Inspection 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 technology]

[0002] Polarizing plates used in liquid crystal displays, organic EL displays, and the like are generally constructed by sandwiching a polarizer between two protective films. To attach the polarizing plate to a 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 protection film) that protects the surface of the other protective film is also attached. Polarizing plates are transported and distributed with the release film laminated in this manner, and the release film is peeled off when the polarizing plate is attached to a display device during the manufacturing process of the display device.

[0003] During the manufacturing process of polarizing plates, foreign matter may be mixed in between the polarizer and the protective film, air bubbles may remain, or, if the protective film functions as a retardation film, alignment defects may be present (hereinafter, these foreign matter, air bubbles, and alignment defects may be collectively referred to as "defects"). When a polarizing plate containing defects is attached to a display device, the defective areas may be visually recognized as bright spots, or the image may appear distorted at the defective areas. In particular, defects that are visually recognized as bright spots are easily visible when the display device is displaying black.

[0004] Therefore, before a polarizing plate is bonded to a display device (when the polarizing plate has a release film attached), an inspection is performed to detect defects in the polarizing plate. This defect inspection is generally an optical inspection using the polarization axis of the polarizing plate. Specifically, as shown in Patent Document 1, a polarizing filter is placed between the polarizing plate to be inspected and a light source, and the polarizing plate or polarizing filter is rotated in a planar direction to establish a specific relationship between the polarization axes of the polarizing plate or the polarizing filter. When the polarization axes are perpendicular to each other (i.e., in a crossed Nicol configuration), linearly polarized light that passes through the polarizing filter does not pass through the polarizing plate. However, if a defect exists in the polarizing plate, linearly polarized light passes through the defect, and the detection of this light reveals the presence of the defect. On the other hand, when the polarization axes of the polarizing plate and the polarizing filter are parallel, linearly polarized light that passes through the polarizing filter passes through the polarizing plate. However, when a defect exists in the polarizing plate, linearly polarized light is blocked at the defect, and the absence of detection of this light reveals the presence of the defect. The presence or absence of defects in polarizing plates can be inspected by an inspector visually detecting the light that has passed through the polarizing plate, or by automatic detection 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 Summary of the Invention [Problem to be solved by the invention]

[0006] When the polarizing plate is a circular polarizing plate and the release film is made of polyethylene terephthalate resin (PET resin), a retardation filter (corresponding to the polarizing filter described above) that is somewhat tuned to the wavelength dispersion of the PET resin is used to inspect the polarizing plate. Here, when the circular polarizing plate and the retardation filter are arranged in a crossed Nicol configuration, defects are visually recognized as bright spots according to the above principle. However, in areas with low retardation values, such as alignment defects or pinholes in the retardation film of the circular polarizing plate, bright spot defects may be visually recognized as black spots, making detection more difficult than detecting them as bright spots. This tendency is particularly pronounced when the circular polarizing plate includes a retardation film made of a cured product of a polymerizable liquid crystal compound.

[0007] Furthermore, the principle of the inspection method disclosed in Patent Document 1 is to observe light transmitted through an object to be inspected. With this principle, if the object to be inspected has a deformation defect (for example, wrinkles that occur when cutting a circularly polarizing plate), it is difficult to optically detect the deformation defect because the optical path length is almost the same between the normal part and the deformation defect part.

[0008] Furthermore, as mentioned above, when a polarizing plate has a release film, the birefringence of this release film impairs the polarization properties of the circular polarizing plate, and conventional inspection devices were unable to accurately detect defects such as bright spots present on the polarizing plate.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a reflection-type inspection method that can easily determine whether or not a circularly polarizing plate has a defect. [Means for solving the problem]

