JP3436704B2 - Birefringence measuring method and apparatus therefor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a birefringence measuring method for measuring the birefringence of a test object made of a plastic product such as a plastic lens used for optical writing or pickup used in a laser printer or the like. Regarding the device.

[0002]

2. Description of the Related Art Conventionally, a phase modulation method and a rotation analyzer method have been known as methods of measuring birefringence of an object such as a lens to be inspected. In these methods, a transparent beam is radiated to a parallel test object, the transmitted light from the test object is received by a light receiving element such as a photodiode, and the polarization state of the transmitted light due to the birefringence of the test object. The birefringence of the test object is obtained by detecting the change in

In the phase modulation method, "optical technology contact" V
ol.27.No.3 (1989) `` Measurement and application of birefringence by phase modulation method '' p.127-p.134, etc.
The irradiation light is phase-modulated by using a photoelastic modulator (PEM), and the birefringence is obtained from the phase of the beat signal and the modulation signal of the light transmitted through the transparent test object.

In the rotation analyzer method, "Optical Measurement Handbook" (published on July 25, 1981) Toshiharu Tayuki, Junpei Tsujiuchi, Shigeo Minami, "Polarization Analysis" p.256-p.265 in Asakura Shoten. As reported in, the transmitted light is received by the light receiving element on the back side of the analyzer while rotating the analyzer placed on the back side of the transparent object, and the received light output from the light receiving element accompanying the rotation of the analyzer. The birefringence is determined by the change of.

Here, as a concrete proposal example, for example, as disclosed in Japanese Patent Application Laid-Open No. 4-58138,
By irradiating the transparent parallel object with the expanded parallel light and receiving the transmitted light with a two-dimensional sensor such as a CCD camera,
There is a device in which the birefringence of a test object is obtained.

According to Japanese Patent Publication No. 6-23689, a phase modulation method is used as a measurement principle, and a plurality of light sources having different wavelengths are used, whereby a large birefringence can be measured.

[0007]

However, Japanese Unexamined Patent Application Publication No.
In the example of Japanese Patent No. 58138, the measurement result of the birefringence is obtained within the range of 0 to λ / 4 (λ: light source wavelength) in terms of the birefringence phase difference. Therefore, the measurement result of the birefringence phase difference exceeding the range is Uncertain. That is, when the measured value of the birefringence phase difference is δ and the true birefringence phase difference is δ ′, δ ′ = m · λ / 2 ± δ (m: integer) ……………… (1) And the value of the integer m becomes uncertain.

After all, in the example of Japanese Patent Laid-Open No. 4-58138, the measurement range is determined by the light source wavelength λ, so that
It cannot handle large birefringence that exceeds the measurement range,
Moreover, when the birefringence distribution exceeds the measurement range, the same cannot be dealt with.

Further, in the case of the example of Japanese Patent Publication No. 6-23689, a so-called "point measurement" is performed in which a thin laser beam is applied to the test object and the transmitted light is received by a photodiode. In order to measure birefringence in a two-dimensional space, it is necessary to adjust the object to be inspected and the measuring device. Also, when measuring a lens (non-parallel plate) instead of a flat plate, the transmitted light of the object is refracted. Therefore, there is a problem that the setting is difficult.

Regarding the object to be inspected, the plastic lens for the optical writing system used in the laser printer or the like is thicker than the plastic lens having the small diameter or the optical disc, and the thickness thereof exceeds the measurement range determined by the light source wavelength. It is sufficiently conceivable that the birefringence and the birefringence distribution are large.

Therefore, the birefringence measuring method is required to be capable of measuring a large birefringence without being restricted by the wavelength of the light source.

Therefore, the present invention provides a birefringence measuring method and apparatus capable of measuring a two-dimensional spatial distribution of a large birefringence phase difference exceeding the measurement range without being restricted by the wavelength of the light source. The purpose is to

In order to achieve the above object, the present invention clarifies a method for detecting the rotation direction of elliptically polarized light that has passed through an object to be inspected, and a method for detecting the direction of elliptical polarized light long axis.

Further, the present invention provides a birefringence measuring device capable of obtaining a clear transmitted image of a test object with little distortion even when the test object is a lens and capable of highly accurate measurement. The purpose is to

Further, the present invention is a birefringence measuring apparatus capable of measuring even in the case where the object to be inspected is a lens group consisting of a plurality of lenses such as an optical writing system lens, in a state in which the lens is actually used. The purpose is to provide.

[0016]

According to the birefringence measuring method of the present invention, a light having a wavelength λ emitted from a light source is arranged at a predetermined position in a substantially circularly polarized state by a polarizing element. A linear retarder that irradiates an object and gives a phase difference of approximately λ / 4 to the phase difference of two-direction vibration components orthogonal to each other in the polarization state of the transmitted light of the object, and the polarization of the transmitted light of the object. Λ / that acts as an optical rotator that rotates the direction of
The four plates and the polarizer for detecting the polarization state of the light transmitted through the λ / 4 plate are rotated about the traveling direction of the light, the rotation angle is detected, and the light transmitted through the polarizer is combined. An image is formed on the light-receiving surface of an array-shaped light-receiving element by an image optical system, and based on the detected rotation angles of the λ / 4 plate and the polarizing element and the light-receiving output detected by the light-receiving element, The birefringence of the test object is calculated, and the two-dimensional spatial distribution of the birefringence phase difference of the test object is measured by detecting the rotation direction of the elliptically polarized light of the light transmitted through the test object and the major axis direction of the ellipse. I decided to do it.

