dark-field microscope
Meaning of dark-field microscope: Turns the usual picture inside out by throwing away the direct light, so unstained living organisms glow against black.
Definition of dark-field microscope
A dark-field microscope is a light microscope fitted with a stop or special condenser that keeps the direct beam out of the objective lens. Only light scattered sideways by the specimen reaches the eye, so objects appear bright against a black background. The arrangement reveals living, unstained structures too thin or too transparent to see by ordinary transmitted light.
How does the background go black?
An ordinary light microscope sends light straight through the specimen and into the lens, so the background is bright and objects show up as shadows. Dark-field inserts a stop, a small opaque disc, into the path below the stage. The disc blocks the central rays and leaves a hollow cone of light that strikes the specimen from angles too steep to enter the objective. With nothing travelling straight in, the field goes black. Anything in the specimen that bends or scatters light redirects a little of that steep cone upward into the lens, and only the scattered portion forms the image. It is the same effect that makes dust visible in a sunbeam crossing a dark room.
What is it used to see?
Living things that staining would kill and plain light would not reveal. The long-standing medical use is showing the corkscrew bacterium of syphilis in fluid taken from an ulcer, an organism too slender for routine stains to display. The method also shows other spiral bacteria, the darting movement of organisms in a fresh sample, and crystals in urine or joint fluid, which blaze because they scatter light so strongly. One limitation deserves naming: the image renders outline and motion but almost no internal detail, and dust, bubbles and scratches scatter light exactly as a specimen does. Claims that live blood viewed this way reveals hidden disease rest on reading those artifacts as findings.
How does it compare with other instruments?
| Instrument | Source of contrast | Suited to |
|---|---|---|
| Bright-field | Light absorbed by stains | Fixed, stained tissue and cells |
| Dark-field | Scattered light only | Live, unstained, very thin objects |
| Phase contrast | Shifts in the light waves | Live cells with internal detail |
| Electron beam | Scattered electrons | Viruses and structures inside cells |
Only the last of these escapes the limit set by the wavelength of light itself. An electron microscope resolves far finer structure, but its specimen must be fixed, dried and held in vacuum, so nothing examined in it is still alive.
Also known as
Used in a sentence
Dark-field microscopy of the lesion exudate was requested.
Good to know
The image is formed entirely from scattered light, so it shows outline and movement rather than internal structure, and dust, bubbles and scratches scatter light in the same way a specimen does.
Word origin
Where “dark-field microscope” comes from
Microscope comes from Greek mikros, small, and skopein, to look at.