The basic principles of the phase contrast microscope
A phase contrast microscope is a special type of microscope that converts the light travelling difference (i.e. phase difference) produced when light passes through the details of a transparent specimen into a difference in light intensity.
When light passes through a relatively transparent specimen, there is no significant change in the wavelength (colour) or amplitude (brightness) of the light. As a result, the morphology and internal structure of unstained specimens (such as living cells) are often difficult to distinguish when viewed with an ordinary light microscope. However, due to differences in the refractive index and thickness of the various parts of the cell, there is a difference in the optical range of direct and diffracted light as it passes through such a specimen. As the light range increases or decreases, the phase of the speeded-up or lagging light waves changes (creating a phase difference). The phase difference of light can not be felt by the human eye, but the phase contrast microscope can make use of the interference phenomenon of light through its special devices - ring diaphragm and phase plate - to transform the phase difference of light into the difference in amplitude (light and darkness) that can be perceived by the human eye, thus making the original transparent objects show obvious differences between light and darkness and enhanced contrast, so that we can The contrast is enhanced, so that we can more clearly observe the living cells and some fine structures inside the cells which cannot be seen under the ordinary optical microscope and dark field microscope or cannot be seen clearly.
The imaging principle of phase contrast microscope: when inspecting the mirror, the light source can only pass through the transparent ring of annular diaphragm, and then converge into a beam of light after the concentrator, when this beam of light passes through the object being inspected, due to the difference in the range of the various parts of the light, the light is deflected to varying degrees (diffraction). As the image formed by the transparent ring falls exactly on the focal plane after the objective lens and the conjugate surface of the phase plate overlap. As a result, direct light that is not deflected passes through the conjugate plane, while diffracted light that is deflected passes through the compensation plane. Because of the conjugate surface on the phase plate and compensate for the different nature of the surface, they will pass through these two parts of the light to produce a certain phase difference and intensity of the attenuation of the two groups of light and then after the convergence of the lens, and then back in the same light travelling, so that the direct light and diffracted light to produce light interference, change the phase difference for the difference in amplitude. In this way, in the phase contrast microscope inspection, the light through the colourless transparent body makes the human eye indistinguishable phase difference into the human eye can distinguish the amplitude difference (light and dark difference).