[0010] The present invention is an inspection method for determining the presence or absence of defects in a film-like object to be inspected, which includes a circular polarizing plate formed by laminating a polarizing film and a retardation film, and a release film made of a polyethylene terephthalate resin and laminated on the retardation film side of the circular polarizing plate, and the method includes arranging a light source, a bandpass filter that transmits light of a predetermined wavelength, a first polarizing unit, and the object to be inspected with the release film side facing the first polarizing unit in this order on the optical path of light emitted from the light source, and arranging a second polarizing unit that forms a crossed Nicol with the first polarizing unit on the optical path of light reflected by the object to be inspected. The first polarizing unit and the second polarizing unit are both left-rotating or both right-rotating circular polarizers, and when the first polarizing unit and the second polarizing unit are viewed from the light source side, the absorption axis of the polarizing film in the first polarizing unit and the absorption axis of the polarizing film in the second polarizing unit are oriented in directions perpendicular to each other; light from the light source is incident on a bandpass filter, the angle of incidence of the light on 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 second polarizing unit side to determine whether or not there is a defect in the circular polarizing plate.

[0011] In this inspection method, the first and second polarizing units are arranged in a crossed Nicol configuration. Light reflected from normal portions of the object under inspection (e.g., light reflected from the surface of a release film) is blocked by the second polarizing unit, sufficiently darkening the observation field, making it easier to observe defects as bright spots. Light reflected from defects within the object under inspection or light reflected after passing through the defects has a phase difference that is different from the ideal (unintended elliptically polarized light) due to the defects. Therefore, the light passes through the second polarizing unit by the amount of this difference, allowing it to be detected as a defect in the object under inspection. While the phase difference of the release film might increase the overall brightness of the observation field, hindering defect detection, this inspection method changes the angle of incidence of light onto the object under inspection to reduce the effect of the phase difference of the release film. This means that the angle of incidence of light is changed so that the phase difference exhibited by the release film approaches an integer multiple of the wavelength of the incident light. Therefore, the observation field can be sufficiently darkened even when the release film has a phase difference. Furthermore, since the optical path in the object to be inspected is longer in such a reflection-type inspection method than in a transmission-type inspection method, deformation defects that are difficult to detect in a transmission-type inspection method can be easily detected. As a result, the inspection method of the present invention can easily determine whether or not there is a defect in a circularly polarizing plate.

[0012] Since this inspection method captures reflected light from multiple types of films, it is advantageous in terms of defect detection to use as inspection light a wavelength at which the interference of the reflected light is relatively strong. From this perspective, it is preferable to calculate the wavelength dependency of the interference reflected light in the front direction from the average refractive index and thickness of the retardation film before inspection, determine the wavelength at which the reflection intensity is maximum in the wavelength range of 500 nm to 600 nm, and determine to use a wavelength within ±20 nm of that wavelength as the predetermined wavelength.

[0013] In this inspection method, it is preferable that the circular polarizer of the object to be inspected and the second polarizing unit are arranged to form a crossed Nicol configuration, which makes it possible to further darken the observation field of view.

[0014] The retardation film may be made of a cured product of a polymerizable liquid crystal compound. When the retardation film is made of a cured product of a polymerizable liquid crystal compound, the possibility of black spot defects being observed increases due to its general thinness. Therefore, it is suitable for application of the present invention. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a reflection-type inspection method that can easily determine whether or not there is a defect in a circular polarizer. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a configuration diagram of an inspection device for performing an inspection method according to a first embodiment. [Figure 2] FIG. [Figure 3] Graphs (A) and (B) both show the wavelength dependence of the reflection index of a λ / 4 film. [Figure 4] FIG. 2 is a diagram showing the positional relationship between polarizing films and retardation films. [Figure 5] FIG. 1 is a diagram illustrating the influence of the retardation of a release film. [Figure 6] FIG. 2 is a diagram showing the positional relationship between polarizing films and retardation films. DETAILED DESCRIPTION OF THE INVENTION

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and redundant explanations will be omitted.

[0018] <Definition of terms and symbols> The definitions of terms and symbols used in this specification are as follows. (1) Refractive index (nx, ny, nz) "nx" is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), "ny" is the in-plane direction perpendicular to the slow axis, 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 a film at 23°C and wavelength λ (nm). Re(λ) is calculated by Re(λ) = (nx - ny) × d, where d (nm) is the thickness of the film.

