WO2001044856A1

WO2001044856A1 - Microscopic device

Info

Publication number: WO2001044856A1
Application number: PCT/SE2000/002521
Authority: WO
Inventors: Anders Rosenqvist; Erik Linderup
Original assignee: Cellavision Ab
Priority date: 1999-12-14
Filing date: 2000-12-14
Publication date: 2001-06-21
Also published as: AU2416601A

Abstract

In a microscope (10) for microscoping an object (14), the object, the objective (16), and the sensor (26) are arranged in relation to each other in such a way that a desired focus change can be performed by moving the object (14) transversely of the optical axis (O) of the objective. For example, the sensor (26) can be offset mounted in relation to the optical axis of the objective, a field curvature aberration of the objective making it possible to provide the focus change by way of a lateral movement of the object.

Description

MICROSCOPIC DEVICE

Field of the Invention

The present invention relates to a microscopic device for microscoping an object. The microscopic device is preferably intended for use in light micro- scopy and especially for automated light microscopy. The invention also relates to a method for mounting a sensor, a method for providing a desired change of focus on an object in a microscope, as well as the use of an objective with field curvature. Background of the Invention

A microscope must be focused on the object that it is imaging in order for the user to obtain a sharp image of the object. In the case of a microscope which is used for scanning an object constant re- focusing is necessary since the object is seldom plane while the focal depth of the lens is relatively small.

Fig. 1 shows an example of a known microscope 10 which can be used for scanning an object. A light source 12 illuminates the object 14, which is imaged by the objective 16 on a retina of a user's eye by the intermediary of one output of the beam splitter 18, the ocular 20, and the lens of the eye, as well as on an image sensor 26 by the intermediary of the other output of the beam splitter 18. A sharp image is obtained on the sensor if the image plane of the objective coincides with the plane of the image sensor. Correspondingly, the user obtains a sharp image if the image generated by the objective/ocular/eye lens system coincides with the retina. The scanning can, for example, be carried out manually by the user by controlling the microscope, on the basis of visual impressions, by the intermediary of a focusing wheel 30, x-y wheels 34, and the respective associated mechanisms 32 and 36, with respect to the focusing as well as the position of the object. The mechanism 36 for position control can, for example, be an x-y stage which is acted upon by the x-y wheels 34. The mechanism 32 for controlling the focusing is a mechanism which modifies the object/objective/sensor system and which in this example moves the object along the optical axis of the microscope.

The scanning can also be effected automatically with the aid of the images recorded by the sensor 26, which are stored in an image memory 40. A processor 42 extracts focusing and positional information from the images and controls, on the basis of this information, the focusing wheel 30 and the x-y wheels 34 by the intermediary of a control mechanism 44, which, for example, can comprise stepping motors and gearboxes.

In order for an automatic scanning microscope to be capable of quickly scanning an object, it must have a fast focusing system which changes the focus . Furthermore, there are special requirements with respect to, for example, the step spacing, the play, and the hysteresis of the focusing system since the system is sensitive to small focusing errors. When the image is viewed by a human eye by the intermediary of an ocular, the eye can compensate for small focusing errors, but in the case of a sensor the focusing system must carry out the compensation. Moreover, it is relatively easy for the human brain to compensate for play and non-linearities of the focusing system, while such problems in an automatic scanning microscope renders the development of a fast auto- focus- ing method more difficult.

There are several known principles for changing the focus of a microscope. The most common principle is to adjust the distance between the object and the objective while the distance between the objective and the sensor/ ocular remains fixed. This principle is illustrated in

Fig. 2a, which schematically shows a sensor 26, an objective 16, and an object 14 with height variations on a microscope slide 13, as well as a graph showing how the distance z between the microscope slide 13 on the one hand and the objective 16 and the sensor 26 on the other must be changed when the object is moved in the x-direc- tion.

