JP5223081B2 - Online retardation measuring device for polarizing plate - Google Patents

Description

  The present invention relates to a retardation measuring apparatus that measures the retardation of a polarizing plate online.

  As a method for measuring the phase difference of the transparent film, the transmission axes of the polarizer and the analyzer are arranged in parallel, a transparent film is placed between the polarizer and the analyzer, and the polarizer and the analyzer are parallel Nicols. There is a method (parallel Nicol rotation method) in which the phase difference and the orientation angle of the object to be measured are determined from the change in transmitted light intensity at that time, while rotating in one state.

  The parallel Nicol rotation method is applied to on-line measurement, and the phase difference and orientation angle of a transparent film / sheet can be measured on-line using a single wavelength measurement light (see Patent Documents 1 and 2). It is actually used in the manufacturing process of retardation films for liquid crystal display devices.

  Further, although not on-line measurement, the phase difference can be measured at a plurality of wavelengths in the visible range using the parallel Nicol rotation method, and the wavelength dispersion characteristic of the phase difference can be obtained from the result (Patent Document 3). As such an apparatus, for example, there is a phase difference measuring apparatus KOBRA-WR manufactured by Oji Scientific Instruments, which is used particularly in the field of optical films.

  Furthermore, a device for measuring the wavelength dispersion characteristic of the retardation of the polarizing plate using a parallel Nicol rotation method using a plurality of wavelengths in the near infrared region is also commercialized by Oji Scientific Instruments, and a retardation measuring device for polarizing plates KOBRA-WX / It is commercially available as IR and is used as an off-line inspection apparatus.

  A basic measurement system for measuring the wavelength dispersion characteristic of the phase difference by the parallel Nicol rotation method is as shown in FIG. There, the transmission axes of the polarizer 54 and the analyzer 55 are arranged in parallel, a sample 57 is placed between the polarizer 54 and the analyzer 55, and the polarizer 54 and the analyzer 55 are kept in a parallel Nicol state. The motor 56 makes one rotation. The above-mentioned retardation measuring device KOBRA-WX / IR for polarizing plates uses a halogen lamp as a light source, disposes a filter holder 60 so as to cross a light beam from the light source, and a plurality of near-infrared regions disposed on the filter holder 60. For example, the six band pass filters 61-1 to 61-6 are switched by the rotation of the filter holder 60, and the chromatic dispersion formula is obtained from the phase difference measurement values for six different wavelengths. The apparatus shown in FIG. 2 was developed for off-line use, and requires a measurement time of about 30 seconds per point, and is not suitable for online measurement.

  All of the above-mentioned apparatuses use the parallel Nicol rotation method as a measurement principle. FIG. 1 is a diagram for explaining the measurement principle of the parallel Nicol rotation method. C (= cos 2πR / λ) (vertical axis) represented by a measurement wavelength λ and a phase difference R is a phase difference R (horizontal axis). Is shown. The figure shown below is a detected intensity figure of transmitted light when the object to be measured having each phase difference is rotated once while the polarizer and the analyzer are kept in a parallel Nicol state.

Japanese Patent No. 2791506 Japanese Patent No. 2927145 Japanese Patent No. 2791480

  As can be seen from the relationship between the wavelength λ and the phase difference R in FIG. 1, when R takes a value in the vicinity of an integer multiple of λ / 2, C becomes 1 or −1, and the position of the crest or valley of the cosine curve Therefore, the measurement error of the phase difference R becomes large. Therefore, when measuring at six different fixed wavelengths as in the case of using a band-pass filter, if the measured value of any wavelength falls within a large error range when | C | = 1.0 or close to it. The error of the phase difference R at that wavelength increases, and the error of the wavelength dispersion formula also increases. This will be specifically described with reference to FIG. FIG. 3 shows the phase difference of a polarizing plate using six different wavelengths every about 50 nm in the wavelength range from 850 nm to 1100 nm (black dots in the graph), and calculates the wavelength dispersion formula to show the dispersion curve in the graph. This is an example. The shaded portion in the graph indicates a range of | C | ≧ 0.92. | C | ≧ 0.92 is shown as an example of a range in which the measurement error increases, and is not exact. If the allowable range of error can be increased, for example, | C | ≧ 0.95 may be set. In this case, the shaded portion in FIG. Become wider. If | C | ≧ 0.92 is handled as a large error range as shown in FIG. 3, the error in the measured values for the two wavelengths of 950 nm and 1000 nm becomes large. Therefore, when trying to obtain the chromatic dispersion formula, it is necessary to obtain from the measured values of the four wavelengths among them even though the measurement is performed at the six wavelengths.

