Restoring image quality in the polarizing microscope: analysis of the Allen video-enhanced contrast method

Abstract

A long-standing problem in polarized light microscopy has been to achieve simultaneously high spatial resolution and high contrast. Rotation of the plane of polarization of light at oblique interfaces between the polarizer and the analyzer results in a pupil function that resembles a dark cross rather than a uniformly dark field. Dramatic improvements were obtained recently by operating the microscope at a large (bias) compensator setting with simultaneous video contrast enhancement [Allen video-enhanced contrast (AVEC)]. A physical-optics model is presented for AVEC, and the fundamental limits of the method are investigated. Expressions for the pupil and point-spread functions are derived; phase errors in the pupil are analyzed with Zernike polynomial expansions. It is found that, for compensator settings beyond one tenth of a wavelength, the pupil and point-spread functions closely approach the ideal diffraction-limited characteristics. Experiments conducted with the microscope confirm the theoretical predictions and provide practical guidance for optimal use of the method.