[0019] <Inspection equipment and inspection object> The inspection device of this embodiment inspects the surface of a circular polarizer, the spaces between the layers that make up the circular polarizer, or the interior of each layer for defects. As shown in Fig. 1, the inspection device 100 includes a light source 4, a bandpass filter 2, and a phase difference filter 3 arranged in this order. The inspection device 100 also includes an inspection table 20 on which an object to be inspected 10 is placed, on the opposite side of the phase difference filter 3 from the light source 4. The surface of the inspection table 20 is treated to suppress light reflection.

[0020] FIG. 1 shows an inspection object 10 placed on an inspection table 20. The phase difference filter 3 is composed of two broadband circular polarizers 3A and 3B arranged side by side on approximately the same plane. These two circular polarizers constitute a first polarizer 3A, which is the region where light transmitted through the bandpass filter 2 enters, and a second polarizer 3B, which is the region where light reflected from the inspection object 10 (described later) enters. Both the first polarizer 3A and the second polarizer 3B employ left-handed or right-handed circular polarizers. The first polarizer 3A and the second polarizer 3B are arranged so that the absorption axes of the polarizing films of the first polarizer 3A and the second polarizer 3B are perpendicular to each other when viewed from the light source 4 side (crossed Nicols). The first polarizer 3A and the second polarizer 3B are so-called defect-free polarizers.

[0021] As shown in FIG. 2, the inspection object 10 is in the form of a film and includes a circular polarizer 1, which is the main body of the inspection target, and a release film 16a laminated to the circular polarizer 1 via an adhesive layer 15. The circular polarizer 1 has protective films 12a and 12b attached to both sides of a polarizing film 11, and further has a retardation film 14 formed on the protective film 12a on the side with the release film 16a via an adhesive layer 13. A surface protective film 16b is laminated on the side of the circular polarizer 1 that does not have the release film 16a. The circular polarizer 1 is generally used in display devices, such as liquid crystal display devices and organic EL display devices, and when in use, the release film 16a is peeled off and the circular polarizer 1 is attached to the display device via the adhesive layer 15.

[0022] The polarizing film 11 is a film that converts light incident from the surface protection film 16b side into linearly polarized light. Examples of the polarizing film 11 include a polyvinyl alcohol film to which iodine or a dichroic dye is adsorbed and oriented, and a polymerizable liquid crystal compound to which a dichroic dye is adsorbed and oriented.

[0023] The protective films 12a and 12b are intended to protect the polarizing film 11. Protective films 12a and 12b are generally used in the technical field of polarizing plates in order to obtain polarizing plates with appropriate mechanical strength. Typical examples include cellulose ester films such as triacetyl cellulose (TAC) films; cyclic olefin films; polyester films such as polyethylene terephthalate (PET) films; and (meth)acrylic films such as polymethyl methacrylate (PMMA) films. Furthermore, additives generally used in the technical field of polarizing plates may be contained in the protective films.

[0024] The protective films 12a and 12b are attached to a display device together with the polarizing film 11 as components of the circular polarizing plate 1, and therefore require strict control of the retardation value. Typically, a film with an extremely small retardation value is preferably used as the protective film 12a. The protective films 12a and 12b are attached to the polarizing film 11 via an adhesive.

[0025] The retardation film 14 is a film that converts light that has entered from the surface protection film 16b side and been converted into linearly polarized light by the polarizing film 11 into circularly polarized light. When viewed from the release film 16a side, the retardation film 14 is a film that converts circularly polarized light that has entered from the release film 16a side into linearly polarized light. Therefore, the retardation film 14 includes at least a λ / 4 film. The retardation film 14 may also be further laminated with a λ / 2 film. In this case, the λ / 2 film and the λ / 4 film may be laminated in this order from the side closest to the polarizing film 11.

[0026] Furthermore, the retardation film 14 is preferably made of a cured product of a polymerizable liquid crystal compound. A retardation film 14 made of a cured product of a polymerizable liquid crystal compound is typically thin, approximately 0.2 μm to 10 μm thick, and if it contains foreign matter, the retardation value is likely to change in that area. In such areas, linearly polarized light is not completely converted into circularly polarized light as is ideal, resulting in unintended elliptically polarized light. Furthermore, as will be described later, what should be observed as a bright spot defect during inspection may be observed as a black spot.

