JPH0777490A - Measuring method for double refraction - Google Patents

Abstract

PURPOSE:To measure the double refraction of an optical element constituted of a material having large optical anisotropy such as a plastic lens two- dimensionally and quantitatively, CONSTITUTION:A laser beam from a laser light source 1 is expanded in width by a beam expander 2, linearly polarized by a polarizer 3, and radiated to an object 4. The beam transmitting the object 4 is converged by a beam compressor 6 through an analyzer 5 and received by an area sensor 7. The two-dimensional data of the light intensity of the optical elastic interference fringes detected by the area sensor 7 are fed to a computer 8 for an analysis. The polarizer 3 and the analyzer 5 are concurrently rotated while the plane of polarization by the polarizer 3 and the analyzer 5 respectively is kept constant. The maximum value and the minimum value of the light intensity changed in response to the rotation angle of the polarizer 3 and the analyzer 5 are obtained for each picture element of the two-dimensional light receiving face of the area sensor 7. The phase angle is obtained based on the difference between the maximum value and the minimum value, and the double refraction quantity is obtained based on the phase angle and the wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring the birefringence of an optical writing lens for a laser printer or the like, a plastic lens such as a camera lens or an optical pickup lens for evaluating the birefringence thereof. It relates to a method of measuring refraction.

[0002]

2. Description of the Related Art In recent years, a plastic material has come to be used for a writing lens for a laser printer, a camera lens, an optical pickup lens and the like. However, the plastic material has an optical anisotropy as compared with a glass material. Since it is large, birefringence occurs when stress is applied.

In particular, when a plastic lens is manufactured by injection molding, expansion and
The stress generated by shrinkage, the internal stress generated in the plastic solidification process, etc. are frozen during cooling and birefringence occurs. Such birefringence deteriorates various imaging performances.

For example, when a plastic material is used for the fθ lens of a laser printer, and if a large birefringence occurs inside the plastic material, the spot diameter of the laser beam irradiated by this fθ lens on the photosensitive member is doubled. Depending on the amount of refraction, the spot diameter becomes thicker or thinner than the ideal spot diameter and changes.

In particular, when a plastic material having a large optical anisotropy such as PC (polycarbonate) is used, a large birefringence is likely to occur. Therefore, it is important to quantitatively control this birefringence amount.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to enable two-dimensional quantitative measurement of birefringence of an optical element made of a material having a large optical anisotropy such as a plastic material. And

[0007]

The method for measuring birefringence of the present invention made to solve the above-mentioned problems is such that light converted into linearly polarized light by a polarizer is incident on a transparent test object and the transparent test object is A method for measuring birefringence, in which photoelastic interference fringes are generated by passing light passing through an analyzer through an analyzer, and the birefringence amount of the transparent test object is measured based on the photoelastic interference fringes, The relative position of the polarizer and the analyzer is set so that the polarization plane of the polarizer and the polarization plane of the analyzer form an arbitrary angle, and the polarizer and the detector are detected while maintaining the angle formed by the polarization plane. The photon and the transparent object are simultaneously rotated around the optical axis relative to the transparent object, and based on the two-dimensional data of the light intensity of the photoelastic interference fringes generated according to the rotation of the polarizer and the analyzer. The two-dimensional measurement of the birefringence amount of the transparent test object And it features.

[0008]

In the method for measuring birefringence of the present invention, the intensity at any point of the photoelastic interference fringe periodically changes with respect to the rotation angle of the polarizer and the analyzer, and the intensity at the maximum value or the minimum value is obtained. The azimuth angle can be determined from the rotation angle, and the direction of stress applied to the transparent test object can be determined. Further, the phase angle indicating the phase difference and the birefringence amount can be obtained based on the difference between the maximum value and the minimum value.

[0009]

FIG. 1 is a diagram showing an optical system according to an embodiment of the present invention. In the figure, 1 is a laser light source that emits laser light such as He-Ne, 2 is a beam expander composed of a lens system, 3 is a polarizer made of a polarizing plate such as Polaroid, and 4 is a test object such as a plastic lens. Reference numeral 5 is an analyzer composed of a polarizing plate such as Polaroid, 6 is a beam compressor composed of a lens system, 7 is an area sensor, and 8 is a calculator.

