the display device.

[0003] By the way, in the manufacturing stage of the polarizing plate, foreign matter may be mixed between the polarizer and the protective film, air bubbles may remain, or when the protective film has the function of a retardation film, there may be alignment defects (hereinafter, these foreign matter, air bubbles and alignment defects may be collectively referred to as "defects"). When a polarizing plate having a defect is laminated on a display device, the defective portion may be visually recognized as a bright spot, or the image may appear distorted at the defective portion. In particular, defects visually recognized as bright spots are likely to be visually recognized when the display device is in black display.

[0004] Therefore, an inspection is performed to detect defects in the polarizing plate before it is bonded to the display device (while the release film is still attached). This defect inspection is generally an optical inspection that utilizes the polarization axis of the polarizing plate. Specifically, as shown in Patent Document 1, a polarizing filter is placed between the polarizing plate (the object under inspection) and the light source, and the polarizing plate or polarizing filter is rotated in a planar direction to establish a specific relationship between their respective polarization axis directions. When the polarization axis directions are orthogonal to each other (i.e., in the case of an arrangement that constitutes crossed nicols), linearly polarized light that has passed through the polarizing filter does not pass through the polarizing plate. However, if a defect exists in the polarizing plate, linearly polarized light will be transmitted at that location, and the presence of the defect is revealed by the detection of that light. On the other hand, when the polarization axis directions of the polarizing plate and the polarizing filter are parallel, linearly polarized light that has passed through the polarizing filter is transmitted through the polarizing plate. However, if a defect exists in the polarizing plate, linearly polarized light will be blocked at that location, and the presence of the defect is revealed by the fact that that light is not detected. The presence or absence of defects in a polarizing plate can be inspected by visually detecting the light that has passed through the polarizing plate, or by automatically detecting it using image analysis processing values ​​obtained by combining a CCD camera and an image processing device. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 9-229817 [Overview of the project] [Problems that the invention aims to solve]

[0006] When the polarizer is a circular polarizer and the release film is made of polyethylene terephthalate resin (PET resin), a phase difference filter (corresponding to the polarizer filter mentioned above) that is somewhat matched to the wavelength dispersion of the PET resin is used to inspect the polarizer. Here, when the circular polarizer and the phase difference filter are arranged to form crossed nicols, according to the above principle, defects will be visible as bright spots. However, in regions with low phase difference values, such as orientation defects or pinholes in the phase difference film of the circular polarizer, bright spot defects may be visible as dark spots, making detection and judgment more difficult than detecting them as bright spots. This tendency is particularly pronounced when the circular polarizer contains a phase difference film made of a cured polymerizable liquid crystal compound.

[0007] Furthermore, the principle of the inspection method shown in Patent Document 1 involves observing the light transmitted through the object being inspected. With this principle, if there is a deformation defect in the object being inspected (for example, wrinkles that occur when cutting a circular polarizing plate), the optical path length hardly changes between the normal part and the part with the deformation defect, making it difficult to optically detect the deformation defect.

[0008] Furthermore, as mentioned above, when a polarizing plate is equipped with a release film, the birefringence of this release film interferes with the polarization characteristics of the circular polarizer. As a result, conventional inspection devices were unable to accurately detect defects such as bright spots present on the polarizing plate.

[0009] Therefore, the present invention aims to provide a reflective inspection method that can easily determine whether or not there are defects in a circular polarizing plate. [Means for solving the problem]

[0010] The present invention provides an inspection method for determining whether or not there are defects in a film-like object to be inspected, comprising a circular polarizer formed by laminating a polarizing film and a phase difference film, and a release film made of polyethylene terephthalate resin laminated on the phase difference film side of the circular polarizer, wherein a light source, a bandpass filter that transmits light of a predetermined wavelength, a first polarizing portion, and an object to be inspected with the release film side facing the first polarizing portion are arranged in this order on the optical path of the light emitted by the light source, and a second polarizing portion that forms a cross nicol with the first polarizing portion is placed on the optical path of the light reflected by the object to be inspected, light from the light source is incident on the bandpass filter, the angle of incidence of the light to the object to be inspected is changed so as to reduce the effect of the phase difference of the release film, and the light reflected by the object to be inspected is observed from the side of the second polarizing portion to determine whether or not there are defects in the circular polarizer.

[0011] In this inspection method, since the first and second polarization sections are arranged to form crossed nicols, light reflected from the normal parts of the object under inspection (for example, light reflected from the surface of the release film) is blocked by the second polarization section, allowing the observation field to be sufficiently darkened, making it easier to observe any defects as bright spots. Light reflected from defects inside the object under inspection, or light reflected after passing through the defects, has a phase difference that is shifted from the ideal due to the defect (resulting in unintended elliptical polarization). This shift allows the light to pass through the second polarization section, enabling detection of defects in the object under inspection. Here, it is expected that the phase difference of the release film will increase the overall brightness of the observation field, hindering defect detection. However, in this inspection method, the angle of incidence of light on the object under inspection is changed so that the effect of the phase difference of the release film is minimized. That is, the angle of incidence of light is changed so that the phase difference produced by the release film approaches an integer multiple of the wavelength of the incident light. Therefore, even if the release film has a phase difference, the observation field can be sufficiently darkened. Furthermore, since this type of reflective inspection method results in a longer optical path through the object being inspected compared to a transmissive inspection method, deformation defects that were difficult to detect with the transmissive inspection method can be easily detected. Therefore, the inspection method of the present invention makes it easy to determine whether or not there are defects in a circular polarizer.

