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light projected at the specimen from a sideways, slanting angle to reveal features with higher contrast

OCC image of a live zebrafish showing fine muscle striations.  Image courtesy of Joe Fetcho, Ph.D., Cornell University


Transparent colorless specimens may appear featureless when viewed using transmitted brightfield illumination. If the light, however, strikes the specimen from only one direction at an oblique (slanting) angle, phase gradients are accentuated to reveal details with much greater contrast. Results are similar to the pseudo-relief images produced in DIC imaging. Oblique illumination is similar to darkfield illumination except that, instead of light being directed from all directions at oblique angles, light is projected from a single angle. Oblique illumination can be employed on most microscope systems but is most commonly exploited on stereomicroscopes and patch-clamp microscopes.


Oblique illumination can be used to reveal the morphology and internal structure of a variety of materials that appear transparent or nearly transparent under brightfield illumination and that cannot be treated with stains or other contrast methods. These may include living cells, tissues and entire organisms, processes such as IVF, chemical crystals, glass or transparent synthetic materials.


For stereomicroscopy, the most versatile and reliable way of providing oblique illumination is with a stereomicroscopes illumination stand. The Nikon Oblique Coherent Contrast (OCC) diascopic illumination stand contains both a high and low numerical aperture condenser enabling utilization of the entire stereomicroscope objective magnification (generally 0.5 x to 2 x) and numerical aperture (0.07 to 0.21) range. This enables improved contrast while maintaining a high condenser numerical aperture capable of matching that displayed by apochromatic objectives. The angle of illumination can be varied from axial brightfield to highly oblique off-axis light by using a simple diaphragm control knob. The degree of oblique illumination can then be adjusted easily to suit the specimen and the required level of detail. Refractive index differences can be observed to 1/30 of the wavelength of light (λ30). Additionally, the fixed stage microscope (FN1) may be configured with oblique illumination, especially for patch-clamp applications and/or physiology related research.


Nikon's SMZ25 or SMZ18 stereomicroscope configured with a C-DSD Diascopic stand with the Oblique Coherent Contrast (OCC) Illumination system or Nikon's FN1 fixed stage microscope for physiology /patch clamp applications. The FN-1 enables easy switching between brightfield, DIC and oblique light illumination techniques by simply rotating the turret. The direction of the contrast in oblique illumination, in addition, can be freely adjusted by rotating the incident illumination through 360º.


Oblique Illumination
Introduction to oblique illumination
Oblique Coherent Contrast Illumination
Introduction and interactive tutorial
SMZ-1500 Oblique Coherent Contrast Illumination
Introduction and interactive tutorial

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