First, the laser beam emitted from the laser light source 1 is expanded to an appropriate size by the beam expander 2. Here, when the outgoing light flux of the laser light source 1 is desired to be randomly polarized light or circularly polarized light and is deviated to linearly polarized light or elliptically polarized light, it is adjusted to circularly polarized light by a ¼ wavelength plate or phase compensator not shown. The randomly expanded or circularly polarized laser light beam is expanded by the beam expander 2 to an appropriate size.

This laser beam is converted into linearly polarized light through the polarizer 3, and this linearly polarized laser beam is applied to the object 4 to be inspected. The light transmitted through the object to be inspected 4 is focused by the beam compressor 6 via the analyzer 5 into a parallel light beam having an appropriate size and guided to the area sensor 7.

The area sensor 7 is arranged so that its two-dimensional light-receiving surface is orthogonal to the optical axis L, and outputs a signal corresponding to the light-receiving intensity at each pixel arranged two-dimensionally on the light-receiving surface. The computer 8 two-dimensionally analyzes the state of internal strain of the test object 4 based on the output signal.

Here, the light linearly polarized by the polarizer 3 is generally elliptically polarized at the portion that undergoes birefringence when passing through the object 4, and when this elliptically polarized light passes through the analyzer 5, this Only the polarization component corresponding to the polarizer axis of the analyzer 5 passes. Therefore, the distribution of the light intensity in the cross section of the parallel light flux that has passed through the analyzer 5 corresponds to the birefringence amount distribution in the two-dimensional surface (the surface orthogonal to the optical axis L) of the test object 4, for example, Bright and dark interference fringes are generated as shown in FIG. The series of interference fringes having the same brightness indicates a portion having the same amount of birefringence.

The polarizer 3 and the analyzer 5 are rotated about the optical axis L by a mechanism to be described later, and are formed by the polarization plane of the polarizer 3 and the polarization plane of the analyzer 5. The angle is preset to any angle before measurement, and the polarizer 3 and the analyzer 5 are simultaneously rotated while maintaining this angle.

When the polarizer 3 and the analyzer 5 are rotated,
The angle of each polarization plane of the polarizer 3 and the analyzer 5 changes with respect to the strain direction of the test object 4, so that the distribution of the light intensity in the cross section of the parallel light flux passing through the analyzer 5 changes. The change in the distribution does not change the shape of the interference fringes, and the continuation of the same brightness indicates a portion having the same birefringence amount.

FIG. 3 is a view showing an example of a rotating mechanism of the polarizer 3 and the analyzer 5, wherein the polarizer 3 and the analyzer 5 are disk-shaped having the same diameter and gears are formed around them. . In FIG. 3A, the motor a is driven by the reference output of the amplifier b, the polarizer 3 is rotated in a fixed direction via the pinion c, and further, the encoder d arranged adjacent to the polarizer 3 is used. The rotation amount of the polarizer 3 is detected.

On the other hand, the motor e is driven by the comparison amplifier f, the analyzer 5 is rotated in the same direction as the polarizer 3 via the pinion g, and further, by the encoder h arranged adjacent to the analyzer 5. The rotation amount of the analyzer 5 is detected. The output of the comparison amplifier f is set to be the same reference output as the output of the amplifier b when the input to the inverting input terminal (-) is 0.

Further, the rotation amount of the polarizer 3 detected by the encoder d is supplied to the computer 8 and the inverting input terminal (-) of the comparison amplifier i, and the analyzer 5 detected by the encoder h.
Is supplied to the non-inverting input terminal (+) of the comparator amplifier i. The output of the comparison amplifier i is supplied to the inverting input terminal (-) of the comparison amplifier f that drives the motor e.

With the above configuration, when the rotation amount of the analyzer 5 is smaller than the rotation amount of the polarizer 3, the output of the comparison amplifier i becomes negative, so that the output of the comparison amplifier f becomes larger than the reference output, and conversely, When the rotation amount of the analyzer 5 is larger than the rotation amount of the polarizer 3, the output of the comparison amplifier i becomes positive, so that the output of the comparison amplifier f becomes smaller than the reference output. Therefore, the analyzer 5 is rotated in synchronization with the rotation amount of the polarizer 3,
The polarizer 3 and the analyzer 5 are simultaneously rotated in the same direction while keeping the angle formed by the respective polarization planes constant.