As an apparatus therefor, the invention according to claim 5 is a light source which emits light of wavelength λ, a polarizing element which irradiates the test object with the light from the light source in a substantially circularly polarized state, and the test object. As a linear retarder that gives a phase difference of approximately λ / 4 to the phase difference of two-direction vibration components orthogonal to each other in the polarization state of the transmitted light of an object, and an optical rotator that arbitrarily rotates the polarization direction of the transmitted light of the object. Λ / 4 plate acting, a polarizer for detecting the polarization state of light transmitted through this λ / 4 plate, and rotation for rotating each of these λ / 4 plate and the polarizer about the traveling direction of light. Means and these λ by this rotating means
Angle detecting means for detecting the rotation angle of the / 4 plate and the polarizer, an array-shaped light receiving element for receiving the light transmitted through the polarizer, and the light transmitted through the polarizer formed on the light receiving element. An image forming optical system, an arithmetic means for calculating birefringence of the test object based on a rotation angle detected by the rotation angle detection means and a light reception output detected by the light receiving element, and the test object. And a measurement range expanding means for measuring the two-dimensional spatial distribution of the birefringence phase difference of the test object by detecting the rotation direction of the elliptically polarized light of the light transmitted through and the major axis direction of the elliptically polarized light.

Therefore, the birefringence phase difference result obtained within the measurement range determined by the wavelength of the light source to be used is based on the rotation direction of the elliptically polarized light of the light transmitted through the test object and the direction of the ellipse major axis. As a result, a two-dimensional spatial distribution of a large birefringence phase difference that exceeds the measurement range can be measured by adding a predetermined conversion process to.

According to a second aspect of the present invention, in the birefringence measuring method according to the first aspect, the direction of the ellipse major axis of the elliptically polarized light passing through the λ / 4 plate is detected to allow the object to pass through. The rotation direction of the elliptically polarized light is detected.
According to a sixth aspect of the present invention, in the birefringence measuring device according to the fifth aspect, the measurement range expanding means detects the elliptical long axis direction of the elliptically polarized light transmitted through the λ / 4 plate,
The rotation direction of the elliptically polarized light of the light transmitted through the test object is detected.

Therefore, focusing on the direction of the major axis of the elliptically polarized light transmitted through the λ / 4 plate, by detecting the point where the sign of the major axis direction of the elliptically polarized light is reversed, the light transmitted through the test object is detected. Since the rotation direction of the elliptically polarized light is known, the invention according to claim 1 or 5 can be easily realized.

According to a third aspect of the present invention, in the birefringence measuring method according to the first aspect, the change of the elliptically polarized light transmitted through the λ / 4 plate between adjacent elements on the light receiving element in the direction of the ellipse major axis is changed. By detecting, the direction of the ellipse major axis of the elliptically polarized light of the light transmitted through the test object is detected. According to a seventh aspect of the present invention, in the birefringence measuring device according to the fifth aspect, the measuring range expanding means is provided between adjacent elements on the light receiving element in the ellipse major axis direction of elliptically polarized light transmitted through the λ / 4 plate. By detecting the change in (3), the ellipse major axis direction of the elliptically polarized light of the light transmitted through the test object is detected.

Therefore, since the tendency of the sign in the direction of the major axis of the ellipse changes depending on the principal axis direction of the object to be measured, the range of the phase difference cannot be uniquely determined only from the sign of the phase of the light intensity change in the measurement pixel, but the received light By comparing the sign of the phase of the light intensity change between adjacent elements on the element, the change of the sign can be detected, and the measurement range can be expanded by correcting based on this detection result. The invention described in 5 can be easily realized.

According to a fourth aspect of the present invention, in the birefringence measuring method according to the first, second or third aspect, the absolute amount of the birefringence phase difference at the reference position in the arbitrarily set two-dimensional test space is estimated. Then, the absolute amount of the birefringent phase difference in the entire two-dimensional space is measured by applying the estimated absolute amount of the birefringent phase difference to all measured values of the two-dimensional spatial distribution of the measured birefringent phase difference. did.

According to the first aspect of the present invention, the change (called distribution) of the measured value at the arbitrary position with respect to the measured value of the birefringence at the position set as the reference in the two-dimensional test space,
It is obtained as a two-dimensional spatial distribution of the birefringence phase difference. In other words, it is a comparison value for the birefringence measurement value at the reference position. Therefore, there is a possibility that the measurement value at the reference position has already exceeded the measurement range determined by the wavelength of the light source used, and the measurement value in this case is not necessarily the absolute amount of birefringence. Here, the absolute amount of the birefringence phase difference at the reference position in the arbitrarily set two-dimensional test space is estimated, and the estimated value is the reference position measured by the method of the invention according to claim 1. It is possible to know the absolute amount of the birefringence phase difference in the entire two-dimensional space by applying it to all the measured values of the two-dimensional spatial distribution of the birefringence phase difference. That is, the absolute amount of birefringence in the entire two-dimensional test space can be known even for a test object having a large birefringence exceeding the measurement range limited by the wavelength of the light source used.

