Darkfield Illumination is a transmitted light technique that uses oblique light to illuminate the sample. Background light (non-sample light) is not collected by the objective, which results in a dark background. Light that interacts with the sample (sample light) is scattered (refracted, reflected, and/or diffracted). Some of this scattered light is collected by the objective and is seen as light spots or areas on a dark background. Contrast is therefore generated and the sample visualized.
Darkfield illumination is provided to the sample by a specialized "DF" condenser. The simplest DF condenser has a central Stop with an illuminating ring (Annulus) larger than the angular aperture. With this condenser, an opaque circle obscures the central portion of the light path. This allows only light in a outer 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 background 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. The gray cone (above) represents the scattered light collected 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.