Transmitted Light Phase Contrast is an illumination technique which enhances the phase difference between the specimen and its surroundings. Specimens which have a refractive index similar to their surroundings can be invisible in Brightfield, but are well defined in Phase Contrast. Phase Contrast requires a special set of objectives with phase rings placed in the back focal plane of the objective. A matching phase ring is placed in the condenser. A Phase Turret condenser will usually have a Brightfield position as well as Phase positions to match each Phase objective. Phase Contrast is normally used to examine unstained biological specimens.