[0012] In this inspection method, it is preferable to perform an inspection using a bandpass filter, and then perform another inspection using a bandpass filter that transmits light of a different wavelength than the wavelength transmitted most by the first bandpass filter. This makes it possible to inspect for the presence or absence of two types of defects, for example, defects with a phase difference value greater than a predetermined value (which often appear blue when visually recognized as a defect, and are therefore referred to as blue spot irregularities below) and defects with a phase difference value less than a predetermined value (which often appear red when visually recognized as a defect, and are therefore referred to as red spot irregularities below).

[0013] In the inspection method of the present invention, before performing the inspection, a light source and two test pieces of circular polarizers having the same configuration as the circular polarizers on the object under inspection are prepared. The two test pieces are arranged so that their phase difference film sides face each other, and the angle between the slow phase axes of the phase difference films is an angle other than 90° when viewed from the direction of the optical path of the light source. Light of various wavelengths is incident from one side of the polarizing film of the test piece so that the optical path passes through a defect-free region on the test piece, and the polarizing film is observed from the other side to determine the wavelength at which the amount of transmitted light is minimized (hereinafter referred to as the "minimum wavelength"). It is preferable to decide to adopt at least one wavelength that is 5 nm to 50 nm larger than that wavelength and at least one wavelength that is 5 nm to 50 nm smaller than that wavelength as the predetermined wavelength. By using light of at least one wavelength that is 5 nm to 50 nm larger than the minimum wavelength and at least one wavelength that is 5 nm to 50 nm smaller than the minimum wavelength determined in this way for the inspection, the blue dot unevenness or red dot unevenness becomes more pronounced while the brightness of the entire observation field is sufficiently darkened. Specifically, by performing two inspections using both light with wavelengths 5nm to 50nm greater than the minimum wavelength determined in this way, and light with wavelengths 5nm to 50nm smaller, the blue and red dot irregularities can be emphasized in each inspection. Note that using light of a predetermined wavelength here means using a bandpass filter that transmits light of that wavelength.

[0014] In the inspection method of the present invention, the first polarizing portion and the second polarizing portion may both be circular polarizers, or they may both be linear polarizers. Furthermore, if they are circular polarizers, they may be composed of a single circular polarizer common to both.

[0015] The phase difference film may be made of a cured polymerizable liquid crystal compound. When the phase difference film is made of a cured polymerizable liquid crystal compound, its typical thinness increases the likelihood of observing black spot defects. Therefore, it is a suitable target for application of the present invention. [Effects of the Invention]

[0016] According to the present invention, there is provided a reflective inspection method capable of easily determining the presence or absence of defects in a circular polarizing plate.

Brief Description of the Drawings

[0017] [Figure 1] It is a configuration diagram of an inspection apparatus for performing the inspection method of the first embodiment. [Figure 2] It is a cross-sectional view of the inspection object. [Figure 3] It is a diagram showing the arrangement of each member in the wavelength selection step by transmission light measurement. [Figure 4] (A) is a diagram showing the relationship between the slow axes of two test pieces. (B) is a view of (A) seen from the optical path side. [Figure 5] It is a diagram for explaining the influence of the retardation of the release film in the inspection method of the first embodiment. [Figure 6] It is a configuration diagram of an inspection apparatus for performing the inspection method of the second embodiment.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In each figure, the same parts or corresponding parts are denoted by the same reference numerals, and duplicate explanations are omitted.

[0019] <Definitions of Terms and Symbols> The definitions of terms and symbols in this specification are as follows. (1) Refractive index (nx, ny, nz) 「nx」 is the refractive index in the direction where the in-plane refractive index is maximum (i.e., the slow axis direction), 「ny」 is the direction orthogonal to the slow axis in the plane, and 「nz」 is the refractive index in the thickness direction. (2) In-plane retardation value The in-plane retardation value (Re(λ)) refers to the in-plane retardation value of the film at 23°C and wavelength λ (nm). Re(λ) is obtained by Re(λ) = (nx - ny) × d when the film thickness is d (nm).

[0020] <First Embodiment> The inspection method of the first embodiment will be described.

[0021] (Inspection Device and Object to be Inspected) The inspection device of this embodiment inspects the presence or absence of defects on the surface of the circular polarizing plate, between each layer constituting the circular polarizing plate, or inside. As shown in FIG. 1, the inspection device 100 has a light source 4, a band-pass filter 2, and a retardation filter 3 arranged in this order. Further, the inspection device 100 also includes an inspection table 20 for placing the object to be inspected 10 on the opposite side of the retardation filter 3 as viewed from the light source 4. The inspection table 20 has a surface that is processed to suppress light reflection.

