The present invention relates to optical instruments, especially microscopes, and in particular optical instruments which have an exit pupil at which an image of an object may be viewed by an observer.

In conventional optical instruments, the size of the exit pupil is determined by a function of the numerical aperture and overall magnification of the optical instrument, and hence the size of the exit pupil is of fixed and relatively-small dimension. Consequently, it is necessary for an observer to accurately align the entrance pupil of his/her eye with the exit pupil of the optical instrument in order properly to view an image.

The present applicant has previously developed a number of different optical instruments which, through the provision of a diffractive element at an intermediate image plane, provide an exit pupil which is effectively enlarged, allowing an observer to view an image by placing his/her eye anywhere within the enlarged exit pupil. These optical instruments are disclosed in US-A-6028704, US-A-6608720 and US-A-7123415.

The present applicant has now developed optical instruments, which still provide an exit pupil which is effectively enlarged, but also provides far greater optical clarity.

The present inventors have surprisingly found that, by providing a suitable diffractive element at the primary image plane as opposed to the secondary image plane, a greater optical clarity can be achieved than with the previous optical instruments.

To date, there has been no recognition that an enlarged exit pupil could be achieved by the provision of a diffractive element at the primary image plane, and, in this regard, the present invention overcomes a technical prejudice. This is not least because it was perceived that the diffractive element and projection optics could not be fabricated to allow for the required resolution.

In one aspect the present invention provides an optical instrument for producing an optical image to be viewed by an observer, the optical instrument comprising : an optical system for producing an optical image of an object which is viewable by an observer at an exit pupil; and a diffractive element located at a primary image plane of the optical system for producing an array of the exit pupils, which are perceivable as a single, enlarged exit pupil by the observer.

In one embodiment the diffractive element comprises a surface which has an array of diffractive sub-elements, each of which generates one of the exit pupils of the array of exit pupils.

In one embodiment the optical system comprises an objective lens for producing a primary image of an object in the primary image plane, and a projection lens for projecting a secondary image of the primary image at the diffractive element to a secondary image plane.

In one embodiment the optical system further comprises a field arrangement at the secondary image plane for relaying the aperture images of the array of exit pupils.

In one embodiment the filed arrangement comprises a pair of field lenses located at the secondary image plane.

In another embodiment the field arrangement comprises a single field lens and a plane mirror located at the secondary image plane.

In a further embodiment the field arrangement comprises a field mirror located at the secondary image plane. In one embodiment the diffractive element is a transmissive element.

In one embodiment the optical instrument is an on-axis instrument.

In another embodiment the optical instrument is an off-axis instrument.

In one embodiment the optical instrument is a microscope.

Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which:

Figure 1 illustrates an optical instrument in accordance with a first embodiment of the present invention;

Figure 2(a) illustrates a fragmentary plan view of the diffractive element of the optical instrument of Figure 1;

Figure 2(b) illustrates a vertical sectional view (along section I-I) of the diffractive element of Figure 2(a);

Figure 3(a) illustrates a fragmentary plan view of an alternative diffractive element for the optical instrument of Figure 1;

Figure 3(b) illustrates a vertical sectional view (along section II-II) of the diffractive element of Figure 3(a);

Figure 3(c) illustrates a vertical sectional view (along section III-III) of the diffractive element of Figure 3(a);

Figure 4(a) illustrates a vertical sectional view (along section II-II) of one modification of the diffractive element of Figure 3(a); Figure 4(b) illustrates a vertical sectional view (along section IH-III) of the one modification of the diffractive element of Figure 3(a);

Figure 5(a) illustrates a fragmentary plan view of another alternative diffractive element for the optical instrument of Figure 1;

Figure 5(b) illustrates a vertical sectional view (along section IV-IV) of the diffractive element of Figure 5(a);

Figure 5(c) illustrates a vertical sectional view (along section V-V) of the diffractive element of Figure 5(a);

Figure 5(d) illustrates a vertical sectional view (along section VI-VI) of the diffractive element of Figure 5(a);

Figure 6(a) illustrates a vertical sectional view (along section IV-IV) of one modification of the diffractive element of Figure 5(a);

Figure 6(b) illustrates a vertical sectional view (along section V-V) of the one modification of the diffractive element of Figure 5(a);

Figure 6(c) illustrates a vertical sectional view (along section VI-VI) of the one modification of the diffractive element of Figure 5(a);

Figure 7 illustrates an optical instrument in accordance with a second embodiment of the present invention;

Figure 8 illustrates an optical instrument in accordance with a third embodiment of the present invention; and

Figure 9 illustrates an optical instrument in accordance with a fourth embodiment of the present invention. Figure 1 illustrates a microscope in accordance with a first embodiment of the present invention.

The microscope comprises an objective lens 3 for producing a primary aperture image in a primary image plane PIP of an object at an object plane OP.

The microscope further comprises a diffractive element 5, in this embodiment a transmissive element, which is located at the primary image plane PIP and is effective to produce an array of exit pupils AEP each corresponding to an exit pupil EP which would be produced in the absence of the diffractive element 5.