[0027] Examples of polymerizable liquid crystal compounds that can form the retardation film 14 include those disclosed in JP 2009-173893 A, JP 2010-31223 A, WO 2012 / 147904 A, WO 2014 / 10325 A, and WO 2017-43438 A. The polymerizable liquid crystal compounds described in these publications can form retardation films with so-called reverse wavelength dispersion, which are capable of uniform polarization conversion over a wide wavelength range. For example, by applying a solution containing the polymerizable liquid crystal compound (polymerizable liquid crystal compound solution) to a suitable substrate and photopolymerizing it, an extremely thin retardation film can be formed as described above. Therefore, a circular polarizer having such a retardation film can be formed with an extremely thin thickness. Such extremely thin circular polarizers are advantageous as circular polarizers for flexible display materials, which have been attracting attention in recent years.

[0028] Examples of the substrate onto which the polymerizable liquid crystal compound solution is applied include those described in the above-mentioned publications. Such a substrate may be provided with an alignment film to align the polymerizable liquid crystal compound. The alignment film may be either one that is optically aligned by irradiation with polarized light or one that is mechanically aligned by rubbing treatment. Such alignment films are also described in the above-mentioned publications.

[0029] However, if foreign matter or the like is present on the substrate to which the polymerizable liquid crystal compound solution is applied, or if the substrate itself has scratches or the like, defects may occur in the coating film obtained by applying the polymerizable liquid crystal compound solution. Furthermore, when an alignment film is subjected to a rubbing treatment, scraps of the rubbing cloth remain on the alignment film, which may cause defects in the coating film of the polymerizable liquid crystal compound solution (liquid crystal cured film-forming composition). Thus, although a retardation film formed from a polymerizable liquid crystal compound can be formed to have an extremely thin thickness, there are factors that can cause defects. Furthermore, as described below, defects in the retardation film may occur that are observed as black spots. The inspection method of this embodiment is particularly useful in detecting the presence or absence of defects in an inspection object having a circular polarizer and a release film that include such a defective retardation film.

[0030] The retardation film 14 can be produced by applying a composition for forming an alignment film onto a substrate, and then applying a composition for forming a liquid crystal cured film containing a polymerizable liquid crystal compound onto the composition for forming an alignment film. The retardation film 14 thus produced is attached together with the substrate to the pressure-sensitive adhesive layer 13 formed on the protective film 12a, and then the substrate is peeled off, thereby transferring the retardation film 14 onto the protective film 12a.

[0031] In the inspection method of the present embodiment, when a circular polarizer to be used as the first polarizing unit 3A and the second polarizing unit 3B is adopted, the phase difference value of the release film 16a is used to determine whether its rotation direction (left rotation or right rotation) is the same as or different from the rotation direction of the circular polarizer 1 in the inspection object 10. Details will be described later.

[0032] The release film 16a is peeled off from the circular polarizer 1 when the circular polarizer 1 is attached to a display device, and the peeled release film 16a is usually discarded. Therefore, unlike the protective films 12a and 12b, strict control of the retardation value is not required. Therefore, if a commercially available film is used as the release film 16a, failure to compensate for its retardation value may result in malfunction during defect inspection. In other words, in defect inspection of a circular polarizer 1 to which a release film 16a whose retardation value is not strictly controlled is attached, the retardation of the release film 16a may cause a decrease in the inspection accuracy of the inspection device 100.

[0033] As described in the background art above, in circular polarizer 1, a surface protective film 16b, which is a type of release film, is often provided on the side opposite release film 16a. In the circular polarizer 1 shown in FIG. 2, surface protective film 16b is attached to the protective film 12b side. This surface protective film 16b is also usually peeled off from circular polarizer 1 when it is attached to a display device, and unlike protective films 12a and 12b, strict control of the retardation value is not required. In FIG. 2, protective film 12b and surface protective film 16b may be attached via an appropriate adhesive layer or pressure-sensitive adhesive layer (this adhesive layer or pressure-sensitive adhesive layer is not shown in FIG. 2).