The present invention can be installed in a polarizing plate production line, can accurately measure the phase difference regardless of the value of the phase difference R, and can obtain a wavelength dispersion characteristic R _p (λ) of an accurate phase difference. An object of the present invention is to provide an on-line measuring device that can be used.

  The on-line phase difference measuring apparatus for a polarizing plate of the present invention is a measurement light that is irradiated with measurement light containing multi-wavelength components in the near-infrared region through a polarizer onto a moving measurement object consisting of a polarizing plate, and transmitted through the measurement object. Is incident on a spectroscope (including a dispersive element and a detector) through an analyzer and is detected light intensity Ii (θ) for each wavelength λi (i indicates that the wavelength is λi, and θ is a polarization azimuth described later. A phase difference measuring unit to be measured, a phase difference plate arranged in a direction perpendicular to the object to be measured, and a detected light intensity Ii (for each wavelength λi measured by the phase difference measuring unit). An arithmetic processing unit is provided for calculating at least the phase difference of the device under test from θ).

  The retardation plate arranged in an orientation that is orthogonally overlapped with the object to be measured has a retardation value of λ / in the range of the wavelength λ of the measurement light when the retardation plate and the object to be measured are orthogonally stacked. The phase difference value is set so as to be 2 or less and | C | <K (C = cos 2πR / λ, K is a constant having a numerical value in the range of 0.90 to 0.95).

| C | <K is a condition for avoiding a large measurement error range. K is a constant indicating a cosine curve peak or valley representing C, that is, a range of how much before and after the order boundary is treated as being large and discarded. If K is set to a large numerical value close to 1, it will be treated as valid data up to the crest or valley of the cosine curve, so that the range of data that can be handled becomes wide, but the error becomes large. Conversely, if you set a small value apart from 1 for K, only the part away from the crest or valley of the cosine curve will be treated as valid data, so the error will be small, but the range of data that can be handled Becomes narrower. For example, assuming that the measurement wavelength λ is 1000 nm, when calculated backward as R = (λ · cos ⁻¹ C) / 2π, if K = 0.95 is set, the phase difference is equivalent to 50 nm, so the order boundary is about twice that. 100 nm is considered to have a large error, and if K = 0.92, it corresponds to a phase difference of 64 nm. Therefore, before and after the order boundary, 128 nm that is twice that is regarded as a large error, and K = If it is set to 0.90, it corresponds to a phase difference of 72 nm, and therefore, 144 nm, which is twice that of the order boundary, is regarded as having a large error. From the result of measurement in the visible range, a range of K = 0.90 to 0.95 is appropriate regardless of the wavelength. If importance is attached to less error, the value of K may be set small. In the preferred embodiment of the present invention, the coefficients a and b of the equation are finally determined by the least square method using the phase difference values for many wavelengths, so K = 0.95 may be set.

  The term “overlapping” refers to a state in which the slow axis of the polarizing plate of the object to be measured (MD direction (moving direction of the object to be measured)) and the slow axis of the phase difference plate are orthogonal to each other.

  In the phase difference measuring unit, the polarizer, the analyzer, and the spectroscope include first, second, and third sets arranged along the moving direction of the object to be measured. Each set of polarizer and analyzer includes The polarizers and analyzers of the pair of polarizers and analyzers are arranged in the parallel Nicols state, and the polarization directions θ (transmission axis directions) with respect to the reference direction are set to 0 °, 45 °, and 90 °, respectively.

In the case of the parallel Nicol arrangement, the detection light intensity is generally expressed by the following equation.
Where θ is the transmission axis orientation of the polarizer / analyzer, I ₀ (θ) is the detected light intensity when there is no object to be measured, and α is the amplitude when linearly polarized light is transmitted in the two optical principal axis directions orthogonal to each other. The transmittance ratio, φ is the orientation angle of the object to be measured (the direction in which the refractive index of the two optical main axes of the object to be measured is large, that is, the direction of the slow axis), R is the phase difference of the object to be measured, and λ is the measurement wavelength It is. θ and φ are angles based on the MD direction selected as the reference orientation.

