Phase Contrast Microscopes
Phase Contrast Microscopy is an optical contrast technique that allows the user to see specimens that are almost completely transparent. This technique is very often used to observe living cells in culture (inverted tissue culture microscope) and is also routinely used to observe any unstained specimens including non-organic specimens that include things like fibers and glass.
A Phase Microscope differs from a Brightfield Microscope in two distinct ways. First, a phase annulus is inserted in the light path at the front focal plane of the sub stage condenser. Second, a specialized objective is used that contains a corresponding phase ring inserted at the rear focal plane of the objective.
Light from the light source passes through the phase annulus in the condenser and then focuses that light to fully illuminate the specimen plane. Light that passes through the specimen is diffracted and shifted in phase while light that passes through the specimen plane without passing through the specimen remains undeviated.
As the undeviated or background light passes through the phase ring at the rear focal plane of the objective, it is phase shifted to more closely match the light that passed through the specimen. This results in significant changes in amplitude between the diffracted light and the undeviated light which causes significant changes in contrast allowing the specimen to be viewed.