JPH08271409A - Photoelastic measuring equipment - Google Patents

Abstract

PURPOSE: To measure the photoelastic constant of an object under relatively low load by measuring the photoelastic characteristics of the object based on the intensity of light detected through a detection means and the modulation characteristics of a modulator. CONSTITUTION: A monochromator 104 selects a specified wavelength component from the light emitted from a light source 102 and a polarizer 106 further selects a specified linearly polarized component which is then subjected to phase modulation, depending on the wavelength, through a phase modulator 108 before impinging on an object 118. A specified load is applied to the object 118 by means of a load cell 120. The light transmitted through the object 118 enters, while containing retardation caused by the load, into a detector 112 through an analyzer 110 and the output from the detector 120 is digitized before being fed to a control means 126. Photoelastic constant can be determined based on the modulation frequency and the output from the detector by irradiating the object 118 with a light modulated through the modulator 108.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelastic measuring device, and more particularly to improvement of its optical system.

[0002]

2. Description of the Related Art When an external force is applied to an isotropic, isotropic and transparent elastic body, a stress is generated, and birefringence, that is, a photoelastic effect is optically exhibited due to temporary anisotropy. This photoelasticity measurement is performed as an evaluation of the homogeneity of the lenses of various optical devices.
Usually, in measuring the photoelasticity, a weight is applied to a transparent elastic body which is an object to be measured, for example, glass, and the Senarmont method is used for the measurement.

That is, in the Senarmont method, FIG.
As shown in, the linear polarizer 12 is arranged on the light incident side of the DUT 10, and the quarter-wave plate 14 and the linear polarizer 16 are arranged on the light outgoing side. Then, the DUT 10
Causes birefringence due to weighting, retardation δ, and its first axis has an angle of 45 degrees with respect to the Ox axis.

On the other hand, the optical axis of the light-incident-side linear polarizer 12 is vertical. The first axis of the quarter-wave plate 14 is horizontal. Further, the light output side linear polarizer 16 is arranged such that its optical axis forms an angle of θ with the Ox axis. According to the photoelasticity measuring device configured as described above, the light from the light source is made into monochromatic light by the monochromator or the like, and then enters the light-incident side linear polarizer 12, and further the predetermined retardation by the object to be measured. After being added, the light is detected by the detector through the quarter-wave plate 14 and the light output side linear polarizer 16.

When the object to be measured is weighted, the retardation changes depending on the weight, and the retardation is calculated based on the rotation angle θ of the light-outgoing linear polarizer 16, and The photoelasticity of the object to be measured is evaluated.

[0006]

However, the Senarmont method has a low sensitivity of about 1 degree, and in the case of glass with small anisotropy, the photoelastic constant must be measured unless a weight is applied just before the glass is broken. I can't.

Therefore, particularly high-precision and high-resolution devices such as large-sized aerial cameras and large-diameter lenses for refracting telescopes, interferometers and large-scale optical systems, and optical systems for steppers (reduction projection exposure apparatus) in semiconductor manufacturing are required. It was difficult to apply the Senarmont method to the evaluation of optical glass having high homogeneity, which is essential for Moreover, the quarter-wave plate is indispensable, and there is a problem that it is difficult to perform the spectroscopic measurement by changing the wavelength of the measurement light due to its wavelength dependence.

Further, there is a problem that a load cell for applying a large weight to the object to be measured is required, and the device becomes large in size. The present invention has been made in view of the above-mentioned problems of the prior art, and an object thereof is to provide a photoelasticity measuring method and a measuring apparatus capable of measuring the photoelasticity constant of an object to be measured with a relatively low weight. is there.

[0009]

In order to achieve the above-mentioned object, the present invention provides a light source for emitting light, a modulating means for modulating the light with a predetermined frequency f, and a predetermined weight F on an object to be measured.
A weighting application means for applying a force, a detection means for detecting light transmitted through the weighted object to be measured, a light intensity detected by the detection means, and a modulation characteristic of the modulator to determine the object to be measured. Arithmetic control means for measuring the photoelastic characteristics.

Here, it is preferable that a spectroscopic unit is provided between the light source and the modulation unit, and the spectroscopic unit is wavelength-scanned by the arithmetic control unit. Further, it is preferable that the calculation means calculates the photoelastic constant C based on the following equation.

[0011]

[Number 2] detection means output _{I out = 1 + sin (Δ} ) sin (δ) sin (δ) = sin (δ 0 sin (ωt)) here, δ _0: amplitude of the retardation of the strange tone ω: 2πf (f Is the modulation frequency) Retardation of the DUT Γ = (Δ / 360) · λ where λ: wavelength of the incident light C = (S / f) · Γ, where S is the area to which the weight is applied.

[0012]

Since the photoelasticity measuring device according to the present invention irradiates the object to be measured with the light modulated by the modulating means as described above, the photoelasticity can be extremely accurately measured even when the anisotropy is small. A property measurement can be performed. Further, since an optical element having a high wavelength dependency such as a quarter-wave plate is not used, it is possible to measure the photoelastic characteristics even when the light from the light source is dispersed and the wavelength is scanned.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. FIG. 1 discloses a photoelasticity measuring device according to an embodiment of the present invention. Device 100 shown in FIG.
Is a light source 102, a monochromator (spectrometer) 104 that disperses the light emitted from the light source 102, a linear polarizer 106, a wavelength-programmed phase modulator (phase modulator) 108, and an analyzer 110. And a detector (detection means)
And 112.

