GB2038500A - Polarising microscope - Google Patents

Abstract

A doubly refracting constructional element 11 for polarising interference apparatus is, in use, positioned in the path of rays of a microscope for the high contrast display of phase objects and produces a differential image splitting. The element 11 comprises a Wollaston prism which consists of two comparatively thin (for example 1 mm.) component prisms 4, 5. The two component prisms 4, 5 are embedded for mechanical stability in glass wedges 3, 6 the refractive index of which corresponds substantially to the mean refractive index of quartz, so that no reflected light is produced at the glass-quartz boundary surfaces. With the element 11 inclined at an angle of approximately 3 DEG to the optical axis of the apparatus the axial ray impinges substantially perpendicularly on the boundary surface of the Wollaston prism. <IMAGE>

Description

SPECIFICATION Doubly refracting constructional element for polarizing interference apparatus This invention relates to a doubly refracting constructional element which is mainly used in conjunction with interference contrast devices for incident and transmitted light microscopes. As a rule such interference contrast devices are constructed as supplementary units for conventional microscopes, but they can also form an integral component of a microscope. They are employed more particularly in biology and medicine and in other technical fields, such as chemistry and metallography for the high contrast display of phase objects-i.e., microscopic objects which have only poor contrast, if any, when observed against a normal bright ground.

Interference microscope arrangements working on the principle of differential image splitting in polarised light and employed for the high contrast display of phase objects mainly use Wollaston prisms. These consist of two cemented component prisms of a doubly refracting material whose optical axes are orientated perpendicularly to one another, and on the image side they are disposed adjacent the exit pupil of the objective and on the illumination side adjacent the front condenser focal plane.

The Wollaston prisms effect an angular splitting of the incident ray, linearly polarised by a polariser connected in front, into its ray components polarised perpendicularly to one another, resulting for each object in two images which are offset by a small amount laterally in relation to one another and produce a relief contrast by interference in the intermediate image plane. As a rule the central thicknesses of the component prisms are so adapted for the two Wollaston prisms on the image and illumination sides, that in the case of the axial ray the phase difference between the ordinary and extra-ordinary ray component in each Wollaston prism is zero.

However, this is then not the case as regards the extra-axial and inclined rays.

These phase differences between the components of the various rays, which have an unfavourable effect on image contrast, can be partially compensated by suitable adaptation of the data of the illumination-side Wollaston prism to the image-side one. Such arrangements were first proposed in British pp"Cn+ Specification No. 639014 and French Patent Specification 1,059,123.According to British Patent Specification No. 639014 the Wollaston prisms are disposed directly in the focal plane of the objective and condenser, where the ray is also split, and according to French Patent Specification No. 1,059,123 an altered orientation of the optical axis of one component prism enables the Wollaston prism thus transformed to some extent to be disposed even outside the focal plane in a manner convenient for use, the ray being so split that the components or their rearward prolongation intersect one another in the focal plane of the objective or condenser.

However, with the arrangements disclosed in British Patent Specification No. 639014 and French Patent Specification No.

1,059,123 the phase differences can be completely compensated for the pupil and image field only with the completely symmetrical structure disclosed in the aforementioned British Patent Specification-i.e., if the focal lengths of the condenser and objective are identicai, since only in the case are the two Wollaston prisms also identical, but in practice such a structure can be put into effect ollEy in special cases. Complete compensation is all the less possible in proportion as symmetry is more heavily disturbed.

Pupil compensation for the rays passing through an axial point of the image field is always successful. All that is necessary is for the prism angle and orientation of the opticalaxes of both Wollaston prisms to be so adapted to one another that the ray components split in the illumination-side Wollaston prism are again united in the image-side Wollaston prism. For extra-axial image points, pupil compensation is also still possible with good approximation. This is not the case with field compensation to achieve uniform contrast over the whole image field, more particularly if the intention is to make full use of the large image fields attainable with the large-field objectives conventionally used nowadays.

Thus it can be shown theoretically and experimentally that an effect dependent on the azimuth of the ray of light passing through the Wollaston prisms occurs which results in a hyperbolic brightness distribution in the image field; German Democratic Republic Patent Specification No. 113271, Etude et application d'un interféromètre à polarisation (Study and application of a polarisation inter ferometer--original in French), Optica Acta (1954) 50-58.With optimum adaptation of the two Wollaston prisms, the value of the brightness differences occurring and of the uniformly contrasted zone depends above all on the thickness of the prisms and, according to French Patent Specification 1 059,1 23, on the orientation angle of the optical axis in the cX.nponent prism orientated at an inclination tc the prism base--i.e., the distance from the focal plane. For reasons of stability the Wollaston prisms cannot be made just as thin as desired. This applies more particularly to the image-side prism, since even a slight deviation from plane-parallel can result in distinct defect in the image quality, so that without additional ancillary means it is impossible to reduce the in homogeneity of the field by reducing prism thickness.

