Unstained and living biological specimens have little contrast with their surrounding medium, although small differences of refractive index (RI) do exist in their structures. Phase contrast overcomes these, problems by using controlled illumination with the full aperture of the condenser and therefore improving resolution. The higher the RI of a structure, the darker it will appear against a light background, i.e. with more contrast. To achieve phase contrast, a microscope requires modification of the objectives and condenser, the specimen to retard light by between 1 ⁄8 and 1 ⁄4 of the light wavelength (λ) and an intense Köhler illumination light.






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