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1、INTERNATIONAL STANDARD ANSI Pnternat Dse Set IS0 11455 First edition 1995-03-I 5 Raw optical glass - Determination of birefringence Verre d optique brut - Dktermination de la bir p and q are related to the type of glass. 2.1 birefringence (of glass): An anisotropy of refractive index in optically ho
2、mogeneous and Isotropic glasses, usually induced by mechanical and/or thermal stress. The refractive index depends on the orientation of the plane of polarization and the propagation vector of the electromagnetic wave with respect to the axis of the principal stresses. From equations (I) and (Z), eq
3、uation (3) for the differ- ence in refractive index may be obtained: Definition taken from IS0 9802.1 “1 - % = b - 4)(0, - 02) (3) The difference in refractive index (n, - 4) is the birefringence An, which is related to the stress ootical coefficient K =p - q at the wavelength used as fol- lows: 3 P
4、rinciple An = K(a, - 02) (4) For the correlation between birefringence, An, and the optical path difference, As, between the wave com- ponents in the principal stress direction, (T, and Us, after passing through the sample thickness u (which Measurement of the optical path differences by com- pensat
5、ion using an optical interference method with polarized light. 1 IS0 11455:1995(E) Figure 1 - Principal stresses shown in a rectangular plate is usually equal to light path a), the following equation may be applied: The birefringence An centimetre. 4 Apparatus 4.1 Instrument the specimen at calliper
6、s. for measuring the thickness of the measuring points to 1 % e.g. . . (5) is given in nanometres per 4.2 Polarimeter with a compensator as described by de SBnarmont and Friedel. 4.2.1 Polarimeter components The polarimeter shall consist of the following compo- nents (see also figure 2). a) Source o
7、f collimated light, e.g. white light source with monochromatic filter for a wavelength ;1 of 546 nm or 589 nm. b) Diffuser, e.g. an opal glass or a uniformly ground glass screen. c) Polarizer, containing a cross-section line at 45” to the polarizing direction. The polarizer is mounted between glass
8、and housed in a rotatable mount capable of being locked in a fixed position. 2 Q IS0 d) Specimen holder. e) Quarter-wave plate, having a retardation equiv- alent to one quarter of the wavelength of the light being used. It shall be housed in a rotatable mount capable of being locked in a fixed posit
9、ion. f) Analyser, identical to the polarizer. It shall be housed in a rotatable mount capable of being locked in a fixed position. This mount shall then be housed within a graduated mount capable of being rotated foufcGmated light .G52- s* - T Polarizer -ul-l, Specimen holder - Llmmersion cell -wave
10、 plate - Quarter-wave plate Monochromatic if needed filter Figure 2 - Test arrangement as described by de Sbnarmont and Friedel, including the specimen the viewer (see figure 2) and the viewer looking at the polarizer. 4.2.2 Adjustment of the polarimeter The polarimeter shall be adjusted so that the
11、 45” line on the polarizer IS parallel to the edge of the sample. The analyser shall be rotated without the full-wave plate and the quarter-wave plate to zero-position to produce homogeneous darkness. Finally, the quarter-wave plate shall be adjusted to maximum darkness through rotation and fixed in
12、 pos- ition. In case of using a full-wave plate to help in determi- nation of tensile or compressive stress at the edge, the full-wave plate shall be adjusted to maximum darkness in the case of monochromatic light. Switch to a white light source with the full-wave plate interference-colour red-viole
13、t (“first order red”). The sensitivity in compensating of very little bright- ening, caused by small birefringence, or small path length, in the glass body depends on the surface pol- ishing and on the quality of polarizer, analyser and quarter-wave plate. Best results are obtained only with paralla
14、x-free long distance optics using the short-focus telescope. 5 Test samples The test samples should be of simple geometrical shapes, e.g. rotationally symmetrical, rectangularly cut plates and blocks, and equilateral, rectangular tri- angles. The test samples should have two opposite planeparallel p
15、olished surfaces. For these test sam- ples, apply the normal test procedure. In case of test samples with non-planeparallel and/or unpolished sur- faces, use the immersion cell see 4.2.1 i). Examples of test samples with polished surfaces and measuring points are given in figure3. The test samples s
16、hall be held in isothermal con- ditions at room temperature to reach an equilibrium temperature before and during measurement. IS0 11455:1995(E) Q IS0 Figure 3 - Examples of test samples, including possible measuring points 6 Procedure 6.1 Place the test sample so that the principal stress direction
