In variable phase bright-field contrast, a bright-field image based on axial or concentric-peripheral light is optically superimposed with a phase-contrast image, so that typical details that are imminent in one or the other technique contribute to the resulting composite image. In particular, complex structured specimens consisting of high-density light absorbing details and additional low-density phase shifting components can be observed with improved clarity. As both partial images interfere with each other, fine details within thin specimens can be highlighted further by additional contrast effects based on interference. Haloing and shade-off are significantly reduced when compared with phase contrast carried out stand-alone. Our method is characterized by several technical means that are relevant for the high image quality that can be achieved: both illuminating light components associated with bright field and phase contrast are filtered at different colors and separated from each other so that they meet the specimen at different angles of incidence. The intensities of the phase-contrast- and bright-field-producing light can be selectively regulated so that the final image can be dominated by phase contrast or bright field, or be equalized. The condenser aperture diaphragm can be used for modulations of the image's appearance.

Variable phase dark-field contrast has been developed as an illumination technique in light microscopy, which promises significant improvements and a higher variability in imaging of several transparent specimens. In this method, a phase contrast image is optically superimposed on a dark-field image so that a partial image based on the principal zeroth-order maximum (phase contrast) interferes with an image that is based on the secondary maxima (dark field). The background brightness and character of the resulting image can be continuously modulated from a phase-contrast-dominated to a dark-field-dominated character. The condenser aperture diaphragm can be used for modulations of the image's appearance. Specimens can either be illuminated concentrically or obliquely (eccentrically) when parts of the illuminating light beams are covered and blocked. Moreover, a bright-field-like partial image can be added. In this way, the illumination can be optimally adjusted to the specific properties of the specimen. The techniques described can lead to improved visual information especially in biological specimens consisting of phase structures and additional light-absorbing or -reflecting components. Moreover, the specimen's three-dimensionality can be accentuated with improved clarity because the illuminating light beams associated with phase contrast and dark field run to the specimen at different angles of incidence.