[0022] FIG. 1 shows a state where the object to be inspected 10 is placed on the inspection table 20. The retardation filter 3 is a single wide-band circular polarizing plate that is wider than the band-pass filter 2, and has both functions of a first polarization part 3A, which is a region where the light transmitted through the band-pass filter 2 is incident, and a second polarization part 3B, which is a region where the light reflected from the object to be inspected 10 described later is incident. That is, the first polarization part 3A and the second polarization part 3B are composed of a common single circular polarizing plate, and the first polarization part 3A and the second polarization part 3B are on the same plane as each other.

[0023] As shown in FIG. 2, the object to be inspected 10 is in a film shape and includes a circular polarizing plate 1, which is the main body to be inspected, and a release film 16a laminated on the circular polarizing plate 1 via an adhesive layer 15. The circular polarizing plate 1 has protective films 12a and 12b laminated on both surfaces of a polarizing film 11, and further, a retardation film 14 is formed via an adhesive layer 13 on the protective film 12a on the side with the release film 16a. And a surface protective film 16b is laminated on the surface of the circular polarizing plate 1 on the side without the release film 16a. The circular polarizing plate 1 is generally used in display devices, such as liquid crystal display devices and organic EL display devices , and during use, the release film 16a is peeled off and attached to the display device via the adhesive layer 15.

[0024] In this specification, "circular polarizer" includes both circular polarizers and elliptic polarizers. Furthermore, "circular polarization" includes both circularly polarized and elliptic polarized light.

[0025] The polarizing film 11 is a film that converts or absorbs light incident from the surface protective film 16a side into linearly polarized light. Examples of the polarizing film 11 include a polyvinyl alcohol film on which iodine or a dichroic dye is adsorbed and oriented, or a polymerizable liquid crystal compound that is oriented and polymerized on which a dichroic dye is adsorbed and oriented.

[0026] The protective films 12a and 12b are for protecting the polarizing film 11. To obtain a polarizing plate with appropriate mechanical strength, protective films commonly used in the field of polarizing plates are used for 12a and 12b. Typical examples include cellulose ester films such as triacetylcellulose (TAC) film; cyclic olefin films; polyester films such as polyethylene terephthalate (PET) film; and (meth)acrylic films such as polymethyl methacrylate (PMMA) film. Additives commonly used in the field of polarizing plates may also be included in the protective films.

[0027] Since the protective films 12a and 12b are laminated to the display device together with the polarizing film 11 as components of the circular polarizer 1, strict control of the phase difference value is required. Typically, a protective film 12a with an extremely small phase difference value is preferred. As for the protective film 12b, for example, one with a phase difference of λ / 4 or a small phase difference value is used to improve visibility when viewing the display device through polarizing sunglasses. The protective films 12a and 12b are laminated to the polarizing film 11 via an adhesive.

[0028] The phase difference film 14 is a film that converts light reflected from the surface protective film 16b side and linearly polarized by the polarizing film 11 into circularly polarized light. Viewed from the release film 16a side, the phase difference film 14 is a film that converts circularly polarized light incident from the release film 16a side into linearly polarized light. The phase difference film 14 is not particularly limited as long as it is a film having a phase difference, but it may be a laminate of a λ / 2 film and a λ / 4 film. In this case, the λ / 2 film may be layered on the side closer to the polarizing film 11, followed by the λ / 4 film.

[0029] Furthermore, it is preferable that the phase difference film 14 is made of a cured polymerizable liquid crystal compound. The phase difference film 14 made of a cured polymerizable liquid crystal compound is usually thin, with a thickness of about 0.2 μm to 10 μm, and the phase difference value is likely to change in areas containing foreign matter. In such areas, linearly polarized light is not completely converted to the ideal circularly polarized light, resulting in unintended elliptical polarization. Also, as will be described later, during inspection, what should be observed as a bright spot defect may be observed as a dark spot.

[0030] Polymerizable liquid crystal compounds capable of forming a phase difference film 14 include, for example, those disclosed in Japanese Patent Publication No. 2009-173893, Japanese Patent Publication No. 2010-31223, WO2012 / 147904, WO2014 / 10325, and WO2017-43438. The polymerizable liquid crystal compounds described in these publications can form a phase difference film having so-called inverse wavelength dispersion, which enables uniform polarization conversion over a wide wavelength range. For example, by coating a solution containing the polymerizable liquid crystal compound (polymerizable liquid crystal compound solution) onto a suitable substrate and photopolymerizing it, an extremely thin phase difference film can be formed as described above, and a circular polarizer having such a phase difference film can be formed with an extremely thin thickness. Such an extremely thin circular polarizer is advantageous as a circular polarizer for flexible display materials, which have attracted attention in recent years.

[0031] Examples of substrates to which polymerizable liquid crystal compound solutions are applied include those described in the aforementioned publication. Such substrates may be provided with an alignment film to orient the polymerizable liquid crystal compound. The alignment film may be photo-oriented by polarized irradiation or mechanically oriented by rubbing. Such alignment films are also described in the aforementioned publication.

