Patent GB-710490-A (GB)
Title: Improvements in or relating to microscopy
IPC: G02B21/14
Filed: 1950-06-20
Abstract: 710,490. Interference microscope. SMITH, F. H., and BAKER, of HOLBORN, Ltd., C. June 20, 1951 [June 20, 1950], No. 15440/50. Addition to 639,014. Class 97(1) In an interference microscope of the kind disclosed in the parent Specification the object-illuminating beam is divided by a diaphragm having one or more light-transmitting regions and positioned in the path of said beam before it reaches the object nearer to a position conjugate with the back focal surface of the microscope objective than to the objection position so that the object is illuminated by light which passes directly through the light-transmitting region or regions of the diaphragm and also by light diffracted by said region or regions and coherent with the direct light, or by coherent light diffracted by different amounts by said region or regions, and the direct and diffracted light is re-combined by a double-refracting member situated at a position optically after the object, a polarizer for polarizing the light before it reaches the double-refracting member, and an analyser disposed at a position optically after the double-refracting member so that interference occurs between the differently polarized pairs of rays which emerge from the double-refracting member. The double-refracting member is shaped and positioned so that the said interference results in an interference figure comprising several fringes which is substantially conjugate with the diaphragm and within, or conjugate with, the exit pupil of the microscope, and the light-transmitting region or regions of the diaphragm are shaped, positioned and dimensioned so that it or they is or are imaged on the fringes and so that when an object is placed at the object position light rays from pairs of portions of the object which are illuminated by coherent light are combined by the double-refracting member, in conjunction with the polarizer and analyser, to produce visible interference effects in the field of view and corresponding to any phase differences introduced by the object. The microscope shown comprises a diaphragm 15 having annular openings 18 whereby the light from a mirror 13 is diffracted before passing through a condenser 12 which may be achromatic, this diaphragm being positioned in the back focal surface of the condenser. The object is placed at o-o<SP>1</SP> and the diaphragm 12 is imaged by the condenser and the objective 11 near the back focal surface 22 of the later. Lenses 23, 24 relay this image to the interface of lens components 27, 28 and, together with the objective 11, form an image of the object o-o<SP>1</SP> in the microscope eyepiece or, if the microscope is of the projection type, on a screen. In front of the lenses 27, 28 is a plane polarizer 25 and behind them are disposed lenses 31, 32 which correct for spherical aberration, a quarter-wave plate 33 and an analyser 34. The lens 28 is formed of birefringent material and doubly refracts rays, such as 19 and 21, diffracted by a diaphragm opening so that the ordinaryray 35 derived from ray 21 is superposed on the extraordinary ray 36 derived from ray 19. Analyser 34 is rotated to bring the combined rays into parallel plane polarized relationship so that they interfere and the quarter wave plate may be adjusted to vary the phase relationship of the superposed rays 35, 36. The central line of each diaphragm opening is arranged so that its image registers with a line of uniform phase in the interference figure and each opening does not exceed one quarter the width of the intervening opaque regions of the diaphragm with the result that the image of each diaphragm opening registers with a part of the interference figure which is produced by interfering rays having values of phase difference which do not differ by more than one-eighth of a wavelength from the single value of phase difference along said line of uniform phase in the interference figure. The diaphragm is preferably produced after the rest of the apparatus has been set up by passing light in the reverse direction through the microscope and photographing the image of the interference figure formed in the back focal plane of the condenser 12. A master position is prepared and a negative taken from the master is cemented between glass plates to form the diaphragm. Alternatively, the diaphragm may be formed by mapping the positions of the interference minima using a transparent squared graticule and graph paper and using the map as an aid in removing portions of an opaque coating on a transparent plate. In another form, the lenses 23, 24, 27, 28, 31 and 32 are omitted and the diffracted rays are combined by a double-refracting plate as of calcite cut parallel to its optic axis disposed between the object o-o<SP>1</SP> and the objective 11, the interference figure being imaged by the objective at its back focal surface. In this case, the front component of the objective may be mounted on the doublerefracting plate and, if it is desired to use a thick double-refracting plate with a high power objective, long working-distances attachments as described in Specification 651,035 may be used. The doublerefracting member may be incorporated in the objective lens train or located at the back focal surface of the objective. A further form is described in which the rays 19, 21 are passed through a collimating lens having one focal plane in common with the back focal plane of the objective, the rays being combined by a double-refracting plate having its surfaces cut parallel or oblique to its optic axis and disposed normally to the microscope axis. The apparatus may also be used for examining reflecting objects, for example, as described in Specification 710,495. Thus, the diaphragm may be disposed on an optical axis normal or oblique to the microscope axis and intersecting the latter between the objective and lens 23, a thin glass plate being disposed at the intersection of the axes to direct the light through the objective on to the object o-o<SP>1</SP>. The glass plate may be set at the polarizing angle and polarizer 25 omitted. A set of different diaphragms for use with a corresponding set of different objectives may be mounted on a slidable or rotatable member provided with a centering adjustment whereby each diaphragm can be registered optically with the interference figure. Specification 643,048 also is referred to.