Abstract:
An apparatus for measuring photoelasticity permitting to control mechanical stress applied to an elastic body, by visualizing phase differences of polarized light transmitted by the elastic body is disclosed. In such a prior art apparatus a quarter wavelength plate was used in order to obtain circularly polarized light. However, the precision of the circularly polarized light is worsened, when it works in a wide wavelength region. To the contrary, in an apparatus according to this invention, circularly polarized light is obtained by means of Fresnel&#39;s rhombic body. As the result good circularly polarized light can be obtained for a wide wavelength region from the visible region to the near infrared region and control of products including thin films and semiconductor substrates can be effected by visualizing mechanical stress therein.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an apparatus for measuring photoelasticity and in particular to an optical system employing circular polarization for light of a wide wavelength region including the infrared region. 
     An apparatus disclosed in a Japanese reference &#34;Oryoku Sokutei Ho (Stress Measuring Methods)&#34; pp. 473-692, particularly pp. 492-503, published by Asakura Publishing Co., November 1964 has been known as an apparatus for measuring photoelasticity. However, the objects to be measured by means thereof are restricted to glass and plastic, which are transparent for the visible light region and white light (wavelength 400-760 nm) emitted by a tungsten lamp, D line (wavelength 589 nm) emitted by a natrium lamp, and light (wavelength 400-580 nm) emitted by a mercury lamp, that have been used as a light source therefor. In order to obtain the circular polarization necessary for the measurement of the photoelasticity, a quarter wavelength plate is used. Among the light sources described above, for the white light and the ultraviolet light, quarter wavelength plates for their center wavelength 580 nm and 490 nm, respectively, are used. Further, in the case where the light emitted by the light source is monochromatic as D line emitted by a natrium lamp, a phase shifter made of mica and corresponding to a wavelength of 147.25 nm is used as a quarter wavelength plate. 
     For this reason, when photoelasticity is measured by using a light source emitting light of wide wavelength region, it is difficult to obtain correctly circularly polarized light and measurement precision is not taken sufficiently into consideration. Further, in the case where a plurality of kinds of monochromatic light sources are used, they give rise to a problem similar to that of the light source stated above. As the result, an operation to interchange quarter wavelength plate in the course of a measurement is necessary, which complicates the measurement. 
     A stress measuring method, using the infrared photoelastic effect is disclosed in, for example, JP-A-No. 57-191504. 
     On the other hand, a catalog of an apparatus for measuring photoelasticity in the infrared region (Photolastic Inc. Model 501, Infrared polariscope) can be cited as a recent publication. However, this apparatus works only for a wavelength of 1.13 μm and doesn&#39;t meet the needs to measure photoelasticity by using a light source emitting light of wide wavelength region. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide an apparatus for measuring photoelasticity permitting a continuous a measurement without interchange any element of the optical system, while using light of wide wavelength region. 
     Another object of this invention is to provide an apparatus for measuring photoelasticity having a simple construction of the optical system and providing correctly circularly polarized light. 
     This invention is characterized in that an apparatus for measuring photoelasticity using a light source emitting light of wide wavelength region or a plurality of kinds of monochromatic light sources and having a simple construction, which can be operated easily, by utilizing a Fresnel rhombic body (FIG. 6) acting as a quarter wavelength plate for light of wide wavelength region. 
     The Fresnel rhombic body is originated in a rhombohedron made of glass used by Fresnel, who has discovered the circular polarization. At present principally a quarter wavelength plate is used in order to obtain circularly polarized light from linearly polarized and the problem pointed out in the description of the prior art of this invention remains. 
     The Fresnel rhombic body (hereinbelow referred to as Fresnel rhomb) produces a phase shift corresponding to quarter wavelength, i.e. phase shift of 90 deg. by two total reflections, as indicated in FIG. 6. When linearly polarized light 3 enters a Fresnel rhomb 1, it is transformed into circularly polarized light 4 and emerges therefrom. Denoting the incident angle 2 in the Fresnel rhomb 1 by θ, the value of the incident angle θ depends on the wavelength of the used light and can be given by; ##EQU1## where n represents the refractive index of the material, of which the Fresnel rhomb is made. Since the refractive index depends on the wavelength of the light, the incident angle depends also on the wavelength. When the incident angle θ is calculated, supposing that the material, of which the Fresnel rhomb is made, is glass BK7 and that the wavelength of the used light is 589 nm (D line), θ=55°14&#39;2&#34; is obtained. When the phase shift in function of the wavelength is calculated for this incident angle of 55°14&#39;2&#34;, the relationship between the wavelength and the phase shift, as indicated in FIG. 7, is obtained and it can be understood that it acts as a quarter wavelength plate for a wavelength region between 500 and 800 nm with a precision in phase difference of 90±0.5 deg. 
