GB2087551A

GB2087551A - Measurement of path difference in polarized light

Info

Publication number: GB2087551A
Application number: GB8132413A
Authority: GB
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1980-10-29
Filing date: 1981-10-28
Publication date: 1982-05-26
Also published as: JPS5788347A; DE3129505A1; DD154039B5; DD154039A1

Abstract

Apparatus for the measurement of path differences in polarized light, is useful for detecting anisotropic substances in polarization microscopy, and for the detection of stresses. The aid is to be able to measure small path differences (R< lambda /2) of double- refracting samples, using apparatus in which the measuring operation can be objective and automated. This is achieved by apparatus comprising a device (2, 3, 4, 5, 6, 7) for illuminating a sample (1) to be tested with circularly polarized light, a lambda /4-phase plate (18) and an analyzer (13), the light, after it has passed through the sample (1) to be tested and the lambda /4- phase plate (18), being split up by the analyzer (13) into two components (14, 15) which are polarized at right angles to one another and whose directions of oscillation are orientate &cirf& d at +/-45 DEG to the major axis of the lambda /4- phase plate (18), and a photometer (19) measuring the intensities of the two light components (14, 15) separately from one another. <IMAGE>

Description

SPECIFICATION Apparatus for the measurement of path differences in polarized light The present invention relates to apparatus for the measurement of path differences in polarized light.

Measurements of path differences in transmitted light serves to detect anisotropic substances in, for example, polarization microscopy and for the detection of stresses in a sample for testing in stress testing apparata. The apparatus in accordance with the invention determines the path difference which results by reason of the double refraction of the sample to be tested between two component beams polarized at right angles to one another.

Measurement of path differences by the compensation methods in accordance with Berek, Ehringhaus, Brase-Köhler, Bear-Schmidt, Senarmont etc. are very time-consuming, since the sample to be tested has to be adjusted relative to the polarization direction of the incident light by azimuthal rotation before compensation can be effected. A further disadvantage of these known methods of measurement is that work is always carried out in the most insensitive range of detection when compensating the path difference to intensity '0'. The half-shadow methods used to avoid these disadvantages require additional technical expense and involve methodic difficulties.

British Patent Specification Nos. 1459410 and 1 525283 and US Patent Specification No.

3988067, describe automated polarization devices in which the path difference and also the azimuth angle are modulated by means of electrooptical light modulators. The beam is detected photo-electrically and two alternating signals are obtained which serve to control motorized drives and adjust the azimuth angle and compensate for the path difference. The alternating signals go to zero when the two measuring parameters are balanced. The path difference and the azimuth angle of the double-refracting sample result from the rotation.

An apparatus is also known in which a sample to be tested is disposed between two circular polarizers in order, for example, to eliminate the isoclines during photoelastic tests and to avoid the azimuthal orientation of the sample to be tested (see L. Föppel and E. Mönch: "Praktische Spannungsoptik" Berlin, Göttingen, Heidelberg 1950). In this apparatus, the magnitude of the stress in the sample to be tested is deduced semiquantitatively from the illumination of the sample, the path difference not being ascertained as a measured variable.

Furthermore, an apparatus for the objective measurement of path difference without compensation is known (see Fliigge: "Handbuch der Physik" Vol. XXV/1 G. N. Ramachandran and S. Rameseshan: "Crystal Optics", Berlin, Gttingen, Heidelberg 1961, page 51). In this apparatus, the sample to be tested is rotatably disposed between a polarizer and a double-image prism (in accordance with, for example, Wollaston). A criterion- for;the azimuthal alignment of the sample to be tested is obtained by photoelectric comparison of the brightness of the linearly polarized part beams produced by the doubleimage prism. The azimuth of the sample is varied until the two part beams have equal brightness.

The double-image prism is subsequently turned through 900 and the intensities of the two part beams are measured. Their ratios are equal to the ratios of the squares of the semiaxes of the ellipse of oscillation and are consequently indicative of the path difference of the sample to be tested.

The aim of the present invention is to provide a simple apparatus for the measurement of small path differences (R < A/2) of double-refracting samples, in which the measurement operation can be objective and automated. The rapidity of the measurement is to be such that image analysis methods can be realised with the path difference serving as a measurement criterion.

The various known forms of apparatus for determining the path difference R requires specific orientation of the sample with respect of the direction of polarization, that is to say, determination of the main direction of oscillation 5 of the sample is a prerequisite for the measurement of the path difference.

Determination of the two measured variables 5 and R, is a relatively expensive and timeconsuming matter, since mechanical movements of the sample to be tested and of the compensation elements are required in all known measuring apparatus. By means of the known apparatus, it is possible, with a justifiable expenditure of time, to measure the path difference at only a few selected locations on the sample to be tested.

According to the present invention there is provided an apparatus for the measurement of path differences in polarized light, comprising a device for illuminating a sample to be tested with circularly polarized light, a A/4-phase plate and an analyzer, the analyzer being adapted to split up the light after the light has penetrated both the sample to be tested and the .l/4-phase plate, the analyzer splitting up the light into two components which are polarized at right angles to one another and whose directions of oscillation are orientated at +450 to the major axis of the A/4-phase plate, a photometer being also provided which measures the intensities of the two components separately from one another.

