Patent GB-885224-A (GB)
Title: Improvements in or relating to optical microscopes
IPC: G02B21/14
Filed: 1958-01-30
Abstract: 885,224. Microscopes; interferometers. BAKER INSTRUMENTS Ltd., C., and SMITH, F. H. April 28, 1959 [Jan. 30, 1958; March 10, 1958], Nos. 3191/58 and 7668/58. Class 97(1). In a phase-contrast microscope, an interferometer is placed between the object position and the final image position and is adjustable or has adjustable means associated with it to vary the relative intensity and/or the phase relationship between direct light and light diffracted by an object at the object position so as to enhance the contrast of detail in the image. In the form shown in Fig. 1 light from an annular aperture 12 illuminates an object at the object position O-O<SP>1</SP> which is imaged by an object lens 14. The image forming beam is reflected by prism 16 on to a concave mirror 17 disposed close to the position of the real image of the object formed by the lens 14 and slightly tilted so that the beam passes to a reflecting prism 18 and thence through a secondary objective lens 19 to a parallel sided beam dividing plate 20 whereby coherent beams 11g and 11h are formed and are reflected back by a mirror 21 to the plate 20 for recombination in a beam 11j which passes to an eyepiece (not shown). Mirror 17 forms an image of the annulus 12 at an opaque annulus 24 on a graticule plate 23 so as to remove the direct light component from the beam 11h. The relative path length of the diffracted component in beam 11h and the direct component in beam 11g is varied by tilting a twin glass plate assembly 22 comprising elements 22a, 22b one of which is slightly thicker or not quite parallel to the other. The diffracted light component in the beam 11g may be removed by interposing, at the real image in that beam of the aperture 12, a diaphragm having an opening which fits the real image of the aperture 12. The back surface 20a of plate 20 is metallised for high reflectivity and the two beam splitting and combining regions of the front surface 20b are coated with high-reflecting dielectric films. In one modification of the microscope shown, plate 23 carries an opaque mask having an opening which fits the image of the aperture 12 in the beam 11 h so as to remove from the beam 11 h the diffracted light component. The optical path length difference between the coherent beams may alternatively be varied by making the plate 20 slightly wedge shaped and mounting plates 20, 21 and 23 so that they may be rotated as a whole about an axis normal to the surfaces 20 and 21. Further, the front surface 20b of plate 20 may be coated with a system of dielectric layers which polarize the reflected and transmitted portions of the incident beam in mutually perpendicular planes, the phase and/or intensity relationship of the combined beams being adjusted by a birefringent compensator. A second form of microscope is shown in Fig. 2 wherein a diffraction plate 32 of alternating opaque and transparent sectors is imaged upon a reflecting graticule 56 of identical pattern by lenses 35, 38 and 45 the transparent sectors of plate 32 being imaged on the reflecting sectors of graticule 56 by direct light from the object at O-O<SP>1</SP>. The direct and diffracted light from this object is plane polarized at 39 and then passed through a prism cube 41 having a cemented interface 44 containing a multi-layer polarizing filter. The light then passes to an interferometer 46, comprising a birefringent compensator 51, 52, of which element 51 is adjustable, and a prism cube containing a further multi-layer polarizing filter 49. This filter splits the beam into components, one of which 57 is reflected back by mirror surfaces 54 and the other of which 58 passes to the face 55 of the prism cube. The face 55 carries the reflecting graticule 56 and a transparent dielectric layer 61 covered, as shown in Fig. 3, by a reflecting film 59 which is spaced from the graticule so as to introduce a phase shift equal to one or an odd integral number of half wave lengths between the diffracted light in split beam 58 which is reflected by the film 59 and the direct light in split beam 58 which is reflected by the graticule 56. The split beams 57, 58 are recombined at 49 and passed through an analyser 69 crossed with respect of polarizer 39 to an eyepiece for viewing the interference figure, the contrast in which can be varied by adjustment of the birefringent compensating element 51. An alternative form of interferometer for use with the arrangement of Fig. 2, comprises a prism with oblique faces having plane parallel plates cemented thereto with a multi-layer polarizing filter at each interface. This results in the exit from the prism of two beams which are crosspolarized. One such beam passes through an adjustable tapered wedge before both beams are reflected back as in Fig. 2. The diffraction plate 32 may have all its sectors transparent, in which case alternate sectors are of different thickness or material or otherwise constructed to impart a phase difference between the light which passes through the alternate sectors respectively. The microscope may be used with vertical illumination.