Definition
Phase contrast microscopy is a special type of optical microscope that enables the observation of the fine structure of clear or translucent living cells, microorganisms, tissue sections, and other samples without staining by converting the phase difference created by light passing through the sample into an intelligible amplitude difference (i.e., chiaroscuro). This technology allows researchers to study the dynamic processes of biological samples in a state close to nature, and has a fundamental role in biology, medicine, materials science and other fields.
Principle
The core principle of phase contrast microscopy is based on the fluctuation of light. When light passes through a transparent sample, different areas inside the sample can cause phase delays in light waves due to small differences in thickness or refractive index, but this phase change cannot be directly recognized by the human eye. Phase contrast microscopy modulates light waves by introducing a ring diaphragm and a phase plate into the optical path. The ring diaphragm is located under the condenser and produces a hollow cone of illumination beam. The phase plate is located on the back focal plane of the objective and is designed to create an additional quarter-wavelength phase difference between direct light that passes directly through the sample without deflection (background light) and diffracted light that diffracts due to sample details, with a moderate attenuation of the direct light intensity. Eventually, direct and diffracted light interfere in the image plane, converting the phase difference into an amplitude difference, resulting in a high-contrast light and dark image in the field of view. The interference effect can be simplified as follows: I = A₁² + A₂² + 2A₁A₂cos (Δφ), where I is the intensity of synthetic light, A₁ and A₂ are the amplitudes of direct light and diffracted light, respectively, and Δφ is the phase difference between the two.
Measurement method
Observation and analysis using phase contrast microscopy typically follow a standardized operating procedure. First, a suitable unstained sample is prepared and placed on a slide to cover the coverslip to ensure uniform thickness. Adjust the microscope illumination source to the appropriate brightness and rotate the condenser dial to the position of the phase contrast ring of the corresponding objective magnification (usually labeled Ph1, Ph2, etc.). Rotate the condenser lift knob while observing the eyepiece until the bright ring of the ring diaphragm and the dark ring of the phase plate coincide exactly, this step is called axis adjustment, which is the key to obtaining a clear phase difference image. The sample is then focused using the coarse adjustment and fine-tuning knobs. Image analysis is usually based on the observed chiaroscuro to qualitatively evaluate the morphology, boundaries, internal structure and dynamics of the sample. Quantitative analysis requires the measurement of gray values in specific areas in combination with image analysis software to indirectly reflect the phase information or thickness changes of the sample.
Influencing factors
Obtaining high-quality phase contrast microscopic images is influenced by several technical factors. The nature of the sample itself is fundamental, and samples that are too thick or whose refractive index differs too little from the surrounding medium may produce too weak or confusing contrast. The thickness of the coverslip must meet the standard, usually 0.17 mm, and deviations from this standard will affect the correction effect of the phase plate. Proper alignment adjustment is crucial, as misalignment can lead to reduced imaging contrast and halo artifacts. The matching of the phase difference ring between the objective lens and the condenser must be accurate, and the high-power objective lens must be matched with a phase difference ring with a larger numerical aperture. In addition, the intensity and color temperature of the light source, the cleanliness of the optical components, and the precision of the operator's focus can also affect the final image quality.
Application
Phase contrast microscopy has a wide range of applications. In cell biology, it is a routine tool for observing the morphology, division, migration, and movement of intracellular particles. In the field of microbiology, it can be used to directly observe the morphology and motility of living microorganisms such as bacteria, fungi, and protozoa. In medical examinations, it is helpful to perform non-staining examinations of formed components and germ cells in urine. In materials science, it can be used to observe the internal structure of transparent materials such as polymer films, liquid crystals, and fibers. Due to its lack of staining and minimal damage to the sample, it is of unique value in studies that require long-term in vivo observation or staining that may interfere with the natural state of the sample.
Selection
When choosing a phase contrast microscope, multiple aspects need to be considered based on specific application needs. The core is the optical configuration, which should focus on the phase correction level, numerical aperture and magnification range of the objective, and the objective lens with a high numerical aperture can provide higher resolution. The condenser should be equipped with a multi-position dial, including a phase contrast ring and brightfield position for different magnification objectives. The lighting system should choose LED light sources with adjustable brightness and stable color temperature. The mechanical stability and user-friendly design of the microscope, such as the accuracy and feel of the focusing mechanism, also affect the user experience. For studies that require documentation and analysis, a microscope-compatible digital imaging system, including camera interfaces, camera sensor performance, and the functionality of accompanying software, should be considered. In addition, the scalability of the system, such as whether it supports other contrast enhancement modules such as fluorescence and differential interference, also provides flexibility for possible future research needs.