In the equation (1), I ₀ (θ) and R depend on the measurement wavelength λ. In the case of a transparent film, α is approximately 1, but when the object to be measured is a polarizing plate, the transmittance of linearly polarized light in two optical principal axis directions on the short wavelength side in the near infrared region due to dichroism of iodine. And α is smaller than 1. Actually, when five types of polarizing plates having different single transmittances of about 36% to about 44% are measured for the amplitude transmittance ratio α at wavelengths of about 50 nm from 850 nm to 1100 nm, the result is as shown in FIG. It can be seen that the value of α is considerably smaller than 1 at wavelengths of 850 nm and 900 nm.

  In general, a polarizing plate is produced by first uniaxially stretching a polyvinyl alcohol film (hereinafter referred to as a PVA film) in the MD direction at a stretching ratio of about 5 times, and adsorbing iodine to the polarizing film to form a polarizing film. A triacetyl cellulose film (hereinafter referred to as TAC film) as a protective film is bonded to both sides. Therefore, considering the orientation angle φ in the equation (1), φ = 0 ° in the PVA film, and iodine is also oriented in the MD direction by the PVA molecules, so φ = 0 °. Further, since the orientation angle of the TAC film is often in the TD direction (direction orthogonal to the MD direction), φ = 90 °. When two or more retardation layers overlap, the phase difference measured as a whole is an additive state if the slow axes of each layer are parallel, and the phase difference if the slow axes of each layer are orthogonal. Is in a declining state. Therefore, in the case of a polarizing plate, the measured phase difference is (PVA film phase difference + iodine phase difference-TAC film phase difference). However, since the TAC film is almost isotropic, the phase difference is only a few nm, which is a negligible value in the near-infrared region which is the measurement wavelength in the present invention.

Using the retardation measuring device KOBRA-WX / IR for polarizing plates, the phase difference for six wavelengths in the near infrared region was measured for the five types of polarizing plates having different single transmittances measured in FIG. When the graph is expressed by the wavelength dispersion formula (3), the result is as shown in FIG.
Here, a, b, and c are constants determined for each object to be measured. For c called the absorption edge wavelength, in the case of an iodine-based polarizing plate, if c = 600 nm, the single transmittance is different. It is empirically known that can be well approximated by equation (3).

  It is well known that the retardation of the PVA film exhibits a flat wavelength dispersion characteristic that hardly changes with respect to the wavelength in the visible range, and it has also been empirically found that it does not change even when the wavelength is extended to the near infrared range. ing. Therefore, since it can be considered that the constant a when the wavelength dispersion characteristic of the retardation of the polarizing plate is expressed by the equation (3) corresponds to the retardation of the PVA film, the five curves in FIG. FIG. 5B is obtained by expressing the value of the second term on the right side of the equation (3) with respect to the wavelength after the equation (3). Furthermore, when the vertical axis of FIG. 5B is redrawn into a ratio graph based on the phase difference of the wavelength of 1000 nm, the graph of FIG. 5C is obtained. From FIG. 5 (c), it can be seen that the ratio graph of the value of the second term on the right side of the equation (3) is expressed as almost one curve even when polarizing plates having different single transmittances are measured. That is, it can be considered that this ratio graph corresponds to the wavelength dispersion characteristic of iodine itself. Therefore, when measuring the wavelength dispersion characteristic of the retardation of the polarizing plate in the near infrared region and expressing the wavelength dispersion using the equation (3), the retardation of the PVA film and the retardation of iodine as shown in FIG. It means that the sum is observed.

When measuring a polarizing plate, the orientation angle φ with respect to the MD direction as an angle reference is approximately 0 °, and even a wide production line is accurately produced so that the orientation angle does not deviate from 0 °. Therefore, if φ = 0 ° in the equation (1), the three detection light intensities I (0), I (45), and I (90) at θ = 0 °, 45 °, and 90 ° The value is obtained.
When the value of C is obtained, the phase difference R can be calculated by the following equation.
Here, m is a natural number, 1, 2, 3,..., And increases every λ / 2 as shown in FIG. 1, and specifically, 1 in the integer part of [2 × R / λ]. It becomes the added number. Assuming that the phase difference of a general polarizing plate is in the order of FIG. 5A, the order m takes a value of 2 to 4 with respect to a wavelength of 850 nm to 1100 nm.

