GB2117532A - Incident-light dark field objective for a microscope and method for adapting such an objective to a microscope - Google Patents

Abstract

An incident-light dark field objective for use in an objective turret of a microscope, in which the front edge of the outer part of a dark field channel surrounding a front lens of the objective is bevelled, so that the objective turret may be pivoted without the front surface of the objective contacting the microscope stage or a stage insert therein. The invention also provides a method of adapting an objective to a microscope using a polishing device which corresponds in structure and dimensions to the micro- scope. The objective is mounted on the part of this device which corresponds to the objective turret and a polishing surface is arranged in the vicinity of the plane corresponding to the specimen plane of the microscope, the front surface of the objective being appropriately polished on the polishing surface. <IMAGE>

Description

SPECIFICATION Incident-light dark field objective for a microscope and method for adapting such an objective to a microscope This invention relates to an incident-light dark field objective for attachment to an objective turret of a microscope. The invention also relates to a method of adapting an incidentlight dark field objective to a microscope which possesses an objective turret.

Incident-light dark field illuminations in microscopes is effected by means of a light cone in such a way that the front lens of the objective receives no reflected light but only light scattered by the specimen. This ensures that very fine details of the specimen to be inspected will appear brightly on a dark background. The illumination may be carried out by way of a dark field channel which is disposed around the front lens of the objective, the light used for the illumination reaching the specimen by way of an annular lens or mirror surface.

For correction reasons the working distance (distance between front lens and specimen) is always very small in the case of large aperture objectives which means that the objectives must be brought correspondingly near to the microscope stage which receives the specimen. The front surface of the outer part of the dark field channel, surrounding the front lens of the objective, projects beyond the front lens of the objective so as to prevent the objective striking the specimen. This is necessary because the holder of the front lens is very thin, so that it will not impede the illuminating light beam and is consequently fragile.

Microscopes having an objective turret for receiving a number of objectives usually have the axis of rotation of the objective turret arranged at an angle to the objective axis.

Rotation of the objective turret, together with the objective, causes the objective to move along an arcuate path whilst the objective is, as it were, at the same time laterally tilted relative to the objective axis, which means that the leading and trailing areas (in the direction of rotation) of the front surface of the outer part of the dark field channel approach the specimen or the microscope stage during the pivotal movement. Dark field objectives are usually so constructed that, when the objective is focussed, the leading and trailing areas of the front surface of the outer part can each pivot past the specimen without touching it, even though the clearance may be very small.

When a stage insert for carrying a specimen is introduced into the microscope stage in a tilted microscope the objective, arranged on the objective turret, can only pivot when the objective turret is moved away from the microscope stage. Also, in the case of upright microscopes, there are occasionally so-called "polishing stages", in which the specimen is pressed by spring force against a stage insert.

The abovementioned problems occur with these microscopes.

The necessity of moving the objective or objective turret, prior to rotation of the objective turret relative to the microscope stage, renders manipulation of the microscope more difficult. When a different objective is brought into use, refocussing must take place. Furthermore, there is a considerable risk that the user will forget to move the objective turret away from the microscope stage prior to rotation of the objective turret which may cause damage if the front surface of the objective or the front lens of the objective collides with the stage insert.

An object of the invention is to provide an incident-light dark field objective for a microscope which is less susceptible to damage than prior art objectives. Another object is to provide such an objective which is more convenient in use.

A further object of the invention is to provide a method which, in a simple manner, enables such objectives to be adapted to specific miroscopes to achieve the foregoing objects.

According to the present invention we provide an incident-light dark field objective for a microscope having an objective turret said objective having a front lens and a dark field channel around the front lens, said dark field channel being defined by an outer part which projects beyond the front lens, the front surface of the outer part being bevelled rearwardly and outwardly with respect to the front lens to facilitate rotation of the objective turret relative to the microscope stage.

Thus, according to the invention, the front surface of the objective is not planar, but is rearwardly bevelled starting from two linear portions of the objective which project the furthest distance in the forward direction, the bevel being sufficiently steep to permit pivotal movement of the objective turret (together with the objectives) relative to the microscope stage or a stage insert even when the objective lies very close to it. At least the linear portions of the front surface may project beyond the front lens to protect it from mechanical damage.

It is possible, in principle, to bevel the front surface of the objective, along a plane, towards both sides, starting from the linear portions of the objective. However, it is particularly advantageous to bevel the front surface of the outer part of the objective in the form of a cylindrical surface, which extends perpendicularly to the radial plane and whose radius of curvature approximately equals the distance between a straight line, lying in the radial plane and interconnecting the linear portions of the objective and the first point of intersection between the axis of rotation of the objective turret and the axis of the objective.