Through suitable design, the diffractive element 5 can be configured to provide that the exit pupils EP in the array of exit pupils AEP are spaced apart or in contact, and the configuration is chosen such that the array of exit pupils AEP appears to the eye of the observer in effect as a single, continuous enlarged exit pupil.

In this embodiment the diffractive element 5 comprises a surface 7 which has an array of diffractive sub-elements 9, each of which generates one of the exit pupils EP of the array of exit pupils AEP. The profile and form of the individual diffractive sub-elements 9 determines the comparative light energy within each of the individual pupil images.

In this embodiment, as illustrated in Figures 2(a) and (b), the diffractive sub-elements 9 comprise replications of a pattern of a plurality of separated areas 11 which are effective to produce diffractive interference of light passing therethrough and generate a plurality of exit pupils EP which are displaced relative to one another in the form of an array of exit pupils AEP, such as to be viewable as a single, continuous enlarged exit pupil. In this embodiment the plurality of separated areas 11 include areas 11 of different sizes and shape. Although in this embodiment the areas 11 are illustrated as being rectangular in shape, the areas 11 can take any shape.

In this embodiment the areas 11 are three-dimensional, in being projections which extend from the surface 7 of the diffractive element 5. In an alternative embodiment the areas 11 could comprise depressions.

In an alternative embodiment the areas 11 could be two-dimensional, in being features on the surface 7 of the diffractive element 5.

In this embodiment the replications of the patterns of areas 11 have a pitch of between 0.5 and 20 μm.

In this embodiment the areas 11 are formed by the patterning and development of an actinic photoresist. In one embodiment the pattern can be formed by holographic exposure of a laser wavefront interference pattern into a deposited actinic photoresist. In another embodiment the pattern could be formed by direct writing a Fourier transform pattern, using an electron beam, into actinic photoresist.

In an alternative embodiment, as illustrated in Figures 3(a) to (c), the diffractive sub-elements 9 are defined by first and second diffraction gratings 15, 17, here defined by first and second sets of parallel diffraction grating lines 19, 21 which extend perpendicularly to one another.

In this embodiment the diffraction grating lines 19, 21 have a pitch of between 0.5 and 20 μm.

In this embodiment the diffraction grating lines 19, 21 are formed by edges, as illustrated in Figures 3(b) and (c). In another embodiment the diffraction grating lines 19, 21 can be formed by grooves, as illustrated in Figures 4(a) and (b).

In a further alternative embodiment, as illustrated in Figures 5(a) to (d), the diffractive sub-elements 9 are defined by at least three, here first, second and third diffraction gratings 31, 33, 34, here defined by first, second and third sets of parallel diffraction grating lines 35, 37, 38 which extend equi- angularly to one another.

In this embodiment the diffraction grating lines 35, 37, 38 have a pitch of between 0.5 and 20 μm.

In this embodiment the diffraction grating lines 35, 37, 38 are formed by edges, as illustrated in Figures 5(b) to (d).

In another embodiment the diffraction grating lines 35, 37, 38 can be formed by grooves, as illustrated in Figures 6(a) to (c).

The microscope further comprises a projection lens 41 for projecting the primary aperture image at the diffractive element 5, as a plurality of aperture images corresponding to the exit pupils EP in the array of exit pupils AEP, to a secondary image plane SIP as a plurality of secondary aperture images, a field lens arrangement 42, in this embodiment comprising a pair of field lenses 43, 45, which is located at the secondary image plane SIP for relaying the secondary aperture images, and a viewing/eyepiece lens 47 for rendering the secondary aperture images as the array of exit pupils AEP to the eye of an observer.

With this configuration, the microscope provides a single, enlarged exit pupil having an effective size corresponding to the array of exit pupils AEP.

Figure 7 illustrates a microscope in accordance with a second embodiment of the present invention. The microscope of this embodiment is very similar to the microscope of the first-described embodiment, and thus, in order to avoid duplication of description, only the differences will be described in detail with like parts designating like reference signs.

The microscope of this embodiment differs from the above-described embodiment in being an off-axis microscope, in which the image is viewed laterally relative to the object.

In this embodiment the image projected by the projection lens 41 is reflected off-axis by a plane mirror 51 downstream thereof.

Figure 8 illustrates a microscope in accordance with a third embodiment of the present invention.

The microscope of this embodiment is quite similar to the microscope of the second-described embodiment, and thus, in order to avoid duplication of description, only the differences will be described in detail with like parts designating like reference signs.

The microscope of this embodiment differs from the second-described embodiment in that the downstream field lens 45 is replaced by a plane mirror 61 at which the secondary image is formed.

Figure 9 illustrates a microscope in accordance with a fourth embodiment of the present invention.

The microscope of this embodiment is quite similar to the microscope of the second-described embodiment, and thus, in order to avoid duplication of description, only the differences will be described in detail with like parts designating like reference signs. The microscope of this embodiment differs from the second-described embodiment in that the field lens arrangement is replaced by a concave, field mirror 71 at which the secondary image is formed.

Finally, it will be understood that the present invention has been described in its preferred embodiments and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims.

For example, in the described embodiments the diffractive element 5 is a transmissive element, but it will be understood that the diffractive element 5 could be provided as a reflective element.