[0032] However, if foreign matter or other imperfections are present on the substrate to which the polymerizable liquid crystal compound solution is applied, or if the substrate itself has scratches or other defects, defects may occur in the coating film obtained by applying the polymerizable liquid crystal compound solution. In addition, when the alignment film is rubbed, debris from the rubbing cloth may remain on the alignment film, which can cause defects in the coating film of the polymerizable liquid crystal compound solution (composition for forming liquid crystal curing films). Thus, although it is possible to form extremely thin phase difference films from polymerizable liquid crystal compounds, there are factors that can cause defects. Furthermore, as will be described later, defects in the phase difference film may occur as black spots. The inspection method of this embodiment is particularly useful in detecting the presence or absence of defects in an object under inspection having a circular polarizing plate and a release film equipped with such a phase difference film.

[0033] The phase difference film 14 can be prepared by applying an alignment film-forming composition onto a substrate, and then applying a liquid crystal curing film-forming composition containing a polymerizable liquid crystal compound on top of that. The phase difference film 14 thus prepared can be bonded together with the substrate to an adhesive layer 13 formed on a protective film 12a, and then the substrate can be peeled off to transfer the phase difference film 14 onto the protective film 12a.

[0034] The release film 16a is peeled off from the circular polarizer 1 when it is attached to the display device, and is usually discarded. Therefore, unlike the protective films 12a and 12b, strict control of the phase difference value is not required. Consequently, if a commercially available film is used as the release film 16a and its phase difference value is not compensated for, it may lead to malfunctions during defect inspection. In other words, in defect inspection of a circular polarizer 1 to which a release film 16a whose phase difference value is not strictly controlled is attached, the phase difference of the release film 16a may cause a decrease in the inspection accuracy of the inspection device 100.

[0035] As described in the background technology above, in the circular polarizer 1, a surface protection film 16b, which is a type of release film, is often provided on the opposite side of the release film 16a. In the circular polarizer 1 shown in Figure 2, the surface protection film 16b is laminated to the protective film 12b side. This surface protection film 16b is also usually peeled off from the circular polarizer 1 when it is laminated to a display device, and unlike the protective films 12a and 12b, strict control of the phase difference value is not required. In Figure 2, the protective film 12b and the surface protection film 16b may be laminated via a suitable adhesive layer or tack layer (this adhesive layer or tack layer is not shown in Figure 2).

[0036] In this embodiment, the release film 16a is made of PET resin. The surface protection film 16b is also made of PET resin. Films made of PET resin (PET resin films) have the advantage of being versatile and inexpensive as release films. On the other hand, as mentioned above, inexpensive PET resin films do not require strict control of phase difference values. Therefore, for example, there may be variations in phase difference values ​​from product lot to product. Also, even with the same PET resin film, there may be variations in phase difference values ​​within the plane. Even with circular polarizing plates laminated with such inexpensive PET resin films as release films, the inspection method of this embodiment can accurately detect the presence or absence of defects.

[0037] The in-plane phase difference value (Re(550)) of the release film 16a in this embodiment is, for example, 1500 nm to 3000 nm.

[0038] Here, we will show how to determine the Re(550) of the release film 16a. As mentioned above, these release films are PET resin films, and such films are readily available on the market. From this film, for example, a piece of about 40 mm x 40 mm in size is taken (e.g., by taking a piece from a long film using a suitable cutting tool). The Re(550) of this piece is measured three times, and the average value of Re(550) is calculated. The Re(550) of the piece can be measured using a phase difference measuring device KOBRA-WPR (manufactured by Oji Instruments Co., Ltd.) at room temperature (approximately 25°C). The same test can be performed to determine the Re(550) of the surface protection film 16b.

[0039] Various commercially available products can be used for the light source 4, but it is advantageous if it is a linear light source (including those that approximate linear light), such as laser light. The light emitted by the light source 4 is unpolarized and becomes circularly polarized after passing through the first polarization section 3A, which will be described later.

[0040] In the first embodiment, both the first polarization section 3A and the second polarization section 3B are broadband circular polarizers, each having a polarizing film that converts to linear polarization and a phase difference film that converts linear polarization to circular polarization. The first polarization section 3A and the second polarization section 3B always form crossed nicols when inspecting the object under inspection 10. To form crossed nicols with circular polarizers, when viewed from the light source side, if the slow phase axis of the phase difference film included in the first polarization section 3A and the slow phase axis of the phase difference film included in the second polarization section 3B are approximately parallel, then the polarizers should be arranged so that the absorption axis of the polarizing film included in the first polarization section 3A and the absorption axis of the polarizing film included in the second polarization section 3B are approximately parallel (arrangement α). In another configuration, when viewed from the light source side, if the slow phase axis of the phase difference film included in the first polarization section 3A and the slow phase axis of the phase difference film included in the second polarization section 3B are approximately orthogonal, the polarizing film included in the first polarization section 3A and the polarizing film included in the second polarization section 3B may be arranged so that their absorption axes are approximately orthogonal (arrangement β). When crossed nicols are configured using the former arrangement α, the first polarization section 3A and the second polarization section 3B can be constructed using a single circular polarizer. The phase difference filters 3 that constitute the first polarization section 3A and the second polarization section 3B are so-called defect-free filters.