With apparatus constructed in accordance with the present invention, the measurement of the path difference is based on measuring the intensity of two components of the beam to be measured which are polarized at right angles to one another. The measurement is independent of the azimuth of the major axes of the sample to be tested. This simplification of the method of measurement renders it possible to automate the measurement of the path difference and offers the possibility of using image analysis methods with the path difference as a measurement criterion.

In apparatus in accordance with the present invention, the sample to be tested is illuminated with circularly polarized light. The state of polarization of the light is changed as a result of the double refraction of the sample, so that elliptically polarized light leaves the sample. The axial ratio of the ellipse of oscillation is a function of the path difference R. The azimuthal position of the ellipse of oscillation is determined by the orientation of the sample. There are disposed beyond the sample to be tested a A/4-phase plate 18 and an analyzer which splits the beam into two components which are polarized at right angles to one another and whose intensities are measured photoelectrically independently of one another.

If the sample to be tested does not exhibit double refraction, the second phase plate, which causes a path difference of A/4 or an odd multiple of A/4, produces linearly polarized light whose direction of oscillation is orientated at 450 to the main direction of oscillation of the phase plate.

The analyzer is adjusted such that this light reaches one of the two beam receivers, while the intensity 12 detected by the other beam receiver is 0.

If a double refracting sample to be tested is introduced into the path of the beam, the distribution of the beam changes in the following manner.

in which A02 represents the irradiated intensity, R represents the pass difference of the sample to be measured, and A represents the length of the measuring wave. Dependence upon the irradiated intensity is eliminated by forming a quotient, resulting in:

The path difference R of the sample to be tested is ascertained from the trigonometrical function. The range of values of the trigonometrical function is exhausted for the range O to .l/2 of the path difference. In the case of larger path difference values, it is possible to obtain a well-defined correlation by combination with other methods of measurement.

The non-uniformity of the function for A 2 2 can be avoided if a component ll or 12 and the total radiation 1G = Ii + 12 are measured during the measuring operation. Quotient formation results in

from which the path difference R can in turn be ascertained.

The present invention will now be further described, by way of example with reference to the accompanying drawing which is a diagrammatic illustration of the combination of a transmitted light polarization microscope with apparatus constructed according to the present invention, for the measurement of path differences.

The construction shown diagrammatically in the accompanying drawing, comprises a light source 2, a collector 3, a monochromatic filter 4, a polarizer 5, a A/4-phase plate 6 and a condenser 7, for illuminating a sample 1 to be tested with monochromatic, circularly polarized light. The .t/4-phase plate 6 is orientated such that its main direction of oscillation lies at 450 to the direction of oscillation of the linearly polarized light.

An objective lens 8 forms an image of the sample 1 to be tested, on a measuring window 9 which limits the detail of the object to be measured. A deflecting mirror 10 is located at the location of the measuring screen and, together with an eyepiece 11 and a polarizer 12, permits observation of the surrounding field of the detail of the object to be measured. The light which passes through the measuring window is split up by a polarization prism 1 3 according to Wollaston, Rochon, Senarmont or Dove, into two components 14 and 15 which are polarized at right angles to one another and which are detected separately by two beam receivers 1 6 and 17.The polarization prism 1 3 is adjusted such that the directions of oscillation of the two components lie at +454 the major axis of a V4-phase plate 1 8. The path difference is ascertained from the signals of the beam receivers 16, 1 7 in a device 1 9 for electronic signal processing, and is passed to a measured value indicator 20.

In a further variant of apparatus in accordance with the present invention, the apparatus detects the intensities 1, and í2, of the two light components 14, 1 5, alternately with one beam receiver. The device illustrated in the accompanying drawing, could be so modified by arranging, beyond the polarization prism, a moduiator which alternately covers the two components, and an optical system which combines the two components on the beam receiver. Modulation is effected by alternately applying 1l and 12, or 11 and IGT or 12 and IG to the beam receiver.

Furthermore, it is possible to alternately introduce into the path of the beam beyond the measuring window 10 a linear polarizer having a polarization direction of +45 to the main direction of oscillation of the .114-phase plate 9, and a linear polarizer having the polarization direction of -450 to the main direction of oscillation of the A/4-phase plate 9. The beam is detected by a beam receiver which alternately detects the intensities 1, and 12. The total beam 1G and one of the component beams 1, or 12 are applied alternately to the beam receiver by interposing and removing only one linear polarizer.

Claims

1. An apparatus for the measurement of path differences in polarized light, comprisiny a device for illuminating a sample to be tested with circularly polarized light, a ;1/4phase plate and an analyzer, the analyzer being adapted, to split up the light after the light has penetrated both the sample to be tested and the A/4-phase plate, the analyzer splitting up the light into two components which are polarized at right angles to one another and whose directions of oscillation are orientated at +450 to the major axis of the < 1/4-phase plate, a photometer being also provided which measures the intensities of the two components separately from one another,

2. Apparatus as claimed in claim 1, in which the analyzer is a polarisation prism.

3. Apparatus as claimed in claim 1 or 2, in which the light components are each fed to a respective beam receiver and subsequently to the photometer.

4. Apparatus as claimed in claim 1 or 2, in which a modulator is arranged beyond the analyzer to alternately cover the two light components, an optical system combining the two components on a beam receiver.

5. Apparatus as claimed in claim 1, in which two linear polarisers which are each out of phase by 450 with the main direction of oscillation of the A/4-phase plate, from the analyzer, one linear polariser being movable out of the light path.

6. An apparatus for the measurement of path difference in polarized light, constructed substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.