Another common principle is the use of a so-called infinity corrected objective together with additional optics placed between the objective and the sensor/ ocular. This principle is illustrated in Fig. 2b where the additional optics are designated as 15. Despite the fact that the objective 16 in such a case moves in relation to both the object 14 and the sensor 26, it is only the movement in relation to the object that is of any importance from an optical point of view. The graph shows how the distance z between the object 14 and the objective 16 must be changed when the object is moved in the x-direction.

When the principles according to Figs 1 and 2 are used for a high resolution objective for light microscopy the objective must be moved in smallest steps of a few tenths of a micrometer. The movement can, for example, be effected with the aid of a geared stepping motor which drives a focusing mechanism integrated into the frame of the microscope. Such a focusing system is relatively costly and, above all, it is seldom fast while also providing small smallest movements. Presently available microscope frames are not optimised for simple motorization of small fast movements, rather they are designed for manual focusing. Alternatively, a fine focusing system can be implemented with the aid of piezoelectric crystals which are placed adjacent to the attachment of either the object or the objective. In this case, a focusing mechanism which is integrated into the frame is used for rough focusing. The piezoelectric crystals are very fast, but, on the other hand, they are expensive and require special control electronics generating a high voltage. A third known principle for changing the focus of the microscope is to adjust the distance between the objective and the sensor by moving the sensor while maintaining the distance between the object and the objective. This is similar to the above-mentioned adjusting role of the eye. This principle is illustrated in Fig. 2c, where the graph shows the movement z of the sensor as a function of the movement x of the object. This principle has the advantage that it uses considerably larger movements than those described above. For example, a 0.1 μm objective movement corresponds to a 1 mm sensor movement when the magnification is 100 times. Since the objective is optimised for a certain distance from the sensor, a focusing system based on this principle can only be used for fine focusing and, consequently, it must be supplemented with another focusing system for rough focusing, which, for example, can operate in the same way as the piezoelectric system described above. One of the drawbacks of the principle is that in today's standard microscope frames there is no mechanism upon which to base this focusing system. Moreover, since a sensor can be fairly heavy, for example when it is implemented as a 3 -chip camera, it is difficult to create a fast and inexpensive fine focusing system based on this principle. An advantage of using separate rough and fine focusing systems, as in some of the above cases, is that the systems can be optimised separately and on the basis of different criteria. For example, the rough focusing system can be optimised with respect to being simple and inexpensive. Since it can have rough steps it can have a wide range while still being fairly fast. The fine focusing system can then be optimised with respect to being fast and yielding small steps, while its range of movement need only be somewhat greater than the step spacing of the rough focusing system.

Hitherto, a drawback of using two separate focusing systems has been that the number of mechanisms, actua- tors, driving units, and cables increases. This may result in higher manufacturing costs and renders the task of ensuring electromagnetic compatibility more difficult. Summary of the Invention It is an object of the present invention to solve the above-mentioned problems associated with the known focusing principles.

This object is completely or partly achieved by a microscopic device according to claim 1, a method for mounting a sensor according to claim 14, a method for providing a desired focus change according to claim 16 and the use of an objective with field curvature according to claim 17.

More specifically, according to a first aspect the invention relates to a microscopic device for microscop- ing an object, the structure of which microscopic device is such that a desired focus change is at least partly performable by moving the object laterally in relation to the optical axis of the microscopic device. All of the known principles for changing the focus of a microscope are based on mutual movements of the object, the objective, and the sensor along the optical axis of the microscope. According to the invention, the focus change is instead carried out by moving the object transversely of the optical axis. This means that the movement should be carried out in such a way that the movement vector will have one component at right angles to the optical axis. One of the advantages of this is that the same mechanical components that are used for moving the object in connection with scanning the same can be used for changing the focus. This reduces the manufacturing cost of the microscopic device.

Furthermore, an exchange occurs between the movement of the object and the focus change so that a fairly large movement of the object may result in a very small focus change, which in turn means that very accurate focus changes can be achieved by relatively simple and inexpensive means.

