“These are compound microscopes with a set of special condensers and objectives that allow you to shift what wavelength the beam of light hits your eye or camera, which makes mostly translucent samples now visible. Commonly used to inspect urine crystals, saliva samples, and water for contaminants (such as sewage treatment plants).”
What is phase contrast used for?
Phase contrast is used for any kind of samples where light is passed through absolutely, and little to no contrast is achieved between the sample and the light background of a brightfield microscope. It’s also used frequently in situations where samples are alive and cannot be stained or dyed, as adding such chemicals to a cell or culture will kill the sample quickly (thus making live viewing impossible).
How does phase contrast work?
Phase contrast is achieved by using specialized objectives and condensers, which are paired up by magnification level, which serve to alter the part of the wavelength of light where it hits the eye. This altered path is what takes a translucent sample and gives it depth, showing in greater detail how the light is altered when travelling through the sample at its various thicknesses and material.
The light is emitted from the light source, where it hits the first condenser. This condenser has an annual ring in it that bends the light the first time, then passes it up through the sample. In the objective, there is another annular ring, which assists in the final modification of the path of light, before sending it up to the prism, and then the eyepiece, as a normal compound microscope.
To ensure that the phase of light is adjusted properly, included in phase contrast kits is a special eyepiece called a centering telescope, or a CT lens. This is inserted in place of the eyepiece and telescopes out in order to visualize the position of the annular ring in the objective and in the condenser. With this, the user can make adjustments to the condenser position (assuming the objective is precentered at the factory) to line them up, as they have to be in perfect position for phase contrast to be achieved. Once finished, the CT lens is removed from the eyepiece and the regular (usually 10x) eyepiece is reinserted. This process is repeated for each magnification change, as the condenser and objectives are marked in pairs–one must use both the 10x objective and 10x condenser, and so on.
Here is a diagram visualizing the path of light in a phase contrast setup:
What does phase contrast look like when achieved in a microscope?
Phase contrast appears mostly gray in nature, with degrees of contrast in the sample shown by darker shades. Here is a sample image for clarification:
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