It would also be possible, in principle, to equip, at the manufacturing stage, dark field objectives with a front surface curved in a specified way. However, this would mean that the bevel would, generally speaking, be too steep as the bevelling of the front surface has to be precisely matched to the geometry of the microscope in order to ensure that the bevel allows the objective turret to rotate satisfactorily with the objective, while at the same time the front edge projects sufficiently far beyond the front lens to prevent damage.

The invention also provides a method for adapting an incident-light dark field objective to a microscope having an objective turret, comprising providing a device which corresponds in its structure and dimensions to the stage and objective turret of the microscope, arranging a polishing surface in a plane corresponding to the specimen plane of the microscope, mounting at least the outer part defining a dark field channel of an objective on a part of the device corresponding to the objective turret of the microscope, polishing the front surface of the outer part of the dark field channel of the objective by rotating back and forth the part of the device correponding to the objective turret while gradually bringing this part closer to the polishing surface; and aligning the objective in the objective turret of the microscope by rotating it about its axis.

If such a procedure is adopted, the curvature of the front edge of the objective is accurately matched, in a simple manner, to the geometry of the particular microscope used. When the objective is inserted into the microscope, it is merely necessary to observe the correct angular position between objective and objective turret, which can easily be ensured, for example, by means of appropriate markings or by using connecting means providing for positive location such as a bayonet fitting.

Further features, details, and advantages of the invention will be apparent from the following description, made with reference to the drawings, of a preferred embodiment of a microscope according to the invention.

Figure 1 is a highly schematic representation of an objective turret with an objective, and of a microscope stage, the microscope being tilted; Figure 2 represents, on a scale enlarged in comparison with Fig. 1 and in cross-section, the area of the front edge of the objective, and also the objective stage shown in Fig. 1; and Figure 3 is a perspective view of an objective according to the invention.

Referring to the drawings, a microscope stage 2 includes a well 1 for receiving a specimen. The objective 4 of the microscope includes a front lens 3.

The specimen is illuminated by way of a dark field channel 5, which contains either an annular lens or a toric mirror surface 6. Fig. 2 shows a toric mirror surface 6 provided on an outer part 7 which defines the dark field channel.

The objective 3 is fixed within the outer part 7 by means of a holder 8.

Prior art incident-light dark field objectives have a front surface 9 which lies in a plane located slightly above the front lens 3 to prevent the front lens 3 from being damaged through contact with a specimen. This is shown in dashed lines in Fig. 2. It will be appreciated that there is considerable risk of such damage as the holder 8 for the front lens 3 is very thin to minimise interference with the passage of the illumination beam through the dark field channel 5.

If the objective 4 is changed by rotating the objective turret 1 0 (see Fig. 1) this results in the front surface 9 of the objective 4 moving along an arcuate path about the first point of intersection 1 2 between axis of rotation 11 of the objective turret 10 and the axis of the objective 1 3 (shown schematically in Fig. 1).

In reality the axis of the objective 1 3 would lie behind the axis of rotation 11 in the plane of the drawing. The axis of the objective 1 3 is shown tilted towards the right in Fig. 1, to improve the illustration.

It can be seen from Fig. 2 that such prior art objectives have edge areas 14 and 1 5 of the front surface 9 which would collide with the stage insert 1 6 if the objective 4 were pivoted about the first point of intersection 12. The edge areas 14 and 15 would, in fact, move along a line 1 7 (Fig. 2).

In practice, such collision is prevented by the user increasing the distance between the objective 4 and the microscope stage 2 prior to rotation of the objective turret 10. There is, however, a considerable risk that the user will forget to do this, in which case quite considerable damage can occur to the objective.

In accordance with this invention the front edge or surface 1 8 of the outer part 7 of the objective 4 is uniformly rearwardly bevelled on both sides starting from the linear portions 20, 21 which are defined by a straight line 1 9 and which lie in a radial plane passing through the axis of rotation 11 of the objective turret 10 and the axis of the objective 1 3.

All points 22, 23 (including the points lying the furthest to the rear) of the front surface 1 8 of the outer part 7 lie at a distance from the first point of intersection 1 2 between the axis of rotation 11 of the objective turret 10 and the axis of the objective 13, which is smaller than or equal to the distance between the first point of intersection 1 2 and a second point of intersection 24 between the axis of the objective 1 3 and the straight lines 1 9 passing through the linear portions 20, 21 of the front surface 1 8 of the outer part 7.