[0034] In this embodiment, the release film 16a is made of a PET resin. The surface protective film 16b is also made of a PET resin. A film made of a PET resin (a PET resin film) has the advantage of being versatile and inexpensive as a release film. On the other hand, as described above, strict control of the retardation value of an inexpensive PET resin film is not required. Therefore, for example, the retardation value may vary between product lots. Furthermore, even the same PET resin film may have in-plane retardation value variations. Even in a circular polarizer to which such an inexpensive PET resin film is attached as a release film, the inspection method of this embodiment can accurately detect the presence or absence of defects.

[0035] The release film 16a of this embodiment has an in-plane retardation value (Re(550)) of, for example, 1500 nm to 3000 nm.

[0036] Here, we will explain how to determine the Re(550) of the release film 16a. As mentioned above, these release films are PET-based resin films, and such films are readily available on the market. A piece measuring, for example, approximately 40 mm x 40 mm is taken from this film (e.g., by cutting from a long film using an appropriate cutting tool). The Re(550) of this piece is measured three times, and the average Re(550) is calculated. The Re(550) of the piece can be measured at room temperature (approximately 25°C) using a phase difference measuring device KOBRA-WPR (manufactured by Oji Scientific Instruments Co., Ltd.). A similar test can be performed to determine the Re(550) of the surface protection film 16b.

[0037] Various commercially available products can be used as the light source 4, but it is advantageous to use linear light (including light that approximates linear light) such as laser light. The light emitted by the light source 4 is unpolarized and becomes circularly polarized light after passing through the first polarization unit 3A described below.

[0038] To observe the light reflected from the object under inspection 10, a detection means 5 including a CCD camera or the like may be disposed on the optical path of the reflected light and on one of the two sides of the second polarization unit 3B where the light source 4 is located. For example, the object under inspection can be automatically detected by image processing analysis using a combination of a CCD camera and an image processing device. Alternatively, the detection means 5 may not be a component, and a person may visually observe the second polarization unit 3B. If necessary, a partition may be disposed between the light source 4 and the CCD camera.

[0039] Furthermore, the inspection device 100 preferably includes a mechanism for tilting or rotating the inspection table 20 so as to change the incident angle θ of light with respect to the inspection object 10, or a mechanism for tilting or rotating the arrangement of the light source 4, bandpass filter 2, and phase difference filter 3. By moving these mechanisms, the phase difference exhibited by the release film 16a can be adjusted, and the brightness of the observation field can be adjusted to be dark enough for inspection.

[0040] <Testing method> The following describes a method for inspecting a circularly polarizing plate using the inspection device 100. The inspection method of this embodiment includes a step (wavelength selection step) of selecting the wavelength (hereinafter referred to as "inspection wavelength") of light (inspection light) to be used in inspection, and a step (defect inspection step) of performing inspection using light of that wavelength.

[0041] (Wavelength selection process) Before starting inspection of the object 10 to be inspected, including the circular polarizer 1, an inspection wavelength is selected. In this embodiment, since the purpose is to detect defects in the retardation film 14, the inspection light is selected based on the optical characteristics of the retardation film. When the retardation film 14 includes multiple layers, such as a λ / 2 film and a λ / 4 film, or a λ / 4 film and a positive C film, the inspection light can be selected for the retardation film to be inspected for defects (e.g., the λ / 4 film). For retardation films, the wavelength dependence of the interference reflected light in the front direction can generally be calculated based on the average in-plane refractive index and thickness. An example of this calculation is shown in Figure 3. Figure 3(A) shows the wavelength dependence of the reflectance index of a λ / 4 film with an average in-plane refractive index ((nx + ny) / 2) of 1.58 and a thickness of 2.85 μm at a wavelength of 550 nm. Figure 3(B) shows the wavelength dependence of the reflectance index of a λ / 4 film with an average in-plane refractive index of 1.62 and a thickness of 2.10 μm at a wavelength of 550 nm. Both graphs are wavy, and many λ / 4 films tend to exhibit such wavy patterns.

