Phase-contrast microscopy
Phase-contrast microscopy is an optical microscopy technique that converts phase shifts in light passing through a transparent specimen to brightness changes in the image. Phase shifts themselves are invisible, but become visible when shown as brightness variations.
When light waves travel through a medium other than a vacuum, interaction with the medium causes the wave amplitude and phase to change in a manner dependent on properties of the medium. Changes in amplitude arise from the scattering and absorption of light, which is often wavelength-dependent and may give rise to colors. Photographic equipment and the human eye are only sensitive to amplitude variations. Without special arrangements, phase changes are therefore invisible. Yet, phase changes often convey important information.
Image:Brightfield [phase contrast cell image.jpg|thumb|left|The same cells imaged with traditional bright-field microscopy (left), and with phase-contrast microscopy (right)]
Phase-contrast microscopy is particularly important in biology.
It reveals many cellular structures that are invisible with a bright-field microscope, as exemplified in the figure.
These structures were made visible to earlier microscopists by staining, but this required additional preparation and death of the cells.
The phase-contrast microscope made it possible for biologists to study living cells and how they proliferate through cell division. It is one of the few methods available to quantify cellular structure and components without using fluorescence.
After its invention in the early 1930s, phase-contrast microscopy proved to be such an advancement in microscopy that its inventor Frits Zernike was awarded the Nobel Prize in Physics in 1953. The woman who manufactured this microscope, Caroline Bleeker, often remains uncredited.
Working principle
The basic principle to make phase changes visible in phase-contrast microscopy is to separate the illuminating light from the specimen-scattered light and to manipulate these differently.The ring-shaped illuminating light that passes the condenser annulus is focused on the specimen by the condenser. Some of the illuminating light is scattered by the specimen. The remaining light is unaffected by the specimen and forms the background light. When observing an unstained biological specimen, the scattered light is weak and typically phase-shifted by −90° relative to the background light. This leads to the foreground and background having nearly the same intensity, resulting in low image contrast.
In a phase-contrast microscope, image contrast is increased in two ways: by generating constructive interference between scattered and background light rays in regions of the field of view that contain the specimen, and by reducing the amount of background light that reaches the image plane. First, the background light is phase-shifted by −90° by passing it through a phase-shift ring, which eliminates the phase difference between the background and the scattered light rays.
When the light is then focused on the image plane, this phase shift causes background and scattered light rays originating from regions of the field of view that contain the sample to constructively interfere, resulting in an increase in the brightness of these areas compared to regions that do not contain the sample. Finally, the background is dimmed ~70-90% by a neutral density filter ring; this method maximizes the amount of scattered light generated by the illumination light, while minimizing the amount of illumination light that reaches the image plane. Some of the scattered light that illuminates the entire surface of the filter will be phase-shifted and dimmed by the rings, but to a much lesser extent than the background light, which only illuminates the phase-shift and neutral density filter rings.
The above describes negative phase contrast. In its positive form, the background light is instead phase-shifted by +90°. The background light will thus be 180° out of phase relative to the scattered light. The scattered light will then be subtracted from the background light to form an image with a darker foreground and a lighter background, as shown in the first figure.