In principle, any angle may be selected for the bevel. For example, the front surface 1 8 could slope in the form of two bevelled surfaces passing through the straight line 19. In the embodiment shown, the front surface 1 8 of the outer part 7 is formed by a generated cylindrical surface which runs perpendicularly to the radial plane which is defined by the axis of rotation 11 and by the axis of the objective 13, i.e. perpendicularly to the plane of the drawing in Figs. 2 and 3.The radius of curvature of this generated cylindrical surface is approximately equal to the distance between the straight line 1 9 through the linear portions 20, 21 and the first point of intersection 1 2 between the axis of the objective 1 3 and axis of rotation 11 of the objective turret 10. This generated cylindrical surface is indicated in Fig. 2 by the curved line 25.

In order to enable an incident-light dark field objective to be accurately adapted to the particular microscope in question, i.e. to ensure that manufacturing tolerances-and differing positions of the objective 4.relative to the objective turret 10 caused by such manufacturing tolerances-are compensated, the front surface 1 8 of the objective 4 may be polished by means of a device which corresponds in its structure and dimensions to the stage and objective turret of the microscope.

For example, a polishing surface may be arranged in the plane of the polishing device which corresponds to the specimen plane of the microscope, i.e. approximately in the plane of the under surface 26 of the stage insert 16.

The polishing surface may be in the form of a surface of a suitably abrasive material or a plate provided with a polishing agent.

The outer part 7 is then fixed to the part of the polishing device which corresponds to the objective turret 10 of the microscope. By pivoting this part back and forth, and at the same time gradually bringing the part closer to the polishing surface, the front surface 1 8 of the objective 4 or of the outer part 7 of the dark field channel may be gradually polished, a radius of the generated cylindrical surface being obtained which accurtely and automatically matches the geometry of the microscope.

As, during the polishing process, only the part 7 is fixed to the part corresponding to the objective turret 10, there is no risk of the front lens 3 of the objective being damaged.

After polishing the objective 4 is then completed and fixed to the objective turret 10 of the microscope. Appropriate alignment must take place here through rotation of the objective 4 about its axis 13, to which end, for example, marks may be provided on the objective turret 10 and on the objective 4.

Tests have shown that comparatively small bevel angles of the front surface 1 8 of the outer part 7 will suffice. For example, in the case of pivot radius of the turret of about 1 50 mm and a stage insert of about 0.1 mm clearance above the microscope stage 2, it suffices if the outer part 7 is polished by less than 0.1 mm in the vicinity of points 22, 23.

Furthermore, it is not always necessary, for alignment purposes, to rotate the whole objective 4 about its axis relative to the objective turret 10. Thus, if the outer part 7 of the dark field channel is separately rotatable, it will generally suffice to rotate the part 7 about the axis of the objective 1 3 in order to align the front edge 18.

Claims (5)

1. An incident-light dark field objective for a microscope having an objective turret said objective having a front lens and a dark field channel around the front lens, said dark field channel being defined by an outer part which projects beyond the front lens, the front surface of the outer part being bevelled rearwardly and outwardly with respect to the front lens to facilitate rotation of the objective turret relative to the microscope stage.

2. An incident-light dark field objective for attachment to an objective turret of a microscope, which turret is rotatable about an axis at an angle to the axis of the objective, said objective having a dark field channel around a front lens and defined by an outer part projecting beyond the front lens, the front surface of the outer part being uniformly rearwardly bevelled on both sides starting from two opposed linear portions defined by a radial plane passing through the axis of rotation of the objective turret, the distance of the front surface from a first point of intersection between the axis of rotation of the objective turret and the axis of the objective being, round the whole circumference of the front surface, at the most equal to the distance between the first point of intersection and a second point of intersection between the axis of the objective and straight lines passing, in the radial plane, through the linear portions of the front surface.

3. An incident-light dark field objective according to Claim 1, wherein the front surface of the outer part is formed by a generated cylindrical surface which is curved perpendicularly to the radial plane and having a radius of curvature approximately equal to the distance between a straight line, lying in the radial plane and interconnecting the linear portions of the front surface and the first point of intersection between the axis of the objective and the axis of rotation of the objective turret.

4. A method for adapting an incident-light dark field objective to a microscope having an objective turret comprising providing a device which corresponds in its structure and dimensions to the stage and objective turret of the microscope arranging a polishing surface in a plane, corresponding to the specimen plane on the microscope, mounting at least the outer part defining a dark field channel of an objective on a part of the device corresponding to the objective turret of the microscope, polishing the front surface of the outer part of the dark field channel of the objective by rotating back and forth the part of the device corresponding to the objective turret while gradually bringing this part closer to the polishing surface; and aligning the objective in the objective turret of the microscope by rotating it about its axis.

5. An incident-light dark field objective for a microscope substantially as herein described with reference to the accompanying drawings.