[0042] Because the inspection method of this embodiment involves capturing reflected light from multiple types of films, it is advantageous for defect detection to use as inspection light a wavelength at which the interference of reflected light from the retardation film 14 is relatively strong. From this perspective, in this embodiment, a wavelength with a high reflection intensity index is used for inspection. For example, in the graphs of Figures 3(A) and 3(B), the reflection intensity index is high at wavelengths near 540 nm and 580 nm, so it is preferable to use these wavelengths. These wavelengths are particularly preferable because they have high luminosity and are in the range of 500 nm to 600 nm, which is also the design wavelength for the retardation film. It is determined that a wavelength within ±20 nm, ±10 nm, or ±5 nm of these wavelengths is used as the wavelength of the inspection light. That is, for example, a bandpass filter that transmits light with a wavelength of 540 nm and a bandpass filter that transmits light with a wavelength of 580 nm are prepared.

[0043] (Defect inspection process) In the defect inspection process, the incident angle of light is adjusted to reduce the effect of the phase difference of the release film 16a. In this case, the incident angle θ of light with respect to the object 10 under inspection (the angle based on the normal to the surface of the object 10 under inspection) is preferably adjusted to 30° or less. More preferably, it is adjusted to 20° or less. If the incident angle θ of light with respect to the object 10 under inspection exceeds 30°, the optical path length through the release film 16a becomes longer, weakening the interference of the wavelength determined as the inspection light, which may result in a decrease in inspection sensitivity. Therefore, in the defect inspection process, one of the following two conditions ([1] and [2]) is selected depending on the phase difference value of the release film 16a for inspection. This selection makes it easier to perform inspection within a range where the incident angle θ does not exceed 30°.

[0044] [1] Inspection when the phase difference value of the release film 16a is adjusted to an integer multiple of the inspection wavelength The inspection is performed using a bandpass filter that transmits light of the wavelength found in the wavelength selection process. FIG. 4 shows the relative positions of the polarizing films and the retardation films. The double-headed arrows in the figure represent the absorption axis of the polarizing film or the slow axis of the retardation film. As shown in FIG. 4, the retardation film 14 is assumed to include only a λ / 4 film 14a as a retardation film. When the retardation value of the release film 16a can be adjusted to an integer multiple of the inspection wavelength, as described above, the first polarizing unit 3A and the second polarizing unit 3B are arranged so that the absorption axis of the polarizing film 3a and the absorption axis of the polarizing film 3c are orthogonal to each other. In addition, the first polarizing unit 3A (comprising the polarizing film 3a and the retardation film 3b) and the circular polarizer 1 in the inspection object 10 are arranged in a paranic-nicol relationship, and the circular polarizer 1 and the second polarizing unit 3B (comprising the polarizing film 3c and the retardation film 3d) are arranged in a crossed-nicol relationship. In this case, if the circular polarizer 1 in the inspection object 10 is a right-handed circular polarizer, left-handed circular polarizers are used for both the first polarizer 3A and the second polarizer 3B (see the double-headed arrows in FIG. 4). Conversely, if the circular polarizer 1 in the inspection object 10 is a left-handed circular polarizer, right-handed circular polarizers are used for both the first polarizer 3A and the second polarizer 3B.

[0045] 1, inside the inspection device 100, the inspection object 10 is placed on the inspection table 20. At this time, the side of the inspection object 10 having the release film 16a and the retardation film 14 faces the light source 4.

[0046] A bandpass filter 2 that transmits light of the wavelength (for example, 540 nm) found in the wavelength selection step is prepared and placed inside the inspection device 100. Light is incident on the bandpass filter 2 from a light source 4.

[0047] Light emitted by the light source 4 passes through the bandpass filter 2 and then enters the first polarizing unit 3A, where it becomes circularly polarized light (optical path 9a). This transmitted light then enters the object under inspection 10. It then passes through the release film 16a in the object under inspection 10, is ideally converted to linearly polarized light by the retardation film 14 that constitutes the circular polarizer 1, and is finally absorbed by the polarizing film 11 (end of optical path 9a). A portion of the light that passes through the first polarizing unit 3A is reflected by the surface of the release film 16a in the object under inspection 10 (optical path 9b). Because the first polarizing unit 3A and the second polarizing unit 3B form a crossed Nicol configuration, this reflected light is blocked by the second polarizing unit 3B (end of optical path 9b). This absorption and blocking of the inspection light causes the field of view of the second polarizing unit 3B observed by the detection means 5 to become dark.