[0041] To observe the light reflected from the object under inspection 10, a detection means 5, including a CCD camera, may be placed on the optical path of the reflected light and on the side of the second polarization section 3B where the light source 4 is located. For example, the object under inspection can be automatically detected by image processing analysis combining a CCD camera and an image processing device. Alternatively, the detection means 5 may not be a component, but rather a person visually observing the second polarization section 3B. Furthermore, a partition plate may be placed between the light source 4 and the CCD camera as appropriate.

[0042] Furthermore, it is preferable that the inspection device 100 includes a mechanism for tilting or rotating the inspection table 20, or a mechanism for tilting or rotating the arrangement of the light source 4, bandpass filter 2, and phase difference filter 3, so as to change the angle of incidence θ of light to the object under inspection 10. By moving these mechanisms, the phase difference generated by the release film 16a can be adjusted, and the brightness of the observation field can be adjusted to be darker to suit inspection.

[0043] (Testing method) The following describes a method for inspecting a circular polarizer using the inspection device 100. The inspection method of this embodiment includes a step of selecting two wavelengths as the light to be used for inspection (wavelength selection step) and a step of performing the inspection using the light of those wavelengths (defect inspection step).

[0044] ·Wavelength selection process Before starting the inspection of the object 10, which includes the circular polarizer 1, the wavelength of light to be used for the inspection is selected. The wavelength of light to be used for the inspection can be selected using the transmitted light from the circular polarizer prepared as a test piece, as described below.

[0045] The wavelength selection process by transmitted light measurement (hereinafter simply referred to as "transmitted light measurement") can be performed using a spectrophotometer (for example, the "V7100" manufactured by JASCO Corporation). Such a spectrophotometer has a light source (transmitted light source) and a means for measuring the amount of transmitted light. By placing a sample on the optical path connecting the transmitted light source and the means for measuring the amount of light transmitted from the transmitted light source to the sample and transmitted through the sample, the amount of light transmitted (transmitted light) can be measured. This transmitted light measurement will be explained with reference to Figures 3 and 4. As shown in Figure 3, the measuring instrument 500 used for transmitted light measurement has a transmitted light source 4A and a means for measuring the amount of transmitted light 5A. Two test pieces (test pieces 1A, 1B) of circular polarizers having the same configuration as the circular polarizer 1 provided in the object under inspection 10 are prepared as samples to be used for transmitted light measurement. Here, "same configuration" means that the material, thickness, and lamination configuration are substantially the same. Test specimen 1A comprises a polarizing film 11A and a phase difference film 14A, while test specimen 1B comprises a polarizing film 11B and a phase difference film 14B.

[0046] The two test specimens are positioned so that the phase difference films 14A and 14B face each other, and the angle between their slow phase axes p and q is not 90° when viewed from the optical path direction of the transmission light source 4A. Figure 4(A) is a schematic perspective view showing the main parts of the placement of the test specimens 1A and 1B. Figure 4(B) is a schematic diagram showing that the angle θ1 between the slow phase axes p and q is not 90° when the test specimens 1A and 1B are viewed from the optical path direction 9, in order to show the angle between the slow phase axes of the phase difference films 14A and 14B. This angle θ1 is preferably 10° to 80°, more preferably 20° to 70°, and even more preferably 30° to 60°. By arranging them at such an angle θ1, it becomes easier to find wavelengths useful for defect inspection.

[0047] Subsequently, light of various wavelengths is incident on either the polarizing film 11A or 11B of test specimens 1A or 1B from one side, such that the optical path 9 passes through a defect-free region on the test specimens 1A or 1B. The polarizing film 11A or 11B is then observed from the other side, and the wavelength at which the transmitted light amount is minimized is determined. Commercial spectrophotometers of this type can vary the wavelength of the transmission light source and can automatically analyze the amount of absorbed light at various wavelengths, thus making it easier to determine the minimum wavelength.

[0048] The wavelength analysis is preferably performed between 500 and 600 nm. Once the minimum wavelength (the wavelength at which the amount of transmitted light is minimized) is determined, two wavelengths are selected for the inspection: one 5 nm to 50 nm larger than that wavelength, and another 5 nm to 50 nm smaller. For example, if the minimum wavelength is 565 nm, then 535 nm and 595 nm are selected as the wavelengths of light used for the inspection, within ±30 nm of that wavelength. In other words, it is decided to use two bandpass filters for the inspection: one that transmits light at a wavelength of 535 nm and another that transmits light at a wavelength of 595 nm. These bandpass filters are preferably those with a full width at half maximum (FWHM) of the spectrum of the transmitted light wavelength of ±10 nm, and more preferably those with a FWHM of ±5 nm. These bandpass filters transmit different light spectra, and the wavelengths of light they transmit most strongly are also different. These bandpass filters are preferably those with a FWHM of ±10 nm, and more preferably those with a FWHM of ±5 nm, of the wavelength they transmit most strongly (peak wavelength).

[0049] • Defect inspection process Once the wavelength of light to be used for the inspection has been determined, the defect inspection of the object 10 to be inspected is then performed.