According to an eighth aspect of the present invention, in the birefringence measuring apparatus according to the fifth aspect, an irradiation optical system for irradiating the light emitted from the light source to the object as divergent light or convergent light, and the object to be measured. The irradiation side moving means for moving and adjusting the position of the irradiation optical system with respect to the specimen in the optical axis direction, the λ / 4 plate, the polarizer, the light receiving element, and the imaging optical system are integrated into almost the optical axis. And a light-receiving side moving means for moving and adjusting in a direction orthogonal to each other.

Accordingly, it is possible to obtain a photoelastic interference fringe with little noise due to optical distortion, light beam shading, and overlap with respect to the transmitted light of the object to be inspected, and even if the object to be inspected is a lens, high-precision birefringence can be obtained. Can be measured. Even when the diameter of the lens as the object to be inspected is large, the light receiving side moving means moves the light receiving element or the like in a direction substantially perpendicular to the optical axis direction of the optical system to measure the entire surface of the optical element. It becomes possible to measure.

According to a ninth aspect of the present invention, in the birefringence measuring apparatus according to the fifth or eighth aspect, the object to be inspected is a lens group including a plurality of lenses, and a holding means for holding the lens group is provided.

In the case of a lens for a writing optical system in a laser printer or the like, the lens group is composed of a plurality of lenses. However, the synergistic effect of the birefringence of each lens causes the lens group to be formed when the writing optical system is actually used. Since the transmitted beam is affected, the birefringence measurement value in the actual use state as the lens group is effective information for evaluating the transmitted beam. By providing a holding means for holding such a lens group here, it is possible to measure the lens group in an actual use state.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a first embodiment of the present invention.
Or, it demonstrates based on FIG. FIG. 1 is a configuration diagram showing a birefringence measuring device of the present embodiment. The object 1 to be measured in the present embodiment is held by a holder (not shown). For such a test object 1, first, a semiconductor laser 2 as a light source that emits light in a linearly polarized state at a wavelength λ and a polarization element that converts linearly polarized light from the semiconductor laser 2 into a substantially circularly polarized state A λ / 4 plate 3 is provided. A beam expander 4 for collimating circularly polarized light is provided between the test object 1 and the λ / 4 plate 3.

A λ / 4 plate 5 is provided on the optical axis of the object 1 to be transmitted and emitted. This λ / 4 plate 5
The linear retarder that gives a phase difference of approximately λ / 4 to the phase difference of two-direction vibration components orthogonal to each other in the polarization state of the transmitted light of the inspection object 1 and the polarization direction of the transmitted light of the inspection object 1 are arbitrarily rotated. Acts as an optical rotator. That is, the λ / 4 plate 5 is the test object 1
The light beam that has become elliptically polarized light close to circularly polarized light due to its birefringence is converted into elliptically polarized light close to linearly polarized light.
Further, a polarizing plate 6 as a polarizer is provided after the λ / 4 plate 5.
Is provided. The polarizing plate 6 has a function of detecting the polarization state of light transmitted through the λ / 4 plate 5. An image forming lens 7 forming an image forming optical system and a CCD camera 8 as an array-shaped light receiving element for receiving the image forming light are provided in the subsequent stage of the polarizing plate 6. The position of the imaging lens 7 is adjusted so that an imaging relationship is established with the vicinity of the DUT 1.
The imaging lens 7 is made of a material, such as a glass lens, from which birefringence inside has been sufficiently removed.

Further, with respect to the λ / 4 plate 5 and the polarizing plate 6, stepping motors 9 and 10 and gear systems 11 and 12 for rotating about the traveling direction of light respectively and rotating means 13 are provided.
Is provided as. A rotation origin position sensor (not shown) is attached to each of the stepping motors 9 and 10, and the rotation angle of each of the λ / 4 plate 5 and the polarizing plate 6 is counted by counting the number of pulses of the stepping motors 9 and 10. Is possible (actually, the λ / 4 plate 5,
The rotation angle of each polarizing plate 6 is detected ... Rotation angle detection means). A motor driver 14 drives these stepping motors 9 and 10, and receives the pulses from the personal computer 15 and the pulse generator 16 to drive the stepping motors 9 and 10.

The image data picked up by the CCD camera 8 is taken into the memory of the personal computer 15 through the image input device 17, and a predetermined calculation method is performed based on the image data and the rotation angle data of the stepping motors 9 and 10. The birefringence phase difference and principal axis azimuth of the lens 1 to be tested are calculated by. A function as a calculation means for calculating the birefringence of the test object 1 by a calculation processing function including a CPU included in the personal computer 15 and a function of a measurement range expanding means described later are executed. By the way, the image taken by the CCD camera 8 is displayed on the monitor of the personal computer 15 or a dedicated monitor.