17、 lies at 45” to the oscillating plane of the light. In this way, the oscillating components in the direction of the principal stresses u1 and (TV have the same amplitude but a different optical path length, AS, when leaving the test sample (elliptical polarized light). The point of birefringence mea
18、surement shall be close to the edge of the test sample, at a distance of approximately 5 % of the diameter of the width from the edge. 6.2 The tangent plane at the measuring point for round test samples shall lie at 45” to the polarizer. For rectangular cross-sections and right-angled triangular cro
19、ss-sections, the point of measurement shall be as shown in figure3. 6.3 Determine the light path by measuring the thickness of the test sample at the measuring point to 1 %. 6.4 Rotate the analyser until the path difference is compensated, this means until the maximum dark- ness has occurred at the
20、selected point of measure- ment. During passage through the quarter-wave plates, lin- early polarized light is produced, whose plane of os- cillation is rotated relative to the original plane of the polarizer by an angle proportional to the optical path difference. 4 Note the angle 4 at this positio
21、n. The sign of the angle (+) depends on the sense (compression or tension) of the principal stress difference (see 6.5). Repeat the measurement for further measuring points (figure 3). 6.5 For information on the sign of the stress close to the edge of the test sample, switch to the white light sourc
22、e by taking out the monochromatic filter and putting in the full-wave plate. Colourless bright- ening caused by minor path differences (smaller than half wavelength) appear in the well-known sequence of white light interference colours from red-violet to blue or yellow in the other direction. The de
23、termi- nation of tension or compression can be made by comparison to single-axis prestressed rods or test discs. Using the full-wave plate, the analyser shall be in zero-position. NOTES 3 The change of colour depends on the orientatron of the principal stress cross in the test sample to the polarizi
24、ng direction. 4 For glasses with negative stress optical coefficients (for instance glasses with a mass fraction of PbO of more than 75 %), interference colours will be reversed from the stress correlate determined with prestressed rods or test discs of glass with positive (normal) stress optical co
25、efficients. Another way to determine the sign of the stress is to introduce a point-like force on the test specimen leading to regions of known stress orientations. 6.6 The compensation range given by the procedure described by de Senarmont and Friedel is limited to a maximum retardation of one wave
26、length (180”). Q IS0 IS0 11455:1995(E) If the path-difference exceeds one wavelength, the interference lines shall be counted starting with the neutral black line as zero which has to be determined previously with the white light source (without the monochromatic filter). The visible region beyond t
27、he highest coloured line (after switching to monochromatic light, it is a black line) shall be compensated in accordance with 6.4 and shall be added to the already counted wavelengths. 7 Expression of results For each measuring point, calculate the birefringence according to the following equation:
28、4 2. =mxa (6) where An is the birefringence, in nanometres per centimetre; AS is the optical path difference, in nanometres; a is the light path (sample thickness), in centimetres; 4 is the rotation of the analyser required to produce darkness at the measuring point, in degrees; .I is the wavelength
29、 of the light used, in nanometres. 8 Test report The test report shall include the following information: a) b) cl d) e) f) 9) t-d reference to this International Standard; identification of the test sample; shape of the test sample; kind of light source used (wavelength 1); kind of polarizer used;
30、position of the measuring points; direction of the light path through the test sample; birefringence, in nanometres per centimetre, rounded to the nearest 1 nm/cm. -,-,- IS0 11455:1995(E) Q IS0 Annex A (informative) Bibliography I IS0 9802:- I, Raw optical g/ass - Vocabulary. 2 IS0 101 lo-2:1995 ),
31、Optics and optical instru- ments - Preparation of drawings for optical el- ements and systems - Part 2: Material imperfections - Stress birefringence. 3 DE SNARMONT, H. Anna/es de Chimie et de Physique (ACPHA). No. 2, 1840, pp. 73 and 337. 4 FRIEDEL, G. Sur un pro&de de mesure des birbfringences. Bu
32、lletin de la Soci& FranCake de Mirkalogie (BSFMA), vol. 16, 1893, pp. 19-33. 5 WOLF, H. Spannungsoptik (Photo-elasticity). Springer-Verlag, Berlin, Gdttingen, Heidelberg. 1) To be published. 6 IS0 11455:1995(E) ICS 81.040.10 Descriptors: glass, OPtiCal glaSS, raw materials, tests, optical tests, determination, birefringence. Price based on 6 pages