Darkfield Illumination is a transmitted light technique that uses oblique light to illuminate the sample. Light that does not impinge on the sample is not collected by the objective and results in a dark background. Light that interacts with the sample is scattered (refracted, reflected, and/or diffracted) and is "bent" toward the objective collection angle. This light is collected by the objective and is seen as light spots or areas (resulting from scattered light) on a dark background. Contrast is therefore generated and the sample visualized.
Darkfield illumination is provided to the sample by a specialized condenser. The simplest DF condenser has a Stop, or Annulus illuminating ring (A). Here, an opaque circle obscures the central portion of the condenser light path. This allows only light in a ring to illuminate the sample. The diameter of the central stop, and thus illuminating annulus, is such that the angle of light is greater than the collecting angle of the objective. Thus without a sample, no light is collected by the objective. This kind of DF stop is useful only for low magnification objectives (<20x).

For higher magnification objectives, modifications of the Annular Stop are: B: Immersion paraboloid; C: immersion double mirror concentric; D: cardioid concentric. Gray cone represents the light reflected and refracted from the specimen and collected by the objective. Hatched areas represent glass. Light blocking stops (s) limit light transmission to a hollow cone. i: Immersion oil.; r: reflecting surfaces. (Ruzin,1999).

The diameter of the central stop is dependent on the NA of the objective.

Variations of DF illumination are Oblique Illumination and Rheinberg Illumination.