In such a structure, first, the principle of the birefringence measuring method in the case of the present embodiment will be described.
As a basic setting, the direction of polarization of the semiconductor laser 2 is horizontal with respect to the ground, and the direction of the λ / 4 plate 3 is set to 45 degrees with respect to the direction horizontal to the ground, and the object 1 is circled. It shall be polarized light. The rotation origin in the measurement is when the azimuths of the λ / 4 plate 5 and the polarizing plate 6 are horizontal to the ground.

First, the inspection object 1 is set at a predetermined position by a holder, and the polarizing plate 6 is rotated from the rotation origin (180 / n) while the azimuth of the λ / 4 plate 5 is rotated 45 degrees from the rotation origin.
Rotate each time. n is the preset number of measurement points. Therefore, every time the polarizing plate 6 rotates by (180 / n) degrees, the CCD image data read by the CCD camera 8 is taken into the memory of the personal computer 15, and the rotation angle data of the polarizing plate 6 and the n pieces of CCD image data are stored. get. next,
Set the azimuth of the λ / 4 plate 5 to 0 degrees with respect to the horizontal (return to the rotation origin), and rotate the polarizing plate 6 from the rotation origin position by (180 / n) degrees in the same manner as described above, C
The CD image data is loaded into the memory of the personal computer 15 and the rotation angle data of the polarizing plate 6 and the n CCD image data are obtained. 2n acquired by the personal computer 15 in this way
The birefringence of the object to be inspected 1 is obtained by performing arithmetic processing in the following procedure by the arithmetic means based on the CCD image data of one sheet and the rotation angle data of the polarizing plate 6.

First, the azimuth of the λ / 4 plate 5 is set to 4 from the origin of rotation.
When rotated by 5 degrees, the light intensity I ₄₅ detected by each pixel of the CCD camera 8 according to the rotation of the polarizing plate 6 is represented by I ₄₅ = (1/2) (1 + cosδcos2θ-sinδcos2φsin2θ) (2) To be done. Further, when the azimuth of the λ / 4 plate 5 is returned to the rotation origin, the light intensity I ₀ detected by each pixel of the CCD camera 8 due to the rotation of the polarizing plate 6 is I ₀ = (1/2) ( 1 + sinδsin2φcos2θ−cosδsin2θ) (3) In these equations (2) and (3), θ is the principal axis direction of the polarizing plate 6, δ is the birefringence phase difference of the test object 1, and φ is the principal axis direction of the test lens 1.

Assuming here that the phases of the light intensity change due to the rotation of the polarizing plate 6 are ψ ₄₅ and ψ ₀ , respectively, from equations (2) and (3),
These phases ψ ₄₅ and ψ ₀ are ψ ₄₅ = tan ⁻¹ (−tan δsin 2φ) …………………………………… (4) ψ ₀ = tan ⁻¹ (−1 / tanδcos 2φ) …… ………………………………… It is expressed as (5).

Further, based on the captured CCD image data and the rotation angle data of the polarizing plate 6, ψ ₄₅ and ψ ₀ are expressed by the following formulas (6) and (7) ψ ₄₅ = tan ⁻¹ {Σ (I _45i · sin2θ _i ) / Σ (I _45i · cos2θ _i )} (6) ψ ₀ = tan ⁻¹ {Σ (I _0i · sin2θ _i ) / Σ (I _0i · cos2θ _i )}… (7) .

Therefore, from these equations (4) to (7), δ = tan ^-1 √ {tan ² ψ ₄₅ + (1 / tan ² ψ ₀ )} …………………… (8) φ = (1/2) tan ^-1 (-tan ψ ₀ tan ψ ₄₅ ) ……………………………………………………………………………………………… (9)

The phase difference measurement result thus basically measured is 0 to λ / 4, as is apparent from the equation (8).
Only the measured value in the range of (0 to π / 2 in terms of phase angle) can be obtained.

Incidentally, FIG. 2 is an explanatory view showing, in the birefringence measuring apparatus, the polarization state of the light transmitted through the object 1 to be examined and the birefringence phase difference of the object 1 to be compared. In this case, the phase difference measuring range of the birefringence measuring device is 0 to λ / 4.
Therefore, 0 to λ / 4 and λ / 4 to λ / in FIG.
The phase difference in the range of 2, λ / 2 to 3λ / 4 to λ results in the same measurement result.

In this respect, paying attention to FIG. 2, the rotation direction of the elliptically polarized light is reversed before and after the phase difference of the object 1 becomes (2m + 1) .lamda. / 4, and when m.lamda./2, the elliptically polarized light is rotated. It can be seen that the major axis direction of changes by 90 degrees. Therefore, in the present embodiment, as the function executed by the personal computer 15, the function of the measurement range expanding means is provided, and the rotation direction of the elliptically polarized light of the light transmitted through the DUT 1 and the change in the ellipse major axis direction are changed. The two-dimensional spatial distribution of the birefringence phase difference of the test object 1 can be measured by expanding the measurement range of the birefringence phase difference in the birefringence measuring device by detecting.