Moreover, because of the exchange it is possible to obtain a step spacing and a play which are much smaller than those usually obtained in motorised focusing systems .

As described below, the new principle for effecting the focus change is based on the fact that the way in which the components of the microscope are arranged in relation to one another is partly new and the way they are utilised is partly new.

This new principle for changing the focus can be used as the only principle for changing the focus or be used for fine focusing and be combined with some other known principle for rough focusing. A desired focus change can thus be carried out entirely or partly by a lateral movement of the object.

The object being microscoped, i.e. imaged and magnified in the microscope, can be any object capable of being microscoped. It can be part of a larger object or structure. Normally, the object is part of a specimen located on a microscope slide. The movement of the object is thus normally carried out by moving the microscope slide . According to one embodiment, the microscopic device comprises a plane sensor for recording an image of an object, which sensor is purposely mounted in such a way that if the object is plane the focus will be different in different parts of the sensor. Normally, one strives to mount a sensor so that a plane object whose image covers the entire sensor can be imaged sharply across the entire sensor, thereby making it possible to utilise the entire area of the sensor for recording the image. According to the invention, a com- pletely different approach is used in that one strives to achieve different levels of sharpness on different parts of the sensor so that it will be possible to provide a focus change by moving the image of the object transversely of the optical axis at the sensor. This, in turn, means that it is not possible to utilise the entire sensor area since the image is not sharp across the entire sensor, but this is not a serious problem. With the exception of certain low intensity applications such as fluorescence microscopy, it is the reading of the pixel values rather than the exposure of the sensor that is time-consuming. A utilised sensor area which is half as large for each image only yields half the number of pixels that must be read for each image, which means that almost twice as many images can be read per second and that the area imaged on the object per second, i.e. the product of imaged area per image and the number of images per second, remains almost unchanged. In applications where the height variation of the object is considerable, the principle of smaller imaged areas on the object will have the advantage that different areas can be exposed (imaged) in different, optimal focusing positions. The magnification may depend on the position of the object, but this can be compensated for, as described in, for example, PCT application W099/31622.

The sensor can be a light-sensitive black and white sensor or a one-chip colour sensor. It can be analog or digital. Advantageously, it is a digital area sensor. In an advantageous embodiment, the focus change is achieved by the fact that the sensor is purposely mounted in such a way that an image plane, in which the object is imaged if it is plane, is inclined in relation to the plane of the sensor. As described below, this can be implemented in several ways. If the image plane is inclined in a known manner in relation to the sensor plane a linear function is obtained for controlling the focus change . According to one embodiment, the microscopic device comprises an objective for providing an image of the object, and at least one focusing system, which is adapt- ed to carry out said movement of the object transversely of the optical axis of the objective.

In this case, the optical axis of the microscopic device is thus the optical axis of the objective. In prior art devices the focusing system changes the focus by moving one or more components in the microscopic devices along the optical axis. This means that the entire image plane in which a plane object is focused is moved farther away from or closer to the plane of the sensor. According to the invention, however, the focusing system moves the object transversely of the optical axis of the objective, preferably at right angles to the same, which means that the image of the object is moved across the sensor, thereby changing the sharpness. The movement can be carried out manually by a user acting on the focusing system or automatically by a control signal acting on the focusing system.

In this context, the term "focusing system" refers to one or more mechanical or electromechanical components or other components which, when acted upon by a user in a manual microscope or a by a control signal in an automatic microscope, change the focus of the microscope, i.e. the sharpness of the image of the object which is to be studied in the microscope. In an advantageous embodiment, the microscopic device comprises means for moving the object with respect to two coordinate axes so that the object can be scanned, whereby the focus change can be carried out by moving the object along one of the two coordinate axes. This embodi- ment has the advantage that the means that are used for scanning the object can also be used for changing the focus. The means for moving the object can be mechanical, electromechanical or some other type which is presently used for providing movement when an object is scanned. The two coordinate axes can be two mutually perpendicular axes, such as an x-axis and a y-axis in a Cartesian coor- dinate system. Other coordinate systems can also be used, for example a polar coordinate system.

