| 摘要 |
1,178,726. Optical apparatus. INTERNATIONAL BUSINESS MACHINES CORP. April 20, 1967 [April 26, 1966], No. 18167/67. Heading G2J. A phase plate 28, Fig.2, for use in phase contrast microscopy or Schlieren systems comprises two diffraction gratings 29, 30, lying in the same plane and having equal grating constants with the slits of one grating displaced relative to those of the other by a distance differing from the common grating constant. As shown applied to a phase contrast microscope, rays from a point source 21 are collimated by lens 22, pass through object 23 and form an image of point source 21 in the Fraunhofer plane 25 by virtue of lens 24. Rays diffracted by points in object 23 are focused in image plane 27 by lenses 24, 26, and are collimated when impinging on phase plate 28 positioned in the plane 25. It is shown in the Specification that phase shift occurs in the images formed at the various orders of diffraction which is independent of wavelength. However the angle of the diffracted ray is wavelength dependent, and this causes chromatic aberration unless the source is monochromatic. To avoid this and allow polychromatic sources to be used, an additional grating (84, Fig.8, not shown) may be provided and a mask (80) positioned in the intermediate image plane (27) to select the particular order of diffracted image which required (e.g. -1 order). The final image is then free of chromatic aberration. The additional grating (80) has the same effective grating constant as gratings 29, 30, i.e. if the collimating lenses (82, 86) associated therewith are of the same focal length as lenses 24, 26, the grating constant is the same but if the focal lengths are different the slit spacing is varied to maintain the same effective grating constant. If it is desired to control the relative amplitudes of the direct and object diffracted beams this may be done by varying the slit widths in the two diffraction gratings 29, 30, but retaining the same grating constant, i.e. the spacing between centres of the slits. As applied to a Schlieren system, Fig. 6, (not shown), the two gratings forming the phase plate are side by side instead of being one within the other as in plate 28. To eliminate chromatic aberration a third grating may be also provided, Fig. 9 (not shown). |
