Patent GB-1355472-A (GB)
Title: Arrangement for focussing
IPC: G02B21/24
Filed: 1972-01-18
Abstract: 1355472 Automatic focusing of microscope ERNST LEITZ GmbH 18 Jan 1972 [22 Jan 1971] 2371/72 Heading G1A In a microscope, auxiliary optical elements are provided so that invisible, e.g. infrared or ultra violet radiation, defining an image of a reference mark, is projected on to the object being viewed, this radiation being confined within one half of the cross-section of the object illuminating beam, and is reflected back from the object in the other half of said illuminating beam to be detected by a photo-electric arrangement whose output is used to control the position of the object so the latter is brought into focus. In the arrangement of Figs. 2, 3 and Fig. 1, (not shown), the object illuminating beam is provided from source 1 and passes via lens 2, beam-splitter 3, aperture diaphragm 4, field diaphragm 5, lens 6, beam-splitters 7 and 14' to objective 8, which focuses the light in a plane in which the object 9 is to be positioned. Viewing of the light reflected from the object is permitted by beam-splitter 7 as beam 8b infrared radiation is split out of the illuminating beam by 3 which however transmits visible light and is used by means of mirror 10 and lens 11 to illuminate a slit 12 in mask 12'. The infrared beam is collimated by lens 13 and directed by mirror on to one side of prism 16. The infrared beam 8c thus enters one side of illuminating beam 8a via mirror 14, (a further infrared lightsplitter) is reflected from the object and is directed to the detection optics (mirror 15, lens 17 etc) by the other side of prism 16. Depending on the deviation of the object from the focal plane of the objective 8, one or other of the photo-cells 18a, 18b divided at 18c receives more radiation and by their differential connection they control a servo motor to move the object into the objective focal plane and thereby centre the infrared beam on to the cells. In the arrangement of Fig. 4a, infrared radiation only from a lamp 19 is reflected towards the object by a plate 21 which also transmits visible light for illuminating the object. Plate 22 also allows visible light to be transmitted but only allows infrared through via slit 12' so that a mark shaped image is formed. In the arrangements of Figs. 5, 6 (not shown), a separate light source for forming the infrared beam is again used, but the mask having a slit to form a mark image, is in an auxiliary optical path and not included in the main illuminating path. Instead of using a pair of photo-cells as described above, a detector of circular form with two photo-sensitive sectors and two insensitive sectors is located behind a rotating, tilted, plane parallel transparent plate, Fig. 7 (not shown). In a further arrangement, Figs. 8a and Figs. 8b, 8c (not shown), again using an auxiliary light source 19, instead of a slit shaped mark, a half diaphragm 35 is used to define a mark. In this arrangement, the photo-electric arrangement is associated with the viewing beam, which is finally separated out by dichromatic splitter 37. The differential photo-electric arrangement may be finely adjustable in position by screw means, Fig. 9 (not shown). The automatic positioning servo-mechanism can optionally be switched off by the microscopist.