METHOD AND APPARATUS FOR FOCUSSING IMAGES

Abstract: Images are focussed with an optical system which includes at least first and second apertures (3, 103) which are provided substantially symmetrically about an optical axis (5, 105) of the system. The apertures each provide separate sub-images which are used to determine the focus condition. The separate sub-images are analysed sequentially in a manner such that the sub-images are superimposed to form a stable image when the image is in focus. The optical system is then adjusted to generate such a stable image formed by the sequential sub-images.

Description: METHOD AND APPARATUS FOR FOCUSSING IMAGES This invention relates to a method and apparatus for focussing images. It can often be difficult in optical systems to determine accurately whether or not an image is in focus. This is especially the case where the image is being focussed through a relatively small aperture which inherently gives rise to a substantial depth of field. It is therefore an object of the present invention to provide a method and apparatus for focussing images which permits accurate identification of whether or not an image is in focus. According to one aspect of the present invention there is provided a method for focussing images, which method comprises the steps of: providing an optical system including at least first and second apertures, the apertures being provided substantially symmetrically about an optical axis of the system so as to provide separate sub-images formed by the at least first and second apertures; - 2 - sequentially analysing the separate sub-images in a manner such that the sub-images are superimposed to form a stable image when the image is in focus; and adjusting the optical system to generate a stable image formed by the sequential sub-images. According to a second aspect of the present invention there is provided an apparatus for focussing images, which apparatus comprises an optical system including: at least first and second apertures, the apertures being positioned substantially symmetrically about an optical axis of the system so as to provide separate sub-images formed by the at least first and second apertures; means for analysing the separate sub-images sequentially in a manner such that the sub-images are superimposed to form a stable image when the image is in focus; and means for adjusting the optical system so as to permit focussing of an image. The separate sub-images may be displayed for manual analysis and/or may be analysed by computer. The separate sub-images may be analysed sequentially by opening and closing the apertures in sequence. - 3 - Alternatively, the separate sub-images may be generated simultaneously and may be analysed sequentially in the same space such that the sub-images are superimposed to form a stable image when the image is in focus. The image may be focussed for immediate viewing by a person or may be generated by an imaging means, such as a CCD device, for display. The sub-images may be displayed sequentially at a rate in the range of about five to fifteen or more sub-images per second, preferably either in the range of about five to ten sub-images per second of in the range of about ten to fifteen sub-images per second. The optical system may be adjusted, for example, by adjusting the position from which the image is viewed. For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of one embodiment of an image focussing apparatus according to the present invention, the image being in focus; - 4 - Figure 2 is a diagrammatic illustration corresponding to Figure 1, with the image being focussed beyond an imaging means ; Figure 3 is a diagrammatic illustration corresponding to Figure 1, with the image being focussed before an imaging means ; Figure 4 is a diagrammatic illustration of another embodiment of an image focussing apparatus according to the present invention, the image being in focus; Figure 5 is a diagrammatic illustration corresponding to Figure 4, with the image being focussed beyond an imaging means; Figure 6 is a diagrammatic illustration corresponding to Figure 4 , with the image being focussed before an imaging means ; Figure 7 shows the image focussing apparatus of Figures 4 to 6 incorporating a computer for processing captured images ; and Figure 8 illustrates one manner in which focussing of the image can be adjusted. - 5 - The image focussing apparatus shown in Figures 1, 2 and 3 includes an opaque plate 1 which is provided with two apertures 3 which are positioned symmetrically to either side of an optical axis 5. The plate 1 is provided with shutter means 7 which is adapted to cover one or the other of the apertures 3 alternately. Thus, the visual effect of the shutter and aperture arrangement is that the shutter oscillates between the two apertures. If desired more than two apertures 3 can be provided positioned substantially symmetrically about the optical axis 5, the shutters 7 being opened and closed in sequence. In a focussed optical system as illustrated in Figure 1, rays forming an image converge to meet at the focal plane which coincides with an imaging means 9 , for example a CCD device employed in a video camera or the like. When the imaging means 9 does not coincide with the focal plane 11 of the optical system, that is when the imaging means is axially offset relative to the focal plane, the rays no longer meet at the plane of the imaging means and the resulting image is blurred. This can be seen from Figure 2 where the focal plane 11 is beyond the plane of the imaging means 9 and from Figure 3 where the focal plane 11 is before the plane of the imaging means 9. - 6 - Where the aperture is small, the depth of field is relatively large and blurring of the image is not readily apparent until the offset between the focal plane 11 and the plane of the imaging means 9 is relatively substantial. The lack of apparent blurring renders focussing of the image by examination relatively difficult. Nevertheless, each aperture 3 gives rise to separate sub- images and the sub-images from the two apertures will only coincide at the focal plane 11. When the imaging means 9 is offset from the focal plane, two (slightly blurred) sub- images are formed which are offset laterally relative to each other in the plane of the imaging means 9. When the shutter means 7 is operated to cover one or other of the apertures 3 alternately, for example the shutter means operating at a rate of about 10 to 15 cycles per second, first one of the laterally offset sub-images is formed and then the