Brightfield and Phase Contrast
The default mode, its one big failure, and the fix.
Brightfield
Brightfield is the standard mode on every teaching microscope. Light passes through the specimen from below and the specimen is seen as a dark object on a bright background. Contrast comes entirely from absorption: parts that absorb light look dark.
It is simple, cheap and correct for anything stained or naturally pigmented — stained tissue sections, blood smears, pigmented algae.
Where brightfield fails
Most living cells are nearly transparent. They absorb almost no visible light, so in brightfield they have almost no contrast and are close to invisible. Staining fixes this and usually kills the specimen, which is no help if the point is to watch living cells.
Phase contrast
Transparent specimens do not absorb light, but they do slow it. Light passing through a cell emerges shifted in phase relative to light passing beside it. The eye cannot see phase, only intensity.
Phase contrast, developed by Frits Zernike, converts phase into intensity. An annular stop in the condenser produces a hollow cone of illumination; a matching phase plate in the objective shifts the undiffracted light by a further quarter wavelength and attenuates it. Diffracted and undiffracted light then interfere, and the phase difference becomes a brightness difference. Living unstained cells appear with strong dark-on-light detail.
Choosing between them
| Brightfield | Phase contrast | |
|---|---|---|
| Contrast from | Absorption | Phase shift |
| Best for | Stained, fixed specimens | Living, unstained cells |
| Extra hardware | None | Annulus plus phase objectives |
| Known artefact | — | Bright halo around edges |
The halo is intrinsic to the method, not a fault. It can be mistaken for a real structure such as a capsule or membrane, so it is worth pointing out before students draw what they see.