German Democratic Republic Patent Speci fication No. 11 3 271 discloses an additional pola?isation-optical element and the possibility of its use to reduce inhomogeneities in the field of an interference microscope provided with Woliaston prisms, but such element can be used only to a very limited extent, since the Wollaston prism is assumed for the sake of simplicity to be a plane-parallel crystal plate with axis parallel-with the base surfaces which is disposed perpendicularly to the optical axis of the apparatus.

In arrangements with incident light, but also to a somewhat lesser extent with transmitted light, there is the risk that due to reflection at the boundary surfaces of the image-side Wollaston prisms undesirable light reflections will get into the image and veil it.

These reflections can be rendered harmless by a slight inclination of the Wallaston prism. For example, an inclination of 4 to the optical axis of the apparatus ensures that the reflections do not get into the image field even with an image field diameter of 32 mm. However, this is then offset by an asymmetrical passage through the Wollaston prism, causing a further disturbance of symmetry and therefore of compensation.

It is an object of the invention to provide a constructional element which makes possible uniform contrast over large image fields accompanied by a substantial reduction in the reflected. light. The result is a clear increase in contrast and image quality and a substantial reduction of field inhomogeneities.

Accordingly, the present invention consists in a doubly refracting constructional element for polarising interference apparatuses, which, in use, is positioned in the path of rays of a microscope for the high contrast display of phase objects and produces a differential image splitting, said element comprising a Wollaston prism which consists of two thin component prisms of doubly refracting material and which is so cemented into glass wedges that when the element is inclined to the optical axis of the apparatus the axial ray impinges substantially perpendicularly on the boundary surface of the Wollaston prism.

In order that the invention may be more readily understood, reference is made to the accompanying drawing which illustrates diagrammatically and by way of example an embodiment thereof, and which shows a constructional element in accordance with the invention in the path of rays of incident illumination.

After reflection on flat glass 2 and passing through a glass wedge 3, incident light linearly polarised by a polariser 1 is split on a Wollaston prism made up of two quartz component prisms 4, 5. After passing through a glass wedge 6, the two ray components, polarised perpendicularly to one another, intersect in the objective focal plane 7, pass through an objective 8 and, after reflection on the object 9, are again caused to intersect by the effect of the objective in the objective focal plane 7 and are united in the Wollaston prism. An analyser 10 transmits only those portions of the ray components which oscillate in the same plane; they interfere with one another, and in the image the object observed appears contrasted in relief due to a slight lateral splitting.The Wollaston prism cemented into two glass wedges is so inclined that the radiant field stop reflection otherwise present no longer gets into the image. The wedge angle of the glass wedges was so selected that the axial ray impinges perpendicularly on the Wollaston prism, since as a result the most favourable compensation conditions are created, due to the symmetrical passage of the beam of rays.

Optimum conditions are created for the contrasting of microscopic phase objects in the interference microscope, the Wollaston prism being very thin, for example of the order of magnitude of 1 mm., and being embedded for mechanical stability in glass wedges, whose refractive index corresponds substantially to the mean refractive index of quartz, so that no reflected light is produced at the glass-quartz boundary surfaces. The glass wedges are so constructed as to produce a plane-parallel plate of a thickness of 3 to 4 mm., which plate is so inclined to the optical axis of the apparatus by an angle of about 3-4 that with both incident and transmitted light the resulting main reflection does not get into the image field.

The wedge angle of the glass wedges is so selected that the axial ray refracted at the first boundary surface impinges substantially perpendicularly on the boundary surface of the cemented-in Wollaston prism, so that due to the symmetrical passage of the light ray through the prism very favourable conditions are created for the compensation of field inhomogeneities. The present constructional element is particularly convenient for incident light arrangements, in which the risk of the image being veiled by reflections is particularly great. Moreover, the special adaptation of the compensation prism to the image-side prism for optimum field compensation is possible, since the illumination and image rays pass through the same Wollaston prism, which therefore acts simultaneously as a compensation prism and as an image-side prism. In this case, therefore, only a reduction of prism thickness is left as an essential means for reducing field inhomogeneities.

However, the constructional element of the invention can also advantageously be used for transmitted light, more particularly when applied to infinite image distance of corrected objectives, for field compensation and the avoidance of reflections.

Claims (2)

1. A doubly refracting constructional element for polarising interference apparatuses, which, in use, is positioned in the path of rays of a microscope for the high contrast display of phase objects and produces a differential image splitting, said element comprising a Wollaston prism which consists of two thin component prisms of doubly refracting material and which is so cemented into glass wedges that when the element is inclined to the optical axis of the apparatus the axial ray impinges substantially perpendicularly on the boundary surface of the Wollaston prism.

2. A doubly refracting constructional element for polarising interference apparatus, substantially as herein described with reference to and as shown in the accompanying drawing.