Next, a method of detecting the rotation direction of the elliptically polarized light transmitted through the object 1 will be described. In the following description, 0
An angle of ˜360 degrees is represented as 0˜λ.

FIG. 3 shows a Poincare sphere for displaying the polarization state. Right-handed elliptically polarized light is located in the upper hemisphere, left-handed elliptically polarized light is located in the lower hemisphere, and left and right circularly polarized light are in the lower and upper poles, respectively. To position. The position on the equator represents linearly polarized light, and S ₁
The intersection of the axis and the sphere represents horizontal linearly polarized light, and the intersection of the S ₂ axis and the sphere represents 45 ° linearly polarized light. In the birefringence measuring apparatus of FIG. 1, since the test object 1 is irradiated with right-handed circularly polarized light, the light elliptized by the birefringence of the test object 1 changes its position from the upper pole to the lower pole. Will move. In addition, λ / 4 in FIG.
The plate 5 is a λ / 4 linear retarder, a 2θ optical rotator (Θ: λ / 4
Azimuth with respect to the horizontal of the plate 5), so λ /
The polarization state of the light transmitted through the four plates 5 forms an angle of 2θ with the S ₁ axis in the S ₁ S ₂ plane of FIG. 3 and is λ / 4 only with the axis extending from the center of the sphere as the rotation axis. It will be in the rotated position.

Here, the azimuth of the λ / 4 plate 5 is set to 0 (Θ = 0:
(Horizontal), the birefringence phase difference δ of the test object is in the range of 0 to λ / 4, that is, when the light transmitted through the test object 1 is right-handed elliptically polarized light, it passes through the λ / 4 plate 5. The polarization state of the light is located in the right lower hemisphere (counterclockwise elliptical polarization) when observing the sphere from the S ₁ axis direction. On the other hand, when the birefringence phase difference δ of the test object is in the range of λ / 4 to λ / 2, that is, when the light transmitted through the test object 1 is counterclockwise elliptically polarized light, the light transmitted through the λ / 4 plate 5 is The polarization state is located in the lower hemisphere on the left side (counterclockwise elliptical polarization) when observing the sphere from the S ₁ axis direction. Therefore, since the signs of the major axis directions of the elliptically polarized light are inverted in both, the rotation direction of the elliptically polarized light transmitted through the test object 1 can be known by detecting it.

Since the major axis direction of the transmitted elliptical polarized light when the azimuth of the λ / 4 plate 5 is ₀ corresponds to the phase ψ ₀ described above, the test object 1 is transmitted by observing the sign of ψ ₀ . The rotation direction of the elliptically polarized light can be detected, whereby the range of the birefringence phase difference of the DUT 1 is 0 to λ / 4 or λ /.
It can be distinguished from 4 to λ / 2. That is, by determining uniquely the _{ψ 0 ≦ 0 → 0 ≦ δ} ≦ λ / 4 ψ 0> 0 → λ / 4 <δ ≦ λ / 2, it is possible to extend the measurement range to the 0~λ / 2 .

Next, a method of detecting the elliptically polarized light transmitted through the test object 1 in the direction of the major axis of the ellipse will be described. As described above, in the birefringence measuring device shown in FIG. 1, the CCD camera 8
The birefringence is measured based on the output of each pixel, and the phase difference δ and the principal axis azimuth φ are calculated as described above. For the detection data at any pixel, the detection data at the adjacent pixel and By comparison, the measured value is λ / 2
It is determined whether it is above or below 0. As a result, measurement in a measurement range of 0 to λ / 2 or more is possible.

For the above-mentioned Poincare sphere, the λ / 4 plate 5
Consider the case where the azimuth is set to 45 degrees. In this case, the transition of the polarization state by the action of the λ / 4 plate 5 changes the position on the Poincare sphere so as to rotate by λ / 4 with the S ₂ axis as the rotation axis. The principal axis direction of the birefringence of the test object 1 is 0
If it is in the range of λ / 8, when the birefringence phase difference of the test object 1 is 0 to λ / 2, it shifts to the right hemisphere when viewed from the direction of the S ₂ axis. Further, when the birefringence phase difference of the test object 1 is λ / 2 to λ, it shifts to the right hemisphere when viewed from the direction of the S ₁ axis, and the sign of the elliptically polarized light in the direction of the ellipse major axis is inverted in both ranges. When the principal axis direction of the birefringence of the test object 1 is in the range of λ / 8 to λ / 4, conversely, when the birefringence phase difference of the test object 1 is 0 to λ / 2, from the direction of the S ₁ axis. When moving to the left hemisphere, when the birefringence phase difference of the test object 1 is λ / 2 to λ, it moves to the left hemisphere when viewed from the direction of the S ₁ axis. The elliptical long axis direction of the elliptically polarized light here corresponds to the phase ψ ₄₅ of the above-described equation (4).

As described above, the tendency of the sign of the elliptically polarized light in the direction of the major axis of the elliptically polarized light changes depending on the principal axis azimuth φ of the test object 1.
Although the range of the phase difference cannot be uniquely determined only from the sign of ψ ₄₅ in the measurement pixel in the D camera 8, the sign change can be detected by comparing the signs of ψ ₄₅ between the adjacent pixels.