In an advantageous embodiment, the microscopic device comprises an objective for providing an image of the object, which objective has an optical axis and a field curvature aberration, and at least one sensor for recording the image of the object, which sensor is purposely offset mounted with a first offset along a first axis essentially perpendicular to the extension of the optical axis of the objective at the sensor.

The field curvature aberration or the image field curvature means that the depth position of the object for optimal focusing depends on the radial distance of the object from the optical axis of the objective. The effect is shown in Fig. 3a. A plane object 14, which is placed at right angles to the optical axis, is imaged by means of an objective 16. If the objective 16 has a field curvature aberration, the image plane B will be curved so that the image of the plane object will be focused in a curved plane. A description of this phenomenon can be found in "Optics" by Eugene Hecht, second edition, pp. 228-230 (1987) .

Fig. 3b shows an alternative perspective on the field curvature. In order for an object to be imaged sharply on a plane sensor 26 with the aid of an objective 16 which has a field curvature aberration, the object must be in the curved field F.

The field curvature is usually a square function of the radial distance from the optical axis O. The existence of a field curvature is normally purely disadvantageous since a plane object, whose image covers a plane sensor, cannot be imaged sharply on the whole sensor at the same time. Typically, and primarily in connection with manual microscopy, one tries to remove or mini- mise the field curvature by means of compensations in the objective. Such compensations can be to consider the task of achieving a small field curvature more important than the task of achieving some other objective characteristic, for example the capability to resolve small object details, when manufacturing the objective. Another way of compensating for field curvature is to add extra lens elements, whereby, among other things, the objective will let through a smaller amount of light, thereby requiring longer exposure times and stronger illumination. Most likely the objective will also be more expensive to produce . The advantage of the above embodiment with an offset mounted sensor is thus that the task of correcting the field aberration in the objective is completely, or at least partially, eliminated, which means that the objective can contain fewer lenses, thus making it simpler and less expensive.

In this embodiment, the sensor is thus offset laterally from its normal location on the optical axis. It should be noted that the extension of the optical axis at the sensor need not be aligned with the optical axis of the objective, rather it can be angled in relation thereto with the aid of, for example, beam splitters and mirrors .

Furthermore, the microscopic device can comprise a focusing system which is adapted to carry out said move- ment of the object along a second axis essentially at right angles to the optical axis of the objective, the first axis being parallel to the second axis. Accordingly, the focusing system moves the object in the same direction as that in which the sensor is offset for pro- viding the focus change.

As mentioned above, the second axis is preferably parallel to or coincides with one of the two coordinate axes for scanning the object.

The microscopic device can advantageously comprise at least a second sensor, which is purposely mounted with a second offset perpendicular to the extension of the optical axis at the sensor, the first and second offsets being different in size. Two sensors can thus be placed in different parts of the image plane so that the inclination of the latter will be different in relation to each sensor. In this way, two different focus change rates can be chosen as a function of the movement of the object. Naturally, additional sensors can be used.

The sensors are advantageously placed in such a way that they are offset along a respective one of said two coordinate axes for scanning the object. An alternative way of enabling a selectable focus change rate is to mount to the first sensor so that it is movable. The user can then choose the location in the image plane where the sensor should be placed, thereby choosing the inclination of the image plane in relation to the sensor. However, this solution will probably be a great deal more expensive.

In an alternative embodiment, the sensor is purposely mounted so that a normal to the plane of the sensor forms an angle with the extension of the optical axis of the objective at the sensor.

This also has the effect that the focus varies across the sensor area. In the prior art one strives to mount the sensor so that the normal to the plane of the sensor is parallel to the optical axis so that the same focus is obtained across the whole sensor. According to the invention, one thus uses a completely different approach in that one strives for an angle between the plane of the sensor and the optical axis.

