Definition
Inverted phase contrast fluorescence microscope is a composite optical instrument that combines inverted optical path design, phase contrast observation technology and fluorescence imaging functions. The objective lens is located below the sample and the illumination system is located above, which is particularly convenient for viewing living samples or thicker samples in culture vessels (e.g., petri dishes, flasks). It enhances the contrast of transparent samples through phase contrast technology and uses fluorescence principles to perform highly specific imaging of specific marker structures, making it widely used in life sciences, materials science, and industrial testing.
Principle
The instrument works on two core optical mechanisms: phase contrast imaging and fluorescence imaging. Phase contrast imaging uses the phase difference created as light passes through the sample, converting the phase difference into an amplitude difference (chiaroscuro) through a ring diaphragm and phase plate, making unstained transparent structures visible. The optical path difference Δ can be expressed as: Δ = (n₁ - n₂) × d, where n₁ is the refractive index of the sample, n₂ is the refractive index of the medium, and d is the thickness of the sample.
Fluorescence imaging relies on the properties of fluorescent substances. When a specific wavelength of excitation light irradiates a sample, the fluorescent material absorbs photons and transitions to the excited state, subsequently releasing longer wavelengths of emitted light. The instrument captures the fluorescence signal by separating the excited and emitted light through a dichroic mirror. The entire process involves the absorption, energy conversion and re-emission of light.
Observation method
When using an inverted phase contrast fluorescence microscope for observation, a systematic procedure should be followed. Firstly, the sample is positioned and focused in phase contrast mode, the region of interest is found using a low-power objective, and the best phase contrast comparison is achieved by adjusting the condenser ring to match the phase plate. When switching to fluorescence mode, select a filter set that matches the fluorescent dye, including excitation filters, dichroic mirrors, and emission filters. Exposure time, light intensity, and gain parameters should be optimized for sample fluorescence intensity to avoid signal quenching or background overpower. For dynamic process observation, long-term live-cell imaging can be performed with a culture system.
Influencing factors
Imaging quality is affected by multiple factors. In terms of optics, the numerical aperture and working distance of the objective lens determine the resolution and sample compatibility; The stability of the light source affects the uniformity of the fluorescence signal. The filter bandwidth and matching determine the signal-to-noise ratio level. In terms of sample preparation, the optical uniformity at the bottom of the culture vessel, sample thickness, and fluorescent label density all have an impact. Environmental factors such as mechanical vibrations, stray light interference, and temperature fluctuations can also reduce image stability. Proper phase contrast ring alignment, proper fluorescence exposure control, and regular calibration and maintenance are the basis for performance.
Applications
In life science research, this instrument is often used to observe the dynamics of living cells, such as cell division, migration, and intracellular organelle movement. Protein localization and cell-cell interaction studies of fluorescently labeled proteins are also typical applications. In the industrial field, it can be used for material surface coating inspection, polymer dispersion observation or microelectronic component quality inspection. It can be used in agricultural science for seed or tissue culture monitoring. Its inverted design is particularly suitable for samples that need to be viewed from the bottom, providing convenience for long-term non-invasive observation.
Key points to consider when selecting
Several parameters should be evaluated according to the application requirements. In terms of optical configuration, it is necessary to pay attention to the numerical aperture of the objective lens, the calibration level, and whether it is optimized for fluorescence. whether the fluorescence filter group covers the commonly used fluorescence bands; The longevity and stability of the light source type (e.g. LED or metal halogen lamps). System scalability includes support for multi-dimensional imaging, software analysis capabilities, and integration with other devices. User-friendly design such as focus mechanism feel, thermal control capabilities, and environmental isolation should also be taken into account. The final choice is a balance between imaging performance, cost of use, and long-term reliability.