On the other hand, an example of the distribution of the measurement results in any one cross section in the two-dimensional space is shown in FIG. 4 (a). In FIG. 4A, the horizontal axis represents the measurement position, and the vertical axis represents the phase difference measurement value. The correct phase difference distribution (original value) and the measurement range in the birefringence measuring apparatus of FIG. In this way, the phase difference distribution (measured value) after being expanded from 0 to λ / 2 is shown in contrast. In FIG. 4A, the tendency of increase and decrease of the measured value changes before and after the phase difference measured value becomes 0 degree or 180 degrees. As described above, a change in the sign of ψ ₄₅ between adjacent pixels is detected before and after the phase difference measurement value becomes 0 ° or 180 °, and if the increase / decrease is corrected based on that change, As shown in (b), the measurement range can be expanded to −λ / 2 to λ / 2.
Then, after that, a step of λ is detected between pixels adjacent to each other, and the step is corrected, whereby a large birefringence phase difference can be measured without being limited by the wavelength of the semiconductor laser 2 which is the light source.

A second embodiment of the present invention will be described with reference to FIG. In the present embodiment, a configuration for estimating the absolute amount of the birefringence phase difference is added at the position set as the reference in the two-dimensional test space. In FIG. 5, 21 is a white light source unit, 22 is a circular polarizer composed of a polarizing plate 23 and a λ / 4 plate 24, and 25 is a circular polarizer composed of a polarizing plate 26 and a λ / 4 plate 27. As a result, the test object 1 is irradiated with circularly polarized light, and when the test object 1 is not birefringent, the respective polarizing plates 23 and 26 are arranged so that the extinction state is obtained when viewed from the direction of the observer in the figure. Has been adjusted.

Here, because of interference due to circularly polarized light, the photoelastic interference fringes observed from the direction of the observer in the figure are affected only by the phase difference of the object 1, and the light source is a white light source unit. Since 21 is used, interference fringes are colored and observed. Since the λ / 4 plates 24 and 27 have wavelength dependence,
Although it causes some error as it deviates from the center wavelength,
Since the interference color and the phase difference have a constant relationship (Newton's color scale, etc.), it is possible to roughly estimate the phase difference at the reference position in the two-dimensional inspection space by looking at this interference fringe. .

Therefore, when the phase difference measurement result obtained by the above-described measurement method is δ, the absolute amount δ ′
Is expressed as δ ′ = m · λ ± δ (m: fringe order) ………………………… (10), and if the fringe order m can be specified, obtain the absolute amount of phase difference δ ′. You can

Since the approximate range of the phase difference can be estimated by observing the interference fringes according to the configuration shown in FIG. 5, the fringe order m of the equation (10) can be determined so as to be within the range. .

Therefore, as a procedure, first, the two-dimensional spatial distribution of the birefringence phase difference of the object 1 is measured by the birefringence measuring apparatus as shown in FIG. It is used to estimate the absolute amount of the birefringence phase difference at the reference position in the arbitrarily set two-dimensional test space, that is, to determine the fringe order m at the reference position. Next, the absolute amount of the birefringence phase difference in the entire two-dimensional space can be known by applying the fringe order m to all the measured values of the two-dimensional space distribution previously measured.

As a method of knowing (estimating) the absolute amount of the birefringence phase difference at the reference position in the two-dimensional space to be examined, other than the discrimination method from the interference color, for example, the rotation analyzer method or the like is used. In the point measurement method, the measurement may be performed using multiple wavelengths for the light source.

A third embodiment of the present invention will be described with reference to FIG. In the present embodiment, first, in the configuration shown in FIG. 1, a test lens 31 as a test object is used instead of the test object 1, and correspondingly, instead of the beam expander 4, The irradiation optical system 32 is used. The irradiation optical system 32 is composed of a lens 33 and a pinhole 34. The lens 33 plays the same role as the objective lens in the microscope, and irradiates the lens 31 to be inspected with divergent light. The pinhole 34 acts as a spatial filter. The lens 33 and the pinhole 34 are mounted on a stage 35 which is movable in the optical axis direction, and is moved forward and backward in the optical axis direction by the rotation of a stepping motor (not shown) for driving the stage 35. . An irradiation side displacement means is constituted by the stage 35 and a stepping motor or the like, and the position (distance) of the lens 33 in the optical axis direction with respect to the lens 31 to be inspected is adjustable. Further, this stepping motor is provided with a rotation origin position sensor, and the lens 33
If the distance between the target lens 31 and the lens 31 to be inspected is set in advance and that state is set as the movement origin of the stage 35, by counting the number of pulses supplied to the stepping motor, the lens 33 according to the movement of the stage 35 A change in the distance from the lens 31 to be inspected can be detected.