  Considering the wavelength range that can be actually used for measurement by the measurement method of the present invention, the light source includes multiple wavelengths in the near-infrared region, and a halogen lamp is suitable in consideration of illuminance stability, lifetime, and price. Considering the wavelength detection range of the instrument, the measurement wavelength is preferably in the range of 850 nm to 1050 nm. Accordingly, it is assumed that the three detection light intensities I (0), I (45), and I (90) necessary for calculating the phase difference by the method of the present invention are detected at a wavelength of 0.5 nm with a spectroscope, for example. Then, (1050−850) /0.5=400 values can be captured with respect to the wavelength. However, even if the degree m is appropriately changed in the range of 2 to 4 using the equation (6) and 400 phase differences are calculated and a dispersion curve is drawn, the vicinity of the order boundary as in FIG. However, there is a problem that the measurement error becomes large, and all 400 values cannot be used in many cases.

Therefore, by utilizing the phase down phenomenon when the overlapped orthogonal two retardation layers, measured overlapping the polarizing plate of the object to be measured phase difference R ₀ having the slow axis in TD direction is a known phase difference plate To do. For example, when considering the polarizing plates B_38 and C_38 in FIG. 5A, a retardation plate manufactured by using a material having almost no wavelength dependency with R ₀ of 900 nm and 800 nm is used for each polarizing plate. When the measurement is performed so as to be orthogonally stacked, the wavelength dispersion characteristic as shown in FIG. 7 is obtained. This curve has a value of m = 1 for all wavelengths in the range of 850 nm to 1050 nm. For example, when 400 phase difference values are obtained, the coefficients a and b of the dispersion curve are obtained using all the values. Can be determined. At this time, how much the phase difference of the phase difference plate used for superposition is to be determined can be easily determined by examining the wavelength dispersion characteristics as shown in FIG.

When a phase difference plate is manufactured using a material having wavelength dispersion characteristics, the wavelength dispersion characteristic R ₀ (λ) of the retardation is measured in the near infrared region in advance, and the characteristics are the same as the formula (3). Where the coefficients a ₀ , b ₀ , and c ₀ are constants, and the phase difference R ′ (λ) that can be measured in a state where the polarizing plate and the phase difference plate are orthogonally overlapped is expressed by the following equation.
Since a ₀ , b ₀ and c ₀ are all known in the equation (7), the coefficients a and b can be determined from the 400 phase differences by the least square method. At this time, if a phase difference plate made of a material having no wavelength dispersion such as a PVA film is used, the phase difference of the phase difference plate becomes a constant value R ₀ as follows instead of the equation (7).
In any case, if the coefficients a and b are obtained, the phase difference of the PVA film and the phase difference of iodine can be obtained simultaneously as described above.

The spectroscope-detected light intensities I _0i (0), I _0i (45), and I _0i (90) by the three sets of polarizers and analyzers in the absence of the object to be measured and the retardation plate are essentially the same at the same wavelength. Should be a value. However, in reality, the same value is not obtained due to a slight difference in the transmittance of each polarizer / analyzer or a difference in detection sensitivity between the three spectrometers. Therefore, the coefficient β is set for each detection wavelength λ _i of the spectroscope so that all of these values become appropriate constant values (for example, when the AD conversion for light quantity detection is 16 bits = 65536, it is often set to 70000). _i (0), β _i (45), β _i (90) are created.
With respect to the detected light intensities I _i (0), I _i (45), and I _i (90) for each wavelength by the three sets of polarizers and analyzers in a state where the object to be measured and the retardation plate are overlapped The value multiplied by each coefficient is calculated.
Instead of I (0), I (45), and I (90) in equations (4) and (5) for each detection wavelength of the spectroscope, I ′ _i (0), I calculated in equation (10) C is calculated using ' _i (45), I' _i (90), and the phase difference for each detection wavelength is set to m = 1 and obtained from equation (6). The value corresponds to R ′ (λ) in the equation (7) or (8).

  According to the present invention, the detected light intensity for each wavelength of the spectroscope by the three sets of polarizers and the analyzer without the measured object and the retardation plate, and the measured object and the retardation plate are superimposed. C [= cos (2πR / λ)] is calculated for each detected wavelength from the three sets of polarizers and the analyzer's detected light intensity for each wavelength, and the phase difference for each detected wavelength is calculated. Thus, an on-line measuring apparatus capable of measuring the retardation of the PVA film and the retardation of iodine constituting the polarizing plate simultaneously and accurately in a short time is obtained. be able to.