[0050] As shown in Figure 1, the object to be inspected 10 is placed on the inspection table 20 inside the inspection device 100. At this time, the side of the object to be inspected 10 that contains the release film 16a and the phase difference film 14 faces the light source 4, and the angle between the slow axis of the phase difference film 14 of the circular polarizer 1 and the slow axis of the phase difference film of the phase difference filter 3 is set to 10° to 80° when viewed from the light source 4. This angle is preferably 15° to 50°, and more preferably 20° to 40°. In this embodiment, the first polarizing section 3A and the second polarizing section 3B are composed of the same circular polarizer (phase difference filter 3), so the first polarizing section 3A and the second polarizing section 3B are arranged in a crossed nicol configuration with respect to the light reflected from the object to be inspected 10.

[0051] One of the two bandpass filters found in the wavelength selection process, bandpass filter 2, is prepared and placed inside the inspection device 100. Light is incident on bandpass filter 2 from light source 4. At this time, the incident angle θ with respect to the object under inspection 10 (angle relative to the perpendicular to the surface of the object under inspection 10) may be, for example, 3° to 30° or 5° to 20°. If the light emitted by light source 4 is light with low directivity, it is preferable to make the reflection angle from the object under inspection 10 (or the observation angle by the detection means 5) fall within the above angle range.

[0052] Light emitted from the light source 4 passes through the bandpass filter 2 and then enters the first polarizing section 3A, where it passes through and becomes circularly polarized (optical path 9a). The light that has passed through the first polarizing section 3A then enters the object under inspection 10. It then passes through the release film 16a in the object under inspection 10, and ideally is converted to linearly polarized light by the phase difference film 14 that constitutes the circular polarizer 1, and is finally absorbed by the polarizing film 11 (end of optical path 9a). Here, a portion of the light that has passed through the first polarizing section 3A is reflected from the surface of the release film 16a in the object under inspection 10 (optical path 9b). This reflected light is blocked by the second polarizing section 3B because the first polarizing section 3A and the second polarizing section 3B constitute a crossed nicol (end of optical path 9b), and therefore the observation field of view of the second polarizing section 3B by the detection means 5 is dark.

[0053] On the other hand, some of the light incident on the object under inspection 10 is strongly reflected at the parts of the object under inspection 10 that have defects (for example, defect D at the interface between the phase difference film 14 and the polarizing film 11, or defect D' in the phase difference film 14) (optical path 9c). Because the phase difference of this reflected light is shifted from the ideal due to defects D and D' (resulting in unintended elliptical polarization), it is not absorbed by the polarizing film, and reflected light is generated at the interface. This reflected light is transmitted without being blocked by the second polarizing part 3B. When this is observed from the detection means 5 side, the defective parts are observed as bright spots.

[0054] Here, the phase difference (in-plane phase difference) of the release film 16a can sometimes hinder this inspection. Specifically, if the phase difference exhibited by the release film 16a is an integer multiple of the wavelength of the light transmitted through the bandpass filter 2, the polarization state of the circularly polarized light incident on the release film 16a is not disturbed. However, in most cases, it is not an integer multiple, so the polarization state of the circularly polarized light is disturbed, and as shown in Figure 5, it cannot be converted to linear polarization by the phase difference film 14 and is not absorbed by the polarizing film, resulting in reflected light at the interface (optical path 9d). As a result, the amount of transmitted light passing through the second polarizing portion 3B increases, and the observation field becomes brighter. This causes the bright spots of the defect that should be observed to be buried in the overall brightness of the observation field, making it difficult to distinguish the defect. Furthermore, due to in-plane variations in the phase difference value of the release film 16a or variations from lot to lot, defects that should be observed as bright spots may be observed as dark spots.

[0055] To solve this problem, the inspection method of this embodiment changes the angle of incidence θ of light to the object under inspection 10 so that the influence of the phase difference of the release film 16a is reduced. That is, by changing the angle of incidence θ, the phase difference produced by the release film 16a changes, so by finding an angle of incidence θ that is an integer multiple of the above, the observation field can be made even darker. Here, in order to change the angle of incidence θ, the object under inspection 10 may be tilted or rotated in various ways (the inspection stand 20 may also be moved), or the light source 4, bandpass filter 2, and phase difference filter 3 may be tilted or rotated in various ways. In this way, by adjusting the relative positional relationship of the components constituting the inspection device 100, the angle of incidence θ is changed in various ways, and an angle in which the influence of the phase difference of the release film 16a is reduced is searched. When tilting the object under inspection 10, the circular polarizer 1 may be tilted with the slow phase axis direction as the axial direction, or with the fast phase axis direction as the axial direction.

[0056] After the inspection is complete, bandpass filter 2 is replaced with the other bandpass filter found in the wavelength selection process, and the same inspection is performed again. Through these two inspections, light with wavelengths 5 nm to 50 nm greater than the wavelength at which the amount of light transmitted by the second polarization unit 3B is minimized, and light with wavelengths 5 nm to 50 nm smaller, are used for the inspection. As a result, blue dot unevenness is emphasized with one bandpass filter, and red dot unevenness is emphasized with the other bandpass filter. Therefore, the presence or absence of both blue dot unevenness and red dot unevenness can be inspected under conditions where the brightness of the entire observation field is sufficiently darkened.