In FIG. 1B, the pinion q and r are coaxially attached to the drive shaft of the motor p, and the polarizer 3 is rotated by the drive of the motor p via the pinion q and the pinion r. Thus, the analyzer 5 is rotated, and the polarizer 3 and the analyzer 5 are simultaneously rotated in the same direction while keeping the angles formed by the respective polarization planes constant. The amount of rotation of the analyzer 5 is detected by the encoder s arranged adjacent to the analyzer 5, and this amount of rotation is supplied to the computer 8.

Next, a method in which the computer 8 two-dimensionally analyzes the internal strain of the test object 4 will be described. Since the polarizer 3 and the analyzer 5 rotate about the optical axis at the same time while keeping an arbitrary angle, the polarizer 3 and the analyzer 5 are respectively rotated according to the rotation angle θ obtained as the output of the encoder d or s. The two-dimensional electric signal of the interference fringes generated by the area sensor 7, that is, the data of the light intensity at each point in the cross section of the light flux that has passed through the analyzer 5 is taken in, and is generated by the birefringence by the calculation processing described below. The phase angle Δ and the azimuth angle ψ are two-dimensionally analyzed.

First, the analyzer 5 is set at an angle α with respect to the polarizer 3.
The electric field vector E at any one point on the area sensor 7 when installed by inclining by (angle of polarization plane) can be expressed by the following equation (1) from the Jones vector.

[Equation 1]

Here, for simplicity, α = 90 ° (π /
2) That is, considering the case where the polarizer 3 and the analyzer 5 are set to crossed Nicols, the equation (1) becomes the following equation (2).

[Equation 2]

Further, by expanding and rearranging equation (2),
It becomes like the following formula (3).

[Equation 3]

On the other hand, the detected light intensity is given by the following equation (4).

[Equation 4]

Then, by substituting the equation (3) into the equation (4) and rearranging, the following equation (5) is obtained.

[Equation 5]

Therefore, the intensity change with respect to the rotation angle θ of the polarizer 3 and the analyzer 5 based on the equation (5) is shown in FIG. 4, for example. As you can see from Figure 4,
The intensity I changes in a cycle of 90 ° with respect to θ, the angle at which I is maximum and minimum in the figure indicates the azimuth angle ψ or the direction orthogonal to the azimuth angle. Understands the direction in which he is under stress.

Here, the following equation (6) is defined.

[Equation 6] However, I _P : The angle between the polarizer and the analyzer is α = 0 ° (parallel Nicol)
Intensity of light transmitted through the analyzer when the object to be inspected is not set I _C : The angle between the polarizer and the analyzer is α = 90 ° (crossed Nicols) and the light is transmitted when the object to be inspected is not placed. Light intensity I _max : Light transmitted through the analyzer when the polarizer and the analyzer are simultaneously rotated while the angle between the polarizer and the analyzer is maintained at α = 90 ° (crossed Nicols). Maximum light intensity I _min : The polarizer and the analyzer were simultaneously rotated while the angle between the polarizer and the analyzer was maintained at α = 90 ° (crossed nicols), and the analyzer was transmitted when the test object was placed. Minimum light intensity of light

Any one of the maximum light intensity I _max and the minimum light intensity I _min obtained based on the output signal of the area sensor 7, preset known values I _P and I _C, and the above equation (6). When the point m is obtained, the phase angle Δ can be obtained by the following equation (7).

[Equation 7]

1. Furthermore, the birefringence amount BR can be calculated by the following equation (8).

[Equation 8] Where λ is the wavelength of the light source

In the above description, the case where α = 90 ° is described for the sake of simplicity, but it is also possible when α is another angle.