In this embodiment, the objective should have essen- tially no field curvature and the sensor should be mounted essentially symmetrically in relation to the extension of the optical axis at the sensor, i.e. not an offset mount. This means that the objective will be more expensive and, consequently, this embodiment is less preferred if an inexpensive objective is desirable. In addition, according to this embodiment, one is more or less obliged to utilise lateral movement for changing the focus, while in the above embodiment with an offset mounted sensor one can choose whether or not to use the option of lateral movement for changing the focus by choosing an objective with or without a field curvature aberration. In another alternative embodiment a plane sensor is mounted at right angles to the optical axis of the objective and the object is adapted to be held so that a normal to the object forms an angle with the optical axis of the objective if the object is plane. Normally, one strives to arrange an object holder so that a plane object is held at right angles to the optical axis. However, according to the invention one seeks a deviation from this relationship so that the image plane of the plane object forms an angle with the plane of the sensor. The microscopic device can be a complete microscope or parts of a microscope having the characteristics stated in the appended claims. The microscopic device is preferably a light microscopy device and especially an automatic scanning light microscopy device. According to a second aspect, the invention relates to a method for mounting a sensor in a microscope having an objective for providing an image of an object, the objective having an image plane in which the image is focused, comprising the step of purposely mounting the sensor so that the image plane is inclined in relation to the plane of the sensor if the object is plane.

The advantage of this method is evident from the above discussion. Naturally, the above statements with respect to the mounting of the sensor are also applicable to the method.

According to a third aspect, the invention relates to a method for providing a desired change of focus on an object in a microscope, comprising the step of moving the object transversely of the optical axis of the microscope for providing the desired focus change.

The advantage of this method is evident from the above discussion. Naturally, the above statements with respect to the movement of the object are also applicable to the method for providing a focus change.

According to a fourth aspect of the invention, it relates to the use of an objective with field curvature in a microscope for enabling control of a focus change by moving an object which is to be studied in the microscope transversely of the optical axis of the microscope.

The advantage of this use and further aspects thereof are described above. Brief Description of the Drawings

The present invention will now be described in more detail by way of exemplifying embodiments with reference to the accompanying drawings, in which:

Fig. 1, which is described above, schematically shows a prior art microscope.

Figs 2a-c, which are described above, schematically show three known principles for changing the focus.

Figs 3a and 3b, which are described above, schematically show the effect of field curvature. Fig. 4 schematically shows a first embodiment of a microscopic device according to the present invention.

Fig. 5 schematically shows a contour line plot of a field curvature.

Fig. 6 schematically shows how a sensor is mounted according to a first embodiment of the invention.

Fig. 7 schematically shows how a sensor is mounted according to a second embodiment of the invention.

Fig. 8 schematically shows how an object is mounted according to a third embodiment of the invention. Description of a Preferred Embodiment

Fig. 4 shows an example of a light microscope according to the present invention. The light microscope essentially corresponds to that shown in Fig. 1 and, consequently, the same reference numerals indicate like parts.

The microscope thus has a light source 12, which illuminates an object 14 which is to be imaged and magni- fied in the microscope. The object 14 can, for example, be a white blood cell in a blood smear on a microscope slide .

Moreover, the microscope comprises an objective 16, which, with the aid of the light from the light source 12 and by the intermediary of one output of a beam splitter, is adapted to image a part of the object 14 which is located in the field of view of the objective on a digital image sensor 26, which creates an image in electronic format, which is stored in an image memory 40. Furthermore, the objective 16 is adapted to image the object 14 by the intermediary of the second output of the beam splitter 18 and an ocular 20 on a retina of a user's eye. The object 14 can be moved along two coordinate axes x and y in a plane perpendicular to the optical axis 0 of the objective with the aid of an x-y mechanism 36 in the form of an x-y stage which is controlled, manually or automatically, by the intermediary of x-y wheels 34.

