Darkfield Illumination

DAVID MALIN
Anglo-Australian Observatory, RMIT University

Darkfield illumination is usually encountered when an image is formed from light entering the subject obliquely and off the optical axis. The most common application of darkfield
illumination is found in optical microscopy, but the technique is widely used or recognized for creating contrast in low-contrast situations. In a perfectly clear, transparent medium the field would remain dark, but in the presence of a refractive index boundary or a scattering surface or particles, light is deflected into the optical axis and is observable, such as dust in a sunbeam. The key component is a method for preventing on-axis light from the main source of illumination flooding the focal plane. The scattering of light by normally unresolved particles or otherwise invisible refractive index boundaries is a key feature of the technique.

Though this sounds rather elaborate, a simple example is a home aquarium set against a dark background and lit from the top. The technique is useful also for macro specimen photography—especially revealing the subtle structures of cataracts in eye lenses, jellyfish, amoeba etc.—using ring-illumination. On a larger scale the zodiacal light (from dust in the solar system) and the detection of the backscattered light in the shadow of the planet that led to the discovery of the rings of Jupiter are also good examples of darkfield illumination at work.