As described above, based on the output signal from any one point on the two-dimensional light receiving surface of the area sensor 7, the object to be inspected 4 is detected.
The azimuth angle ψ, the phase angle Δ, and the birefringence amount BR for any one point of are measured, and these processes are performed on the two-dimensional plane of the object 4 to be measured based on the two-dimensional electric signal of the area sensor 7. Then, the state of internal strain of the test object 4 is two-dimensionally analyzed.

FIG. 5 is a diagram showing an output example of the above-mentioned analysis result by graphic display and the like. FIG. 5A shows an output example in the direction of distortion (optical axis direction), and FIG. It is an output example of an isometric ray diagram of a refraction amount.

The phase angle Δ obtained as described above
Is a different value depending on the wavelength of the laser beam to be irradiated, but by measuring with a predetermined wavelength as the reference wavelength,
It goes without saying that the state of internal strain can be analyzed.

Further, in the above embodiment, the analysis is performed by using the light of a single wavelength by one laser light source 1. However, as in the next embodiment, the light from a plurality of light sources having different wavelengths is analyzed. When used, it is possible to measure even when the phase angle (phase difference) exceeds π.

That is, as can be seen from the above equation (7), when m = 1, Δ = π, and when m = 0, Δ = 0,
The phase angle can be measured in the range of 0 to π, but cannot be measured in the case of the actual phase angle exceeding this range.

Therefore, an embodiment using light of two wavelengths by the device of FIG. 6 will be described. FIG. 6 shows the beam expander 2 of the above embodiment, which is installed in the preceding stage in place of the laser light source 1, 11 is a laser light source which emits a laser beam of wavelength λ ₁ as linearly polarized light, and 12 is a laser of wavelength λ ₂ . A laser light source that emits a light beam as linearly polarized light, 13 is a polarization beam splitter, and 14 is a quarter-wave plate.

As shown in FIG. 6, the laser beam from the laser light source 11 is set so as to be p-polarized with respect to the reflection / transmission surface of the polarization beam splitter 13.
Is set so as to be s-polarized with respect to the reflection / transmission surface of the polarization beam splitter 13, and the optical axes of the laser light sources 11 and 12 are arranged so as to be orthogonal to the reflection / transmission surface of the polarization beam splitter 13. Has been done.

The polarization beam splitter 13 linearly transmits most of the laser light from the laser light source 11, guides it to the quarter-wave plate 14, and reflects most of the laser light from the laser light source 12 at a right angle. And guides it to the quarter-wave plate 14 with the same optical axis as the laser light source 11. By arranging the laser beams so that the directions of linear polarization of the laser beams are 90 ° to each other, the polarization beam splitter 13 can guide the laser beams to the same optical path with almost no loss.

The laser light guided from the polarization beam splitter 13 is converted into circularly polarized light by the quarter-wave plate 14 and is incident on the beam expander 2. The laser light source 11 and the laser light source 12 are turned on / off by a switch (not shown) to select the laser light.
The wavelength of the laser light incident on is switched between λ ₁ and λ ₂ .

Thus, the laser light source 11 and the laser light source 1
Since the wavelength can be selected by switching on / off of 2, there is no need for mechanical switching,
There is little deviation in the optical paths of the two light fluxes, and switching is easy.

As described above, the laser light sources 11 and 12 are switched, and the analysis is performed for each wavelength λ ₁ and λ _{2 in the same} manner as in the above-mentioned embodiment. Then, wavelength λ ₁ , wavelength λ ₂ , wavelength λ ₁
For each phase Δ _{1 for} each wavelength λ ₂ for each phase Δ ₂
From, the phase angle exceeding π is obtained.

[0043] As shown in FIG. 7, the phase angle delta, measured at a wavelength lambda ₁ and wavelength lambda ₂ for an arbitrary phase angle delta _real phase angle delta ₀ with respect to the reference wavelength lambda ₀ is a predetermined wavelength typically Differently, Δ ₁ and Δ ₂ , respectively. Therefore, in the case of FIG. 7, delta _1, reference than delta ₂ value, from the relation to the phase of the relationship and the wavelength lambda ₂ and the reference wavelength lambda ₀ of the phase at the wavelength lambda ₁ and the reference wavelength λ _0, Δ _1, Δ ₂ and It is converted to the phase angle at the wavelength, and the one with the same phase angle is the true phase angle Δ _real
Can be asked as

When two wavelengths λ ₁ and λ ₂ are used,
If the least common multiple of lambda ₁ and lambda ₂ and lambda _{1, 2,} the reference wavelength lambda
_{For 0} ,

[Equation 9] Up to the measurement is possible.