It is a figure for demonstrating the basic principle of a parallel Nicol rotation method. It is a figure of the measurement system in the case of measuring the wavelength dispersion characteristic of a phase difference offline by the parallel Nicol rotation method. It is a figure explaining the error area | region at the time of an example of the wavelength dispersion curve of a polarizing plate, and a parallel Nicol rotation method. It is a measurement example of the amplitude transmittance ratio α for each wavelength of various polarizing plates. (A) It is an example of a measurement of the wavelength dispersion curve of various polarizing plates. (B) It is the figure which represented the measurement result of (a) with the wavelength dispersion type, and made the value of the 2nd term into a graph. (C) It is the figure redrawn by making the vertical axis | shaft of the graph of (b) into the ratio. It is a figure explaining the phase difference of the PVA film in a polarizing plate, and the phase difference of an iodine. It is a figure of the wavelength dispersion curve obtained when the polarizing plate and phase difference plate of the method of this invention are measured by orthogonal superimposition. It is a schematic block diagram which shows one Example. It is a block diagram which shows the arithmetic processing part in an Example.

  FIG. 8 is a schematic configuration diagram of an embodiment of an on-line phase difference measuring apparatus for polarizing plates according to the present invention. From the phase difference measuring unit and the detected light intensity for each wavelength measured by the phase difference measuring unit, an object to be measured An arithmetic processing unit 11 is provided for calculating the phase difference of the polarizing plate. The portion excluding the arithmetic processing unit 11 and the DUT 7 constitutes a phase difference measuring unit.

  In the phase difference measuring unit, the light source 1 is, for example, a light source that guides light from a halogen lamp with a light guide, and supplies white light as measurement light including multi-wavelength components in the near infrared region.

  In order to irradiate the measurement object 7 with linear polarization to the moving object 7 to be measured, three polarizers 4a, 4b, 4c are arranged along the moving direction of the object 7 to be measured, facing one surface of the object 7 to be measured. ing. Three analyzers 5a, 5b, and 5c are arranged on the other surface side of the object to be measured 7 so as to face the polarizers 4a, 4b, and 4c with the object to be measured 7 interposed therebetween. The polarizer 4a and the analyzer 5a are arranged in a parallel Nicol state, and the polarizer 4b and the analyzer 5b, and the polarizer 4c and the analyzer 5c are also arranged in a parallel Nicol state, respectively. With the reference azimuth as the MD direction, the first set of polarizers 4a and 5a is set to 0 ° with respect to the reference azimuth, and the second set of polarizers 4b and 5b is polarized with respect to the reference azimuth. The azimuth is 45 °, and the third set of polarizers 4c and analyzer 5c is set to have a polarization azimuth of 90 ° with respect to the reference azimuth. Further, a retardation plate 6 having a known phase difference is provided between the polarizers 4a, 4b, 4c and the device under test 7 so that its slow axis is in the TD direction (perpendicular to the moving direction of the device under test 7). Is located in

  The measuring light from the light source 1 is guided by the light guide 2 and irradiated to the polarizers 4a, 4b, and 4c through the condenser lenses 3a, 3b, and 3c. The measurement light transmitted from the polarizers 4a, 4b, and 4c through the phase difference plate 6, the object to be measured 7, and the analyzers 5a, 5b, and 5c is collected by the respective condensing lenses 8a, 8b, and 8c, and the light guides 9a, The light is guided to the spectroscopes 10a, 10b and 10c through 9b and 9c. Each of the spectrometers 10a, 10b, and 10c includes a dispersion element such as a grating and a detector such as a CCD camera.

The detected light intensities detected by the spectroscopy by the spectroscopes 10a, 10b, and 10c are taken into the arithmetic processing unit 11, and as described above, the phase difference plate 6 and the object to be measured 7 are superimposed orthogonally. Then, the phase difference R ′ (λ) is calculated, and then the phase difference of the PVA film and the phase difference of iodine are obtained based on the equation (7) or (8).
The arithmetic processing unit 11 is realized by a dedicated computer or a general-purpose personal computer.