other. Thus, an out of focus image oscillates laterally, the image appearing to jump from side to side, with a period corresponding to the cycle time of the shutter means, while a focussed image remains stationary. It has been found that oscillation of an out of focus image provides a particularly sensitive method of analysing the image for focussing such an image. - 7 - Focussing of the image can be accomplished in any conventional manner, preferably by adjusting the position of the imaging means 9 along the optical axis, that is by adjusting the position from which the image is viewed. This is accomplished, for example as illustrated in Figure 8, by mounting the imaging means on a rack assembly 11, the rack being driven by a pinion 13 on which there is provided an adjusting knob 15 for manual operation and/or which is connected to a stepper motor for computer control . As an alternative (not illustrated) , the lens and plate 1 can be moved along the optical axis, with or without the imaging means 9. Nevertheless, it has been found focussing by moving the imaging means 9 alone to be most satisfactory because the change in magnification is least. The shutter means 7 may comprise any suitable mechanism such as those described in GB-A-2 283 828. That is, the shutter means 7 could comprise one or more mechanical shutters operable to close the apertures 3 in sequence. Alternatively, the shutter means 7 could comprise a pair of polarising filters for each aperture 3 and arranged such that the planes of polarisation are mutually perpendicular. As a further alternative, the shutter means 7 could comprise a liquid crystal shutter arrangement incorporating two liquid crystal shutters controllable to allow light to pass through each shutter in turn. - 8 - The shutters need not be provided in a plate of any substantial dimensions, the need being only to provide a means for defining apertures of the required size. The image focussing apparatus can be used, for example with a wide range of optical instruments and apparatus including ophthalmic apparatus and instruments and microscopes, especially electron microscopes. The image focussing apparatus described in relation to Figures 1 to 3 is relatively simple to construct and operate. However, it suffers the disadvantage that the two sub-images are analysed or displayed alternately and it is desirable to eliminate the possibility of the image changing, for example as a result of a subject moving, between alternate sub-images. The image focussing apparatus shown in Figures 4, 5 and 6 enables two sub-images to be analysed or displayed simultaneously. Instead of occupying the same space on the imaging plane as in the embodiment of Figures 1 to 3 , the sub-images are optically directed to different areas of the imaging plane thus eliminating the need for a shutter. The image focussing apparatus shown in Figures 4, 5 and 6 includes an opaque plate 101 which is provided with two apertures 103 which are positioned symmetrically to either side of an optical axis 105. - 9 - In a focussed optical system as illustrated in Figure 4, rays forming two sub-images converge to meet at the focal plane which coincides with an imaging means 107, for example a CCD device employed in a video camera or the like. The two sub-images falling on the CCD device can be processed by a computer or the like to display two focussed sub-images in registry with each other. When the imaging means 107 does not coincide with the focal plane 109 of the optical system, that is when the imaging means is axially offset relative to the focal plane, the rays no longer meet at the plane of the imaging means and the resulting sub-images are blurred. This can be seen from Figure 5 where the focal plane 109 is beyond the plane of the imaging means 107 and the sub-images are further apart at the imaging means 107 than in Figure 4 and from Figure 6 where the focal plane 109 is before the plane of the imaging means 107 and the sub-images are closer together at the imaging means 107 than in Figure 4. As with the embodiment of Figures 1 to 3 , where the aperture is small, the depth of field is relatively large and blurring of the sub-images is not readily apparent until the offset between the focal plane 109 and the plane of the imaging means 107 is relatively substantial. Nevertheless, each aperture 103 gives rise to separate sub- images and the images from the two apertures will only - 10 - coincide at the focal plane 109 otherwise they will be laterally offset relative to each other. That is, when the imaging means 107 is offset from the focal plane, two (slightly blurred) sub-images are formed which are offset relative to each other in the plane of the imaging means 107. In this case the two sub-images are gathered simultaneously and can be analysed alternately by computer or displayed alternately on a display such as a computer screen, albeit without the need for a shutter. This has the advantage of eliminating subject movement between the two sub-images. The overall image is in focus when the two sub-images correspond each other without any lateral movement between the sub-images when they are displayed alternately on a computer screen. As with the embodiment of Figure 1, we have found it is acceptable to alternate the sub-images at a rate of about 5 to 10 (preferably about 6) times per second and an out-of-focus image oscillates laterally, the image appearing to jump from side to side, with a period corresponding to the rate of change of the sub-images, while a focussed image remains stationary. However, it would also be possible to alternate the sub-images at the video frame rate (25 to 30 times per second) . As with the embodiment of Figure 1 to 3 , focussing of the image can be accomplished in any conventional manner, such - li as that shown in Figure 8. Of course, fresh sub-images are gathered as the image is focussed. However, as an alternative in the case where focussing is accomplished by means of a stepper motor as described above in relation to Figure 8, the sub-images can be processed by computer to determine whether or not they are in focus and the focussing can also be controlled by the computer and adjusted until the sub-images correspond each other without any lateral movement therebetween. However, supplemental manual control would enable manual override of the computer if required. Figure 7 additionally illustrates camera means 111 incorporating the imaging means 107 and a computer 113 for processing captured images and displaying such images alternately on a display 115.