Furthermore, the microscope 10 comprises a processor 42, a control mechanism 44, a rough focusing wheel 48 and a rough focusing mechanism 46, which together form a rough focusing system of a previously known type, which modifies the distance between the object 14 and the objective 16 for providing rough focusing. Moreover, the processor 42, the control mechanism 44, the x-y wheels 34 and the x-y mechanism 36 together form a fine focusing system based on lateral movement of the object 14 and a positioning system which makes it possible to scan the object 14. The following is a more detailed description of how the sensor 26 and the object 14 can be arranged in relation to each other to enable fine focusing with the aid of lateral movement.

Traditionally, one aligns the objective 16 with the ocular 20/sensor 26 in order to use the centre of the image plane B of the objective. This is illustrated in Fig. 5 which schematically shows a contour line plot of the field curvature in the image plane B. A sensor is thus usually placed as indicated by the rectangle a in Fig. 5 since the effects of the field curvature are not as noticeable in the centre of the image plane of the objective. However, the focus aberration is different in the corners and in the centre of the edges respectively of the rectangle a. By offset mounting the sensor so that it is arranged with an offset f in relation to the optical axis 0 of the objective, one can instead use the rec- tangle b in Fig. 5. The offset mount means that essentially one only obtains a focus change with respect to one coordinate axis in the image, hereafter referred to as the "focus axis", and that for a given objective with a fixed field curvature one can to some extent choose the "inclination" of the image plane by choosing the size of the offset f . Preferably, the main scanning direction of the microscope for scanning the object is parallel to the focus axis.

The scanning operation of the microscope can, for example, be carried out by two independent mechanisms, such as the x-y mechanism 36, translating the object along a respective axis or by one mechanism rotating the object while another mechanism translates it. In the first case, the scanning can essentially be carried out by way of the two translation mechanisms. In the second case, the scanning is preferably carried out in circles or arcs with the aid of the rotation mechanism, the current position of the translating mechanism determining the radius of the arc. The rotating motion can then, locally in the area imaged by the sensor and for radii of sufficient magnitude, be approximated by a translatory motion parallel to the focus axis.

The offset mount is calculated with respect to the desired offset at the object and is made at the sensor and is typically a few millimetres, which is easy to carry out with sufficient relative accuracy. In order to provide usable fine focusing, the inclination of the image plane will be a compromise. In order to obtain a sufficiently small focus change within an object, which is equivalent to a sufficient exchange in the relation between a lateral movement and a corresponding focus change, it is desirable to have a small inclination of the image plane with respect to the focus axis. At the same time, in order to obtain a large focus change within the surface of the sensor, thereby enabling the rough focusing system to have large steps, it is desirable to have a large inclination of the image plane with respect to the focus axis.

To enable a varying focus change rate, a second sensor can be arranged with a different offset f ' in rela- tion to the optical axis and perpendicular to the offset of the first sensor, i.e. along the second coordinate axis. This is illustrated by the rectangle c in Fig. 5.

Yet another alternative is to make the first sensor movable along the focus axis so that the user can choose the inclination of the image plane.

A further alternative is to use a large sensor covering the entire image plane and to utilise different parts of this sensor. However, this is probably an expensive alternative. Examples

Suppose that a sensor has a sensor side which is n pixels, where the centre distance between adjacent pixels is p, and that the objective has a magnification m. This means that the side of the sensor corresponds to the dis- tance np/m in the object. Furthermore, suppose that the centre of a circular object having the diameter d shall be in focus and that the border of the same is permitted to be out of focus by a maximum of e. This means that, locally in the relatively plane field, the inclination can maximally be such that the focus changes 2e over the distance d. Using the approximation that the field is an inclined plane across the sensor area, the maximum focus change t along the side of the sensor is t=2npe/ (md) .