  If three pairs of the light projecting part and the light receiving part in FIG. 8 are fixed and measured at, for example, three positions at both ends and the center with respect to the film width direction, changes in the MD direction at each position are closely monitored. it can. Further, if a pair of the light projecting unit and the light receiving unit in FIG. 8 is placed on an automatic table and measured while moving in the TD direction, the width direction profile can be monitored.

DESCRIPTION OF SYMBOLS 1 Light source 4a, 4b, 4c Polarizer 5a, 5b, 5c Analyzer 6 Phase difference plate 10a, 10b, 10c Spectrometer 11 Arithmetic processing part 110 Light quantity correction coefficient calculation part 112 Phase difference calculation part 114 Dispersion curve calculation part

Claims (5)

A measuring object including a polarizing plate is irradiated with measuring light including a multi-wavelength component in the near-infrared region through a polarizer, and the measuring light transmitted through the measuring object is incident on a spectroscope through the analyzer and has a wavelength λi. A phase difference measuring unit for measuring the detected light intensity Ii (θ) (where i indicates that the wavelength is λi, and θ is the polarization orientation described later), the polarizer, the analyzer, The spectroscope includes first, second, and third sets arranged along the moving direction of the object to be measured, and each set of polarizer and analyzer is arranged in a parallel Nicol state, and each set The polarizer and analyzer of the phase difference measuring unit in which the polarization direction θ with respect to the reference direction is set to 0 °, 45 °, 90 °,
A phase difference plate arranged in an orientation that is orthogonally stacked with respect to the object to be measured, wherein the phase difference value of the phase difference plate is a phase difference value R when the object to be measured and the phase difference plate are orthogonally stacked. Becomes λ / 2 or less in the range of the wavelength λ of the measurement light, and | C | <K (C = cos 2πR / λ, K is a constant taking a numerical value in the range of 0.90 to 0.95). A retardation plate that is set as follows:
An arithmetic processing unit that calculates at least the phase difference of the object to be measured from the detected light intensity Ii (θ) for each wavelength λi measured by the phase difference measuring unit,
The arithmetic processing unit calculates a phase difference at a plurality of wavelengths from the detected light intensity Ii (θ) for each wavelength λi with respect to three polarization directions θ based on the principle of the parallel Nicol rotation method, and calculates the phase difference of the object to be measured. An on-line phase difference measuring device that obtains chromatic dispersion characteristics R (λ). The object to be measured is a polarizing plate in which iodine is adsorbed and oriented on the base film,
The arithmetic processing unit represents the wavelength dispersion characteristic R _p (λ) of the phase difference of the object to be measured by the following wavelength dispersion formula:
The first term on the right side is a retardation of the substrate film of the object to be measured, and b / (λ ₀ ² -c ² ) (c is a constant called the absorption edge wavelength) in the second term on the right side at a certain reference wavelength λ ₀ . The on-line phase difference measuring apparatus according to claim 1, wherein the value of) is the phase difference of iodine. The retardation plate is made of a material having wavelength dispersion characteristics, and the wavelength dispersion characteristics R ₀ (λ) of the phase difference when measured in the near infrared region is expressed by the equation (3) according to known constants a ₀ , b ₀ , c _0. Is represented in the same form as
The arithmetic processing unit represents a phase difference R ′ (λ) that can be measured in an orthogonal superposition state of the object to be measured and the phase difference plate by the following equation:
The on-line phase difference measuring apparatus according to claim 2, wherein the coefficients a and b are determined by a least square method using phase difference calculated values at a plurality of wavelengths. The retardation plate does not have wavelength dispersion characteristics, and the retardation is a constant value R ₀ .
The arithmetic processing unit represents a phase difference R ′ (λ) that can be measured in an orthogonal superposition state of the object to be measured and the phase difference plate by the following equation:
The on-line phase difference measuring apparatus according to claim 2, wherein the coefficients a and b are determined by a least square method using phase difference calculated values at a plurality of wavelengths. The arithmetic processing unit calculates [constant value / (spectrometer detection light intensity I _0i (θ))] when there is no object to be measured as a correction coefficient β _i (θ) of the light amount for each detection wavelength λ _i. A calculation unit;
The arithmetic processing section replaces the spectroscope detection light intensity I _i (θ) with the object to be measured and the phase difference plate in place of each other, and detects light intensity β _i for each wavelength corrected with β _i (θ). The online phase difference measuring apparatus according to claim 1, wherein the phase difference is calculated using (θ) × I _i (θ).