[0057] According to the inspection method described above, it is possible to easily determine whether or not there are defects in the circular polarizer. Furthermore, since this inspection method is a reflective type, the optical path in the object under inspection 10 is longer compared to a transmissive type inspection method, and deformation defects such as wrinkles, which are difficult to detect with a transmissive type inspection method, can be easily detected. Although Figure 1 shows the case where there is a defect in the phase difference film 14 of the circular polarizer 1, the inspection method of this embodiment can also detect defects in the polarizing film 11.

[0058] In order to increase the detection sensitivity of the inspection method of the present invention, it is preferable to perform the inspection in a dark room or other environment where external light is blocked. Furthermore, from the viewpoint of minimizing reflected light that is produced when light transmitted through the object to be inspected 10 is reflected off the inspection table 20, it is preferable that the surface on the inspection table 20 on which the object to be inspected 10 is placed is treated to reduce reflectivity.

[0059] <Second Embodiment> The inspection method of the second embodiment will now be described. The difference between the inspection method of the second embodiment and the inspection method of the first embodiment is that linear polarizers are used instead of circular polarizers for the first polarizing section 3A and the second polarizing section 3B.

[0060] (Inspection equipment and object being inspected) As shown in Figure 6, the inspection device 200 is configured such that the light source 4, the bandpass filter 2, and the first linear polarizer 7A are arranged in that order, and the second linear polarizer 7B is arranged next to the first linear polarizer 7A. The first linear polarizer (first polarizing portion) 7A and the second linear polarizer (second polarizing portion) 7B have surfaces that are parallel to each other and are arranged on substantially the same plane. The other configurations of the inspection device 200 are the same as those of the inspection device 100 in the first embodiment.

[0061] The first linear polarizer 7A and the second linear polarizer 7B are always oriented so as to form crossed nicols when inspecting the object under inspection 10. It should be noted that the light incident on the second linear polarizer 7B is reflected light from the object under inspection 10. Furthermore, so-called defect-free linear polarizers are used for both the first linear polarizer 7A and the second linear polarizer 7B.

[0062] (Testing method) The following describes the inspection method for circular polarizers using the inspection device 200. Before starting the inspection of the object 10 to be inspected, which includes the circular polarizer, the wavelength of light to be used for the inspection is selected.

[0063] ·Wavelength selection process The wavelength selection process is the same as in the first embodiment.

[0064] • Defect inspection process Once the wavelength of light to be used for the inspection has been determined, the defect inspection of the object 10 to be inspected is then performed.

[0065] The inspection method using the inspection device 200 is as follows. First, the object to be inspected 10 is placed on the inspection table 20 inside the inspection device 200. Then, the object to be inspected 10 is positioned on the other side of the first linear polarizing plate 7A and the second linear polarizing plate 7B as viewed from the light source 4. At this time, the side of the object to be inspected 10 that contains the release film 16a and the phase difference film 14 faces the light source 4, and the angle between the absorption axis of the polarizing film 11 and the absorption axis of the first linear polarizing plate 7A is 45° as viewed from the light source 4. This angle can take values ​​between 0° and 90°, and angles exceeding 90° are expressed as values ​​between 0° and 90°. Here, the positional relationship between the light source 4 and the detection means 5 is adjusted so that the light before it enters the object to be inspected 10 is transmitted through the first linear polarizing plate 7A, and the light reflected from the object to be inspected 10 is incident on the second linear polarizing plate 7B. Then, the first linear polarizer 7A is adjusted to form a crossed nicol with the second linear polarizer 7B.

[0066] One of the two bandpass filters found in the wavelength selection process, bandpass filter 2, is prepared and placed inside the inspection device 200. Light is incident on bandpass filter 2 from light source 4. At this time, the incident angle θ with respect to the object under inspection 10 (angle relative to the perpendicular to the surface of the object under inspection 10) may be, for example, 3° to 30° or 5° to 20°. If the light emitted by light source 4 is light with low directivity, it is preferable to make the reflection angle from the object under inspection 10 (or the observation angle by the detection means 5) fall within the above angle range.

[0067] Light emitted from light source 4 passes through bandpass filter 2 and then enters the first linear polarizer 7A, where it passes through and becomes linearly polarized (optical path 9a). Next, it enters the object under inspection 10 (optical path 9a). Then, it passes through the release film 16a in the object under inspection 10 and is converted to circularly polarized light by the phase difference film 14 constituting the circular polarizer 1, and the absorption axis component of this circularly polarized light is absorbed by the polarizing film 11 (end of optical path 9a). At this point, a portion of the light that has passed through the first linear polarizer 7A is reflected from the surface of the release film 16a in the object under inspection 10 (optical path 9b). This reflected light is blocked by the second linear polarizer 7B because the first linear polarizer 7A and the second linear polarizer 7B are arranged to form crossed nicols (end of optical path 9b). As a result, the observation field of view of the second linear polarizer 7B by the detection means 5 is dark enough to observe defects.