Claims: - 12 - CLAIMS 1. A method for focussing images, which method comprises the steps of: providing an optical system including at least first and second apertures (3, 103), the apertures being provided substantially symmetrically about an optical axis (5, 105) of the system so as to provide separate sub-images formed by the at least first and second apertures; sequentially analysing the separate sub-images in a manner such that the sub-images are superimposed to form a stable image when the image is in focus; and adjusting the optical system to generate a stable image formed by the sequential sub-images. 2. A method according to claim 1, characterised in that the separate sub-images are displayed for manual analysis. 3. A method according to claim 1 or 2 , characterised in that the separate sub-images are analysed by computer. 4. A method according to claim 1, 2 or 3, characterised in that the separate sub-images are analysed - 13 - sequentially by opening and closing the apertures (3, 103) in sequence. 5. A method according to claim 1, 2 or 3, characterised in that separate sub-images are generated simultaneously and are analysed sequentially in the same space such that the sub-images are superimposed to form a stable image when the image is in focus. 6. A method according to any preceding claim, characterised in that the sub-images are generated sequentially at a rate in the range of about five to fifteen sub-images per second. 7. A method according to claim 6, characterised in that the sub-images are generated at a rate in the range of about ten to fifteen sub-images per second. 8. A method according to claim 6, characterised in that the sub-images are generated at a rate in the range of about five to ten sub-images per second. 9. A method according to any preceding claim, characterised in that the optical system is adjusted by adjusting the position from which the image is viewed. - 14 - 10. An apparatus for focussing images, which apparatus comprises an optical system including: at least first and second apertures (3, 103), the apertures being positioned substantially symmetrically about an optical axis (5, 105) of the system so as to provide separate sub-images formed by the at least first and second apertures; means for analysing the separate sub-images sequentially in a manner such that the sub-images are superimposed to form a stable image when the image is in focus ; and means for adjusting the optical system so as to permit focussing of an image. 11. An apparatus as claimed in claim 10, characterised in that display means (9) , (107) is provided for displaying the separate sub-images for manual analysis. 12. An apparatus as claimed in claim 10 or 11, characterised in that a computer (113, 115) is provided for analysing the separate sub-images. 13. An apparatus as claimed in claim 10, 11 or 12, characterised in that the apertures (3, 103) are - 15 - opened and closed in sequence to analyse the separate sub-images. 14. An apparatus as claimed in claim 10, 11 or 12 and including means for generating the separate sub-images simultaneously and for analysing the separate sub- images sequentially in the same space such that the sub-images are superimposed to form a stable image when the image is in focus. 15. An apparatus as claimed in any one of claims 10 to 14, characterised in that the sub-images are analysed sequentially at a rate in the range of about five to fifteen sub-images per second. 16. An apparatus as claimed in claim 15, characterised in that the sub-images are analysed at a rate in the range from about ten to fifteen sub-images per second. 17. An apparatus as claimed in claim 15, characterised in that the sub-images are analysed at a rate in the range from about five to ten sub-images per second. 18. An apparatus as claimed in any one of claims 10 to 17, characterised in that the optical system is adjusted by adjusting the position from which the image is viewed.

Inventors: DAWSON TIMOTHY RICHARD HOLBROO, DAWSON, TIMOTHY, RICHARD, HOLBROOK
Applicants: DAWSON TIMOTHY RICHARD HOLBROO, MARCHER ENTERPRISES LIMITED, DAWSON, TIMOTHY, RICHARD, HOLBROOK
Application year: 1999
Main classification: G02B7
IPC codes: G02B7, G03B13