On the other hand, the λ / 4 plate 5, the polarizing plate 6, the image forming lens 7, the CCD camera 8 and the rotating means 13 are mounted on the base 36, and the guide 37 makes the direction substantially orthogonal to the optical axis of the measuring optical system. It is movable in the vertical direction (indicated by the arrow in the drawing). The base 36 is displacement driven by a stepping motor 38. Here, by these base 36, guide 37, stepping motor 38, etc.,
λ / 4 plate 5, polarizing plate 6, imaging lens 7, CCD camera 8
A light-receiving side displacement means 39 is integrally configured to move and adjust in a direction orthogonal to the optical axis.

Here, the light transmitted through the lens 31 to be inspected is
The lens 33 is expressed by the equation (11) Δ = f ₀ + f ₁ …………………………………… (11) where f _{0 is} the focal length f _{1 of the} lens 33 and the focus of the lens 31 to be inspected. Distance Δ; By installing the lens 31 and 33 at a position that satisfies the interval (that is, the lens 33 and the lens 31 to be inspected form an afocal system),
It becomes almost parallel light flux.

Therefore, by collimating the transmitted light of the lens 31 to be inspected in this way and observing the vicinity of the lens 31 to be inspected by the imaging lens 7 and the CCD camera 8, optical distortion and light beam shading, It is possible to obtain a photoelastic interference fringe image with less noise due to overlapping. Therefore, even when the object to be inspected is the lens 31 to be inspected, highly accurate birefringence measurement can be performed. Further, even when the diameter of the lens 31 to be inspected is large, the measurement can be performed over the entire surface of the lens 31 to be inspected by performing the measurement while moving the base 36 in a direction substantially perpendicular to the optical axis of the optical system.

A fourth embodiment of the present invention will be described. This embodiment is applied to the case where the object to be inspected is a lens for a writing optical system in a laser printer or the like.
In such a case, the test object is composed of a lens group including a plurality of lenses. Here, the synergistic effect of the birefringence of each lens affects the beam that passes through the lens group in the actual use state of the writing optical system, so the birefringence measurement value in the actual use state of the lens group indicates the transmitted beam. This information is useful for evaluation. Therefore, in the present embodiment,
The lens group as described above is held by a holding means such as a holder (not shown), and the lens 33 and the lens 3 to be inspected in FIG.
1 is set so as to correspond to the distance between the point light source and the lens group in actual use in the writing optical system, and then the measurement is performed. As a result, the lens group can be measured in a state according to the actual use state.

[0061]

According to the first and fifth aspects of the invention, the measurement is determined by the wavelength of the light source to be used, based on the rotation direction of the elliptically polarized light of the light transmitted through the test object and the major axis direction of the elliptically polarized light. By applying a predetermined conversion process to the birefringence phase difference result obtained within the range, as a result, a two-dimensional spatial distribution of a large birefringence phase difference exceeding the measurement range can be measured.

According to the invention described in claims 2 and 6, λ /
The direction of rotation of the elliptically polarized light of the light transmitted through the test object can be known by focusing on the direction of the elliptically polarized long axis of the elliptically polarized light passing through the four plates and detecting the point where the sign of the long axis direction of the elliptically polarized light is reversed. Therefore, the invention according to claim 1 or 5 can be easily realized.

According to the third and seventh aspects of the present invention, the tendency of the sign in the direction of the major axis of the ellipse changes depending on the principal axis direction of the object to be measured, so that the sign of the phase of the change in the light intensity at the measurement pixel is unique. Although the range of the phase difference cannot be determined, the sign change can be detected by comparing the sign of the phase of the light intensity change between adjacent elements on the light receiving element, and the measurement range can be corrected by correcting it based on this detection result. Therefore, the invention according to claim 1 or 5 can be easily realized.

According to the invention described in claim 4, the absolute amount of the birefringence phase difference at the reference position in the arbitrarily set two-dimensional test space is estimated, and the estimated value is calculated in accordance with claim 1. By applying to all measured values of the two-dimensional spatial distribution of the birefringence phase difference at the reference position measured by the method of the invention, it is possible to know the absolute amount of the birefringence phase difference in the entire two-dimensional space. That is, the absolute amount of birefringence in the entire two-dimensional test space can be known even for a test object having a large birefringence exceeding the measurement range limited by the wavelength of the light source used.

According to the eighth aspect of the present invention, it is possible to obtain a photoelastic interference fringe with less noise due to optical distortion, light beam shading, and overlap with respect to the transmitted light of the test object, and the test object is a lens. Even in the case, it is possible to measure the birefringence with high accuracy. Even when the diameter of the lens as the object to be inspected is large, the light receiving side moving means moves the light receiving element or the like in a direction substantially perpendicular to the optical axis direction of the optical system to measure the entire surface of the optical element. It becomes possible to measure.

According to the invention described in claim 9, in the case of the lens for the writing optical system in the laser printer or the like, the lens group is composed of a plurality of lenses. It becomes possible to measure.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a birefringence measuring device according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a polarization state of light transmitted through an object to be examined and a birefringence phase difference of the object to be compared.

FIG. 3 is a perspective view showing a Poincare sphere displaying a polarization state.

FIG. 4 is a characteristic diagram showing a distribution of measurement results and a distribution after correction on any one cross section in a two-dimensional space.

