Principle of Dark Field Microscope

PRINCIPLE:

The compound microscope may be fitted with a dark field condenser that has a numerical aperture (resolving power) greater than the objective. 

The condenser also contains a dark-field stop. The compound microscope now becomes a dark-field microscope. 

Light passing through the specimen is diffracted and enters the objective lens, whereas undiffracted light does not, resulting in a bright image against a dark background. Objects are seen as light objects against a dark background.

PARTS OF DARK FIELD MICROSCOPE:

These are three requisites for adopting an ordinary microscope for Dark ground illumination:

1. Dark ground condenser

2. Suitable light intensity lamp

3. Funnel stop (to reduce numerical aperture of objective)

USES:

1. To visualize Trepenoma pallidum in exudates.

2. To visualize Leptospira species in blood & urine.

3. To visualize Spirillum minus in blood.

4. To detect motility of other bacteria.

5.To identify sheathed microfilaria & other parasites.