The driver 114 of the monochromator 102 and the controller 11 of the phase modulator 108
6. The load cell 12 for applying a desired weight to the DUT 118 placed between the phase modulator 108 and the analyzer 110.
0, a lock-in amplifier 122 that amplifies the output of the detector 112 is connected to the arithmetic control means 126 via each predetermined interface 124.

The photoelasticity measuring device according to the present embodiment is constructed as described above, and its operation will be described below.
First, the arithmetic and control unit 126 uses the interface 12
The driver 114 is controlled via 4 to cause the monochromator 104 to output predetermined monochromatic light while sequentially changing the wavelength. Similarly, the control means 126 controls the phase modulator 108 via the interface 124 and the controller 116.

Therefore, of the light emitted from the light source 102, a predetermined linear polarization component is selected by the polarizer 106 from the predetermined wavelength components selected by the monochromator 104, and the phase modulator 108 responds to the wavelength. The phase is modulated and incident on the DUT 118.

The load cell 1 is attached to the object to be measured 118.
A predetermined weight is applied by 20 and anisotropy is generated according to the weight. Therefore, the light incident on the DUT 118 has a retardation corresponding to its weight. The weighted data by the load cell 120 is
It is sent to the control means 126 via the interface 124.

The light transmitted through the object to be measured 118 is incident on the detector 112 via the analyzer 110 while containing the retardation due to the weighted addition, and the detector output is transmitted to the interface 124 via the lock-in amplifier 122. It is digitized by the provided A / D converter and is further sent to the control means 126. Each characteristic of the above optical system is expressed by the determinant shown in the following Expression 1.

[0019]

## EQU00001 ## As is clear from the above equation 1, the light emitted from the analyzer 110, that is, the light intensity _I.sub.out detected by the detector 112 is expressed by the following equation 2.

[0020]

[Number 2] _{I out = 1 + sin (Δ} ) sin (δ) sin (δ) = sin (δ 0 sin (ωt)) = sin2J 1 (δ 0) sin (ωt) + 2J 3 (δ 0) sin (3
ωt) + ... Here, δ ₀ : amplitude of retardation of modulator ω: 2πf (f = modulator frequency) Then, when f is 50 kHz and a standardized signal of 50 kHz is a (ω),

[0021]

## EQU00003 ## a (.omega.) = Sin .DELTA. Retardation .GAMMA. (Nm) is calculated from the phase difference .DELTA.

[0022]

Γ = (Δ / 360) · λ where λ is the wavelength of the light output from the monochromator. Therefore, the photoelastic constant C is defined as F is the weight and S is the cross-sectional area that receives the weight.

[0023]

## EQU00005 ## .GAMMA. = C. (F / S) C = (S / F) .GAMMA. As described above, according to the photoelasticity measuring apparatus of the present embodiment, the light modulated by the modulator is measured. By irradiating an object, the photoelastic constant can be calculated from the modulation frequency and the detector output.

Further, according to the present embodiment, since the quarter wavelength plate having a high wavelength dependency is not used, it is possible to perform wavelength scanning by the monochromator. Therefore, for example, as shown in FIG. 3, not only the measurement of the photoelastic constant due to the weight change but also the wavelength dependence of the photoelastic constant can be measured.

[0025]

As described above, according to the photoelasticity measuring apparatus of the present invention, the photoelasticity characteristics can be accurately measured by irradiating the object to be measured with the light modulated by the modulator. It will be possible.

[Brief description of drawings]

FIG. 1 is an explanatory view of a conventional photoelasticity measuring mechanism.

FIG. 2 is a structural explanatory view of a photoelasticity measuring device according to an embodiment of the present invention.

FIG. 3 is a measurement example of wavelength dependence of a photoelastic constant by a photoelasticity measuring device according to an embodiment of the present invention.

[Explanation of symbols]

 102 light source 108 modulator 120 load cell (weight applying means)

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[Procedure amendment]

[Submission date] May 10, 1995

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

FIG.

[Fig. 2]

[Figure 3]

Claims (3)

[Claims] 1. A light source for emitting light, a modulation means for applying a modulation of a predetermined frequency f to the light, a weight applying means for applying a predetermined weight F to an object to be measured, and a measured object to which the weight is applied. Photoelasticity comprising a detection means for detecting light transmitted through an object, a light intensity detected by the detection means, and an arithmetic control means for measuring the photoelasticity of the object to be measured from the modulation characteristics of the modulator. measuring device. 2. The photoelasticity measuring device according to claim 1, further comprising a spectroscopic unit between the light source and the modulation unit, wherein the spectroscopic unit is wavelength-scanned by the arithmetic control unit. 3. The photoelasticity measuring device according to claim 1, wherein the calculator calculates the photoelastic constant C based on the following equation. [Equation 1] Detector output I _out = 1 + sin (Δ) sin (δ) sin (δ) = sin (δ ₀ sin (ωt)) where δ ₀ : amplitude of retardation of the modulator ω: 2πf (f is a modulation frequency) Retardation of DUT Γ = (Δ / 360) · λ where λ: wavelength of incident light C = (S / f) · Γ, where S is an area to which a weight is applied.