FIG. 5 is a configuration diagram showing a second embodiment of the present invention.

FIG. 6 is a configuration diagram showing a birefringence measuring device according to a third embodiment of the present invention.

[Explanation of symbols]

1 Object 2 light sources 3 Polarizing element 5 λ / 4 plate 6 Polarizer 7 Imaging optical system 8 Light receiving element 13 Rotating means 31 Object 32 Irradiation optical system 39 Light receiving side displacement means

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01N 21/00-21/01 G01N 21/17-21/61 G01J 3/00-3/52 G01J 4 / 00-4 / 04 G01M 11 / 00-11 / 08 JISST file (JOIS) Practical file (PATOLIS) Patent file (PATOLIS)

Claims (9)

(57) [Claims] 1. The polarization state of the transmitted light of the test object is obtained by irradiating a test object arranged at a predetermined position with a light having a wavelength λ emitted from a light source in a substantially circularly polarized state by a polarizing element. , A linear retarder that gives a phase difference of approximately λ / 4 to the phase difference of two-direction vibration components orthogonal to each other, and λ / 4 that functions as an optical rotator that arbitrarily rotates the polarization direction of the transmitted light of the test object.
The plate and the polarizer for detecting the polarization state of the light transmitted through the λ / 4 plate are rotated about the traveling direction of the light, the rotation angle is detected, and the light transmitted through the polarizer is imaged. An image is formed on the light receiving surface of an array of light receiving elements by an optical system, and the test is performed based on the detected rotation angles of the λ / 4 plate and the polarizing element and the light receiving output detected by the light receiving element. The birefringence of the object is calculated, and the two-dimensional spatial distribution of the birefringence phase difference of the object is measured by detecting the rotation direction of the elliptically polarized light of the light transmitted through the object and the long axis direction of the ellipse. Method for measuring birefringence. 2. The rotation direction of the elliptically polarized light of the light transmitted through the object to be detected is detected by detecting the direction of the ellipse major axis of the elliptically polarized light transmitted through the λ / 4 plate. Birefringence measurement method. 3. The elliptically polarized light of the light transmitted through the test object is detected by detecting a change between adjacent elements on the light receiving element in the direction of the ellipse major axis of the elliptically polarized light transmitted through the λ / 4 plate. The birefringence measuring method according to claim 1, wherein the ellipse major axis direction is detected. 4. The absolute amount of the birefringence phase difference at a reference position in a two-dimensional test space that is arbitrarily set is estimated, and the absolute amount of the estimated birefringence phase difference is calculated from the measured birefringence phase difference. The birefringence measuring method according to claim 1, wherein the absolute amount of the birefringence phase difference in the entire two-dimensional space is measured by applying it to all measured values of the two-dimensional space distribution. 5. A light source that emits light of wavelength λ, a polarizing element that irradiates the test object with the light from the light source in a substantially circularly polarized state, and the polarization state of the transmitted light of the test object is orthogonal to each other. A linear retarder that gives a phase difference of approximately λ / 4 to the phase difference of the directional vibration component, and a λ / 4 plate that acts as an optical rotator that arbitrarily rotates the polarization direction of the transmitted light of the test object; A polarizer for detecting the polarization state of the light transmitted through the four plates, a rotating means for rotating each of the λ / 4 plate and the polarizer about the traveling direction of the light, and a λ / Rotation angle detecting means for detecting rotation angles of the four plates and the polarizer, an array-shaped light receiving element for receiving the light transmitted through the polarizer, and an image of the light transmitted through the polarizer on the light receiving element The imaging optical system and the rotation detected by the rotation angle detection means. An arithmetic means for calculating the birefringence of the test object based on the angle and the light reception output received and detected by the light receiving element, and the rotation direction of the elliptically polarized light of the light transmitted through the test object and the ellipse major axis direction. 2 of the birefringence phase difference of the object to be detected is detected.
A birefringence measuring device, comprising: a measurement range expanding means for measuring a dimensional spatial distribution. 6. The measurement range expanding means detects the rotation direction of the elliptically polarized light of the light transmitted through the test object by detecting the direction of the elliptic long axis of the elliptically polarized light that transmits the λ / 4 plate. The birefringence measuring device according to claim 5. 7. The measuring range expanding means detects the change between the adjacent elements on the light receiving element in the direction of the ellipse major axis of the elliptically polarized light transmitted through the λ / 4 plate to detect the object to be inspected. The birefringence measuring device according to claim 5, wherein the ellipse major axis direction of the elliptically polarized light of the transmitted light is detected. 8. An irradiation optical system that irradiates the object to be inspected with light emitted from the light source as divergent light or convergent light, and adjusts the position of the irradiation optical system with respect to the object in the optical axis direction. 6. An irradiation side moving means, and a light receiving side moving means for integrally adjusting the λ / 4 plate, the polarizer, the light receiving element, and the imaging optical system in a direction substantially orthogonal to an optical axis. The birefringence measuring device described. 9. The birefringence measuring apparatus according to claim 5, wherein the test object is a lens group including a plurality of lenses, and a holding unit that holds the lens group is provided.