If n=512, p=12 μm, m=100, e=0.2 μm and d=20 μm t will be about 1.2 μm. With a more accurate, square model, the maximum focus change along the side of the sensor will be somewhat smaller. As a general rule, the plane approximation is better the smaller the object. In the example, the inclination is 0.4 μm/20 μm = 1/50. Thus, instead of moving the object 0.1 μm along the optical axis using a traditional focusing system, one can move it about 5 μm along the focus axis. The latter movement can be effected using a much simpler device which is already available and which is used for scanning the object. By way of comparison, a focus change as small as 2 nm can be achieved by means of a 0.1 μm movement along the focus axis, which is quite possible even with a relatively simple, motorised x-y stage.

An "Olympus Nea lOOx/1.25 oil" has a curvature of about z=2 μm at a radial distance of r=100 μm from the optical axis. Assuming that the curvature is a square function of the distance from the optical axis, we get z=k(r/l00)² with k=2. At a radial distance of 50 μm the inclination of the plane local- ly is dz/dr/=2k/l00 (r/100) =2*2/100*50/100 μm per μm =2/100=1/50. In order for the centre of the sensor to be in that location, the sensor should be mounted with a 5 mm offset, which is 50 μm magnified 100 times. The operation of the microscope in Fig. 4 with an offset mounted sensor will now be described with reference to Fig. 6, which shows the offset mounted sensor 26, the objective 16, and a microscope slide 13 with a preparation with height variations. The objective 16 has a field curvature aberration, which means that the plane microscope slide 13 is imaged sharply in a curved image plane B. The plane of the sensor 26 is thus inclined in relation to the image plane B. Now, suppose that the dot- shaped object 14 is to be imaged by the sensor 26 and that the microscope slide 13 is in the position indicated by dashed lines. In this position, the dot-shaped object 14 is imaged sharply in a point which is located a short distance above the sensor 26. One or more images are recorded by the sensor 26 and are stored in the image memory 40. The image (s) is/are analysed by the processor 42 with the aid of known methods for determining a value for the quality of the focusing. On the basis of this focusing value, the processor 42 determines a control signal for the control mechanism 44 which in turn causes the x-y wheels 34 and, by the intermediary of the same, the x-y mechanism 36 to move the microscope slide 13 with the preparation in the x-direction to the position indi- cated by solid lines. As a result of the field curvature, the point where the dot -shaped object is imaged sharply will be moved closer to the sensor, in this case ending up exactly in the plane of the sensor, the dot-shaped object 14 and a small area around it at approximately the same height thus being imaged sharply on the sensor.

Thus, the entire area of the sensor 26 cannot be utilised for sharp imaging, but as mentioned above this is not a major problem.

The above embodiment with an offset mounted sensor is most preferred.

As an alternative, the sensor can be mounted with an inclination in relation to the optical axis of the objective. Normally, a plane sensor is mounted so that it is perpendicular to and centred on the optical axis. In order to enable fine focusing with the aid of lateral movement, preferably movement perpendicular to the optical axis, the sensor is instead mounted so that the normal to the sensor forms an angle with the optical axis. The principle of this embodiment is shown in Fig. 7. The sensor 26 has a normal N which forms an angle with the optical axis O of the objective. If the objective essentially lacks field curvature aberration, a plane object 14 will be imaged sharply in an image plane B which is perpendicular to the optical axis. This means that only a small part of the plane object 14 is imaged sharply on the sensor 26, but that a focus change can be effected on the basis of a lateral movement in the same way as described above with reference to Fig. 6.

In a third variant, shown in Fig. 8, a plane sensor 26 is mounted in the traditional manner, i.e. at right angles to and centred on the optical axis O of an objec- tive 16. However, an object 14 is mounted in an inclined plane in such a way that it can be moved in the inclined plane. If the object 14 is plane it is imaged sharply in an image plane B which is inclined in relation to the plane of the sensor 26. This means that only a small part of the object is imaged sharply on the sensor, but that a focus change can be effected by moving the object in the inclined plane.