The above example is an example in which two wavelengths are used, but the measurement range and the measurement accuracy can be improved by increasing the number of wavelengths to three and four.

[0046]

As described above, according to the method for measuring birefringence of the present invention, the light converted into linearly polarized light by the polarizer is incident on the transparent test object, and the light transmitted through the transparent test object is detected. A photoelastic interference fringe is generated by passing an analyzer, and
The relative position of the polarizer and the analyzer is set so that the polarization plane of the polarizer and the polarization plane of the analyzer form an arbitrary angle, and the polarizer and the detector are detected while maintaining the angle formed by the polarization plane. The photon and the transparent object are simultaneously rotated around the optical axis relative to the transparent object, and based on the two-dimensional data of the light intensity of the photoelastic interference fringes generated according to the rotation of the polarizer and the analyzer. Since the amount of birefringence of the transparent test object is measured, the birefringence of an optical element made of a material having a large optical anisotropy such as a plastic material can be quantitatively measured two-dimensionally. You can

[Brief description of drawings]

FIG. 1 is a diagram showing an optical system in an example of the present invention.

FIG. 2 is a diagram showing an example of a photoelastic interference fringe according to the embodiment of the present invention.

FIG. 3 is a diagram showing an example of a rotation mechanism of a polarizer and an analyzer in the embodiment of the present invention.

FIG. 4 is a diagram showing an example of a change in light intensity with respect to a rotation angle of a polarizer and an analyzer 5 in an example of the present invention.

FIG. 5 is a diagram showing an output example of an analysis result in the example of the present invention.

FIG. 6 is a diagram showing an embodiment using a two-wavelength laser light source of the present invention.

FIG. 7 is a diagram showing a difference between measured phase angles of two wavelengths with respect to a phase angle at a reference wavelength in the case of an embodiment using a two-wavelength laser light source of the present invention.

[Explanation of symbols]

1 ... Laser light source, 3 ... Polarizer, 4 ... Test object, 5 ... Analyzer, 7 ... Area sensor, 8 ... Calculator.

Claims (5)

[Claims] 1. A photoelastic interference fringe is generated by making light, which is converted into linearly polarized light by a polarizer, incident on a transparent test object, and passing light passing through the transparent test object through an analyzer. A birefringence measuring method for measuring a birefringence characteristic amount of the transparent test object based on interference fringes, wherein the polarization plane of the polarizer and the polarization plane of the analyzer form an arbitrary angle. The relative position of the polarizer and the analyzer is set, and the polarizer and the analyzer are simultaneously rotated about the optical axis relative to the transparent object while maintaining the angle formed by the polarization plane. A feature amount of birefringence of the transparent test object is measured based on two-dimensional data of light intensity of the photoelastic interference fringes generated according to rotation of the polarizer and the analyzer. Method of measuring birefringence. 2. The polarization and the analyzer are rotated by independent motors, respectively, and the rotation angles of the two motors are monitored by two encoders, and the polarizations of the polarizations are synchronized with each other. The method for measuring birefringence according to claim 1, wherein the probe and the analyzer are simultaneously rotated. 3. The polarizer and the analyzer are simultaneously rotated by one motor, and the rotation angle of the motor is monitored by an encoder.
The method for measuring birefringence described. 4. The birefringence characteristic amount exceeding a phase angle π is measured by using light having a plurality of wavelengths as light converted into linearly polarized light by the polarizer. 1. The method for measuring birefringence according to 1. 5. Two laser light sources that emit linearly polarized light having different wavelengths, a polarization beam splitter that guides the light from the two laser light sources to the same optical axis, and 1 provided on the optical axis. 5. The method for measuring birefringence according to claim 4, wherein a / 4 wavelength plate is used to generate the lights of the plurality of wavelengths by switching on / off of the two laser light sources.