[0048] On the other hand, if there is a defect D in the object 10 to be inspected (for example, a defect at the interface between the retardation film 14 and the polarizing film 11, or a defect in the retardation film 14), reflection will be strong at that part (light path 9c). The phase difference of this reflected light will be shifted from the ideal due to the defect D (it will be unintended elliptically polarized). This reflected light will be transmitted without being blocked by the second polarizing unit 3B. When this is observed from the detection means 5 side, the defective part will be observed as a bright spot.

[0049] However, 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 many cases, this is not an integer multiple, so the polarization state of the circularly polarized light is disturbed, preventing it from being converted into linearly polarized light by the retardation film 14. This makes it difficult for the light to be absorbed by the polarizing film 11, resulting in reflection at the interface (light path 9d in Figure 5). This increases the amount of light transmitted through the second polarizing unit 3B, increasing the brightness of the observation field. As a result, the bright spots of the defects that are the intended target of observation are obscured by the overall brightness of the observation field, making it difficult to identify the defects.

[0050] To solve this problem, this embodiment has two measures. First, in this embodiment, the circular polarizer 1 and the second polarizing unit 3B are arranged in a crossed Nicol state, so even if the reflected light occurs (optical path 9d in FIG. 1), most of it is absorbed by the second polarizing unit 3B and is unlikely to interfere with defect observation.

[0051] Second, in the inspection method of this embodiment, the incident angle θ of light relative to the object under test 10 is changed to reduce the effect of the phase difference of the release film 16a. In other words, changing the incident angle θ changes the phase difference exhibited by the release film 16a, and therefore, by finding an incident angle θ that is an integer multiple of the above, the observation field can be made darker. To change the incident angle θ, the object under test 10 may be tilted or rotated (the entire inspection table 20 may be moved), or the light source 4, bandpass filter 2, and phase difference filter 3 may be tilted or rotated. In this way, the relative positions of the components constituting the inspection apparatus 100 are adjusted to change the incident angle θ in various ways, thereby finding an angle at which the effect of the phase difference of the release film 16a is reduced. When tilting the object under test 10, the circular polarizer 1 may be tilted with the slow axis direction or the fast axis direction as the axial direction. The tilt angle is preferably 20° or less. If the tilt angle needs to exceed 20°, it is preferable to replace the bandpass filter 2 with a different type of filter found in the wavelength selection step.

[0052] According to the inspection method described above, it is possible to easily determine whether or not a circular polarizer has a defect. Furthermore, since this inspection method is a reflection-type inspection method, the optical path in the object 10 to be inspected is longer than in a transmission-type inspection method, and deformation defects such as wrinkles that are difficult to detect using a transmission-type inspection method can be easily detected. Note that while FIG. 1 shows a case where the retardation film 14 of the circular polarizer 1 has a defect, the inspection method of this embodiment can also detect defects in the polarizing film 11.

[0053] In order to increase the detection sensitivity, the inspection method of this embodiment is preferably performed in a darkroom or other environment where external light is blocked. In addition, from the viewpoint of minimizing reflected light that occurs when light that has passed through the object 10 is reflected by the inspection table 20, it is preferable that the surface of the inspection table 20 on which the object 10 is placed is subjected to low-reflection treatment.