[0068] On the other hand, some of the light incident on the object under inspection 10 is strongly reflected at the parts of the object under inspection 10 that have defects (for example, defect D at the interface between the phase difference film 14 and the polarizing film 11, or defect D' in the phase difference film 14) (optical path 9c). Because the phase difference of this reflected light is shifted from the ideal due to defects D and D' (resulting in unintended elliptical polarization), the amount of light absorbed by the polarizing film 11 and the second linear polarizer 7B is smaller than in the normal parts by the amount of this shift, and the light passes through the second linear polarizer. When this is observed from the detection means 5 side, the defective parts are observed as bright spots.

[0069] In this embodiment, the method for reducing the effect of the phase difference of the release film 16a and the principle by which the effects of this embodiment are achieved are the same as in the first embodiment.

[0070] Although preferred embodiments of the present invention have been described above, the present invention is not limited in any way to the above embodiments. For example, in the first embodiment, the first polarization section 3A and the second polarization section 3B were carried out by a single phase difference filter 3, but the first polarization section 3A and the second polarization section 3B may be prepared as separate phase difference filters.

[0071] Furthermore, in the wavelength selection step of the above embodiment, the minimum wavelength was determined using transmitted light, but instead, the minimum wavelength may be determined using reflected light. For example, a laminate is prepared in which a circular polarizer 1 is placed on a reflector such as a mirror, and light of an arbitrary wavelength is incident on the laminate from a light source. The light reflected from the laminate is observed with a detection means and the amount of transmitted light is confirmed. Next, light of a different wavelength is irradiated and the amount of transmitted light is confirmed. In this way, the amount of transmitted light can be measured with light of various wavelengths, and the wavelength at which the amount of transmitted light is minimized can be determined. [Industrial applicability]

[0072] This invention can be used for quality inspection of circular polarizing plates. [Explanation of Symbols]

[0073] 1...Circular polarizer, 1A, 1B...Test piece (circular polarizer), 2...Bandpass filter, 3...Phase difference filter, 3A...First polarization section, 3B...Second polarization section, 4...Light source, 4A...Transmitting light source, 5...Detection means, 5A...Transmitted light quantity measuring means, 7A...First linear polarizer (first polarization section), 7B...Second linear polarizer (second polarization section), 9(9a,9b,9c)...Optical path, 10...Object under inspection, 11 (11A, 11B)...Polarizing film, 12a, 12b...Protective film, 13...Adhesive layer, 14 (14A, 14B)...Phase difference film, 15...Adhesive layer, 16a...Release film, 16b...Surface protective film, 20...Inspection table, 100, 200...Inspection device, 500...Measuring instrument, D, D'...Defect, p, q...Late phase axis direction of phase difference film, θ...Incident angle, θ1...Angle between the late phase axes.

Claims

1. An inspection method for determining whether or not a film-like object to be inspected has defects, comprising a circular polarizing plate formed by laminating a polarizing film and a phase difference film, and a release film made of polyethylene terephthalate resin laminated on the phase difference film side of the circular polarizing plate, Light source and A bandpass filter that transmits light of a predetermined wavelength, The first polarization part and The object to be inspected, with the release film side facing the first polarizing portion side, is arranged in this order on the optical path of the light emitted by the light source, and The first polarizing portion and the second polarizing portion that constitutes a crossed nicol are placed on the optical path of the light reflected by the object under inspection. The light from the light source is incident on the bandpass filter. The angle of incidence of the light on the object to be inspected is changed so that the effect of the phase difference of the release film is reduced. An inspection method for determining whether or not there are defects in the circular polarizer by observing the light reflected by the object under inspection from the second polarizing portion side.

2. The inspection method according to claim 1, wherein, after performing an inspection using the bandpass filter, an inspection is performed using a bandpass filter that transmits the most light of a wavelength other than the wavelength of light transmitted most by the bandpass filter.

3. Before performing the inspection described in Claim 1, Light source and Two test pieces of circular polarizing plates having the same configuration as the circular polarizing plate found in the object under inspection are prepared. The two test pieces are arranged so that their phase difference film sides face each other, and the angle between the slow phase axes of the phase difference films is an angle other than 90° when viewed from the direction of the optical path of the light source. The inspection method according to claim 1 or 2, wherein light of various wavelengths is incident on the polarizing film of the test piece from either side such that the optical path passes through a defect-free region on the test piece, the polarizing film is observed from the other side, the wavelength at which the amount of transmitted light is minimized is determined, and it is decided to adopt at least one wavelength between 5 nm and 50 nm greater than that wavelength and between 5 nm and 50 nm less than that wavelength as the predetermined wavelength.

4. The inspection method according to any one of claims 1 to 3, wherein both the first polarizing portion and the second polarizing portion are circular polarizers.

5. The inspection method according to any one of claims 1 to 4, wherein the first polarizing portion and the second polarizing portion are composed of a common circular polarizing plate.

6. The inspection method according to any one of claims 1 to 3, wherein both the first polarizing portion and the second polarizing portion are linear polarizing plates.

7. The inspection method according to any one of claims 1 to 6, wherein the phase difference film is made of a cured product of a polymerizable liquid crystal compound.

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