In the above embodiments the sensor is plane, which presently is the only existing shape of a microscope sen- sor. However, the principle of the invention is equally applicable to non-plane sensors.

Claims

1. A microscopic device for microscoping an object (14), c h a r a c t e r i s e d in that the structure of the microscopic device is such that a desired focus change is at least partly performable by moving the object (14) laterally in relation to the optical axis (O) of the microscopic device.

2. A microscopic device according to claim 1, further comprising a plane sensor (26) for recording an image of the object (14) , which sensor is purposely mounted in such a way that, if the object is plane, the focus will be different in different parts of the sensor.

3. A microscopic device according to claim 1, comprising a plane sensor (26) for recording an image of the object (14) , which sensor is purposely mounted in such a way that an image plane (B) , in which the object is imaged sharply if it is plane, is inclined in relation to the plane of the sensor.

4. A microscopic device according to claim 1, comprising an objective (16) for providing an image of the object, and at least one focusing system (34, 36, 42, 44) , which is adapted to carry out said movement of the object transversely of the optical axis of the objective.

5. A microscopic device according to any one of claims 1-4, further comprising means (34, 36, 42, 44) for moving the object with respect to two coordinate axes so that the object can be scanned by the microscopic device, the focus change being performable by moving the object along one of the two coordinate axes .

6. A microscopic device according to claim 1, further comprising an objective (16) for providing an image of the object, which objective has an optical axis (O) and a field curvature aberration, and at least one sensor (26) for recording the image of the object, which sensor is purposely offset mounted with a first offset (f) along a first axis essentially perpendicular to the extension of the optical axis of the objective at the sensor.

7. A microscopic device according to claim 6, further comprising a focusing system (34, 36, 42, 44) which is adapted to carry out said movement of the object along a second axis essentially perpendicular to the optical axis of the objective, the first axis being optically parallel to the second axis.

8. A microscopic device according to claim 7, where- in the microscopic device has means for moving the object

(34, 36, 42, 44) with respect to two coordinate axes so that the object can be scanned, the second axis being parallel to one of the two coordinate axes.

9. A microscopic device according to claim 6, com- prising at least a second sensor, which is purposely mounted with a second offset perpendicular to the extension of the optical axis at the sensor, the first and second offsets being different in size.

10. A microscopic device according to claims 8 and 9, wherein the second offset is effected along the other of said two coordinate axes .

11. A microscopic device according to any one of claims 6-10, wherein the first sensor (26) is movably mounted.

12. A microscopic device according to claim 1, further comprising an objective (16) for providing an image of the object, which objective has an optical axis (0), and at least one sensor (26) for recording the image of the object, which sensor is purposely mounted in such a way that a normal to the plane of the sensor forms an angle with the extension of the optical axis of the objective at the sensor.

13. A microscopic device according to claim 1, further comprising an objective (16) for providing an image of the object, which objective has an optical axis (0) , and at least one sensor (26) for recording the image of the object, the object being purposely mounted in such a way that, if the object is plane, a normal to the object forms an angle with the optical axis of the objective .

14. A method of mounting a plane sensor in a micro- scope which has an objective for providing an image of an object, the objective having an image plane in which the image is focused, c h a r a c t e r i s e d by the step of purposely mounting the sensor in such a way that the image plane is inclined in relation to the plane of the sensor if the object is plane.

15. A method according to claim 14, wherein the step of mounting the sensor so that the image plane is inclined in relation to the plane of the sensor comprises the step of offset-mounting the sensor in relation to the optical axis of the objective.

16. A method for providing a desired change of focus on an object in a microscope, comprising the step of moving the object transversely of the optical axis of the microscope for providing the desired focus change.

17. Use of an objective with a field curvature aberration in a microscope for enabling control of a focus change by moving an object which is to be studied in the microscope transversely of the optical axis of the microscope .