     This invention is characterized in that the characteristics of the Fresnel rhomb that variations in the refractive index due to variations in the wavelength are relatively small are applied to a device in practice. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view illustrating an embodiment of apparatuses for measuring photoelasticity according to this invention. 
     FIG. 2 is a scheme indicating the construction of the above embodiment. 
     FIG. 3 is a graph representing the phase shift produced by the Fresnel rhomb used in the above embodiment. 
     FIG. 4 is a scheme illustrating the construction of another embodiment of apparatuses for measuring photoelasticity according to this invention. 
     FIG. 5 is a graph representing the phase shift produced by the Fresnel rhomb used in the embodiment in FIG. 4. 
     FIG. 6 is a scheme illustrating a Fresnel&#39;s rhombic body, which can be used for this invention. 
     FIG. 7 is a graph representing the phase shift produced by the Fresnel rhomb in FIG. 6 with respect to the light incident thereto. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an apparatus for measuring photoelasticity, which is an embodiment of this invention, in which the light source part consists of a spectroscopic light source, which emits light for different wavelengths between 400 and 2600 nm, while analyzing spectroscopically light coming from a halogen lamp. The light of the relevant wavelength region corresponds to the regions from visible light to near infrared radiation. The optical system is composed of a troidal mirror 2-a, a polarizer 2-b consisting of a Glan-Taylor prism having an extinction ratio of 10 -5  a Fresnel rhomb 2-c, lenses 2-d and an analyzer 2-e consisting of a Glan-Taylor prism. The loading part is composed of a load element 3-a, which can effect tension, compression, bending and shearing with a maximum load of 500 kg, and a thermal chamber 3-b, which can set the temperature between -130° and 250° C. with a precision of ±1° C. The detector part is composed of a visible-infrared camera 4-a, which is sensible to light in the wavelength region between 400 and 2200 nm and a camera controller 4-b controlling the camera. The data processor calculates the principal stress difference σ 1  -σ 2 , the principal stress values σ 1 , σ  2 , the principal stress direction, and the shearing stress τ xy  and is composed of a CPU 5-a, a television monitor 5-b, a console 5-c, a floppy disc drive 5-d, a digitizer 5-e, a video printer 5-f, and a plotter 5-g. 
     Hereinbelow this invention will be explained, by referring to FIG. 2 and the following illustrating the construction of different embodiments, where various items are indicated by new reference numerals. 
     In FIG. 2, a tungsten lamp is used as the light source 6, which emits white light (visible light λ=400-760 nm). Light 7 emitted by the light source 6 is transformed into linearly polarized light, passing through a polarizer 8 and further into circularly polarized light by a quarter wavelength plate 9 consisting of a Fresnel rhomb. It is further transformed into a parallel light beam having a large diameter through a concave lens 10 and a convex lens 11 and projected to a specimen to be measured 12. Light transmitted through the specimen to be measured 12 is collected by a convex lens 11&#39; and a concave lens 10&#39; and enters a television camera 14 through a quarter wavelength plate 9&#39; consisting of a Fresnel rhomb and an analyzer 13. The signal (clear and dark) representing photoelastic fringes produced by the photoelastic effect on the specimen to be measured 12 is imaged on a television monitor through an amplifier 15 by the television camera 14. In this way a photoelastic measurement on the specimen to be measured is effected. In this case, the incident angle θ on the quarter wavelength plate consisting of a Fresnel rhomb is so set that the phase shift is 90±1 deg. for the visible light region. When the incident angle θ is calculated, supposing that the material, of which the Fresnel rhomb is made, is glass BK7 (trade name of an article commercialized by Schott Co. in West Germany) and that the refractive index n=1.521 for the wavelength λ=537 nm, θ=55°31&#39;40&#34; is obtained. The quarter wavelength plate consisting Fresnel rhomb having the incident angle described above of 55° 31&#39;40&#34; gives phase shifts for different wavelengths, as indicated in FIG. 3, and acts as a quarter wavelength plate producing a phase shift of 90±1 deg. for light of the wavelengths in the whole visible light region. 