[0054] [2] Inspection when the phase difference value of the release film 16a is adjusted to (an integral multiple + λ / 2) of the inspection wavelength The inspection is performed using a bandpass filter that transmits light of the wavelength found in the wavelength selection process. FIG. 6 shows the relative positions of the polarizing films and the retardation films. The double-headed arrows in the figure represent the absorption axis of the polarizing film or the slow axis of the retardation film. As shown in FIG. 6, the retardation film 14 is assumed to have only a λ / 4 film 14a as a retardation film. If the retardation value of the release film 16a can be adjusted to (an integer multiple + λ / 2) of the inspection wavelength, as described above, the first polarizing unit 3A and the second polarizing unit 3B are arranged so that the absorption axis of the polarizing film 3a and the absorption axis of the polarizing film 3c are orthogonal to each other. In addition, the first polarizing unit 3A (comprising the polarizing film 3a and the retardation film 3b) and the circular polarizer 1 in the inspection object 10 are arranged in a paranic-nicol relationship, and the circular polarizer 1 and the second polarizing unit 3B (comprising the polarizing film 3c and the retardation film 3d) are arranged in a crossed-nicol relationship. In this case, if the circular polarizer 1 in the inspection object 10 is a right-handed circular polarizer, then right-handed circular polarizers are used for both the first polarizer 3A and the second polarizer 3B (see the double-headed arrows in FIG. 6). Conversely, if the circular polarizer 1 in the inspection object 10 is a left-handed circular polarizer, then left-handed circular polarizers are used for both the first polarizer 3A and the second polarizer 3B.

[0055] Inspection can be performed in the same manner as in the above-mentioned [1]. However, in this embodiment, when the incident angle θ of light on the inspection object 10 is changed, if the phase difference exhibited by the release film 16a is an integer multiple + (λ / 2) of the wavelength of the light transmitted through the band-pass filter 2, the polarization state of the circularly polarized light incident on the release film 16a is not disturbed, and the observation field of view can be made darker.

[0056] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. [Industrial Applicability]

[0057] The present invention can be used for inspecting the quality of circularly polarizing plates. [Explanation of symbols]

[0058] 1...circular polarizer, 2...bandpass filter, 3...phase difference filter, 3A...first polarizing section, 3B...second polarizing section, 3a, 3c...polarizing film, 3b, 3d...phase difference film, 4...light source, 5...detection means, 9 (9a, 9b, 9c, 9d)...optical path, 10...object to be inspected, 11...polarizing film, 12a, 12b...protective film, 13...adhesive layer, 14...phase difference film, 14a...λ / 4 film, 15...adhesive layer, 16a...release film, 16b...surface protection film, 20...inspection table, 100...inspection device, D...defect, θ...incident angle.

Claims

1. An inspection method for determining whether or not there is a defect in a film-like object to be inspected, the object comprising: a circular polarizing plate formed by laminating a polarizing film and a retardation film; and a release film made of a polyethylene terephthalate resin and laminated on the retardation film side of the circular polarizing plate, A light source and a bandpass filter that transmits light of a predetermined wavelength; a first polarization unit; the object to be inspected, with the release film side facing the first polarization unit side, and the object to be inspected, so as to be arranged in this order on the optical path of the light emitted by the light source; and a second polarizing unit that forms a crossed Nicol configuration with the first polarizing unit is disposed on an optical path of the light reflected through the object to be inspected; the first polarizing unit and the second polarizing unit are both left-rotating or right-rotating circular polarizers, and when the first polarizing unit and the second polarizing unit are viewed from the light source side, an absorption axis of a polarizing film included in the first polarizing unit and an absorption axis of a polarizing film included in the second polarizing unit are oriented in directions perpendicular to each other; Light from the light source is incident on the bandpass filter; changing the incident angle of the light onto the object to be inspected so as to reduce the effect of the phase difference of the release film; the light reflected by the object to be inspected is observed from the second polarizing unit side to determine whether or not there is a defect in the circular polarizing plate.

2. Before carrying out the test, 2. The inspection method according to claim 1, further comprising: calculating a wavelength dependency of the interference reflected light in the front direction from the average refractive index and thickness of the retardation film; determining a wavelength at which the reflection intensity is maximum in a wavelength range of 500 nm to 600 nm; and determining whether to adopt a wavelength within ±20 nm of that wavelength as the predetermined wavelength.

3. 3. The inspection method according to claim 1, wherein the circular polarizer provided in the object to be inspected and the second polarizing unit are arranged to form a crossed Nicol.

4. 4. The inspection method according to claim 1, wherein the retardation film is made of a cured product of a polymerizable liquid crystal compound.

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