     Now, another embodiment having the construction indicated in FIG. 4 will be explained. In FIG. 4, the light source 6 is composed of a halogen lamp 17 and a spectroscope separating light having an arbitrary wavelength by means of a diffraction grating 18 from the light emitted by the halogen lamp 17. In this way the apparatus for measuring photoelasticity according to this embodiment permits to effect photoelastic measurements by using arbitrary light in a wavelength region between 400 and 2000 nm (from visible light to near infrared light). In the construction illustrated in FIG. 4, the downstream from the polarizer 8 is basically identical to that described in the preceding embodiment, except that 2 kinds of Fresnel rhomb quarter wavelength plates are used, one for λ=400-760 nm (visible light) and the other for λ=760-2000 nm (near infrared radiation) and that the television camera 14&#39; is sensitive to light of λ=400-2200 nm. The Fresnel rhomb quarter wavelength plates 9a and 9a&#39; for λ=400-760 nm are made of glass BK7, similarly to that described in the preceding embodiment and the incident angle θ is 55° 31&#39;40&#34;. On the other hand, for the Fresnel rhomb quarter wavelength plates 9b and 9b&#39; for λ=760-2000 nm those, for which the incident angle θ giving a phase difference of 90° is 54°18&#39;14&#34; for a refractive index n=1.507 for a wavelength λ=1100 nm, when they are made of glass BK7,  are used. FIG. 5 indicates a graph representing phase shifts vs. wavelength for the Fresnel rhomb quarter wavelength plates 9b, 9b&#39; stated above. From FIG. 5 it can be understood that a phase shift of 90±1 deg. can be surely obtained for the wavelength region between 700 and 2000 nm. Consequently it is possible to effect photoelasticity measurements for a wide wavelength region between 400 and 2000 nm by mounting in the apparatus and using one of the two kinds of Fresnel quarter wavelength plates 9a, 9a&#39; and 9b, 9b&#39;, as described in the preceding embodiment, depending on the wavelength of the light used. 
     A plurality of laser lights or a die laser can be used as the light source in the embodiments described above. Further, it is possible to ameliorate the precision on the phase shift by using more than 3 kinds of Fresnel rhombs. In addition, although Glan-Taylor prisms made of calcite having a high extinction ratio and absorbing no light for a wavelength region between 310 and 2300 nm were used for the polarizer 8 and the analyzer 13 in the above embodiments, Glan-Thomson prisms can be used instead thereof. 
     Furthermore it is required that the Fresnel rhombs are made of a material, whose refractive index n is not smaller than about 1.496, preferably, 1.49661. This is a result obtained from the calculation formula described previously under the condition to obtain a precise phase shift of 90° . Therefore the Fresnel rhombs indicated in Table 1 can be used arbitrarily, depending on the required precision on the phase shift. In the table the materials are identified by trade names of Schott Co. in West Germany, which are well known by those skilled in the art. 
     
                       TABLE 1______________________________________     Materialrefractive index       BK7      quartz   SF6    BaSF7n.sub.D     1.517    1.458    1.805  1.702______________________________________visible  incident 50°31&#39;40&#34;                    51°25&#39;06&#34;                           61°59&#39;11&#34;                                  60°44&#39;06&#34;region angle θ400-760  phase    89.1˜                    84.0˜                           89.4˜                                  89.5˜nm     shift    90.0     86.0   91.6   91.1  [deg]near in-  incident 54°18&#39;14&#34;                    51°25&#39;06&#34;                           61°36&#39;54&#34;                                  60°21&#39;28&#34;frared angle θregion phase    89.0˜                    81.0˜                           89.4˜                                  89.4˜760-   shift    90.5     84.0   90.5   90.52000 nm  [deg]______________________________________ 
    
     Since, in the apparatus for measuring photoelasticity according to this invention, permitting to effect measurement of stress in specimens, a Fresnel rhomb quarter wavelength plate is used as a quarter wavelength plate constituting a part of the apparatus and it acts as a quarter wavelength plate for a wide wavelength region, it is possible to obtain purely circularly polarized light in photoelasticity measurements using light including components of wide wavelength region such as white light as the light source and thus to ameliorate measurement precision. Further, in the case where photoelasticity measurements are effected by using a plurality of monochromatic lights, it is not needed to prepare a number of quarter wavelength plates corresponding to various wavelengths and in this way it is possible to simplify the measurement apparatus and the measurement operation. 
     In addition an effect can be obtained to control products including not only Si monocrystal substrates but also polycrystalline Si semiconductor substrates, amorphous Si substrates, transparent objects, molded articles of opaque resins such as ABS resins or HIPS (High Impact Polystyrene) resins, and furthermore all sorts of thin films, whose thickness is about 1.0 μm, by visualizing stress therein.