Faasikontrastissa

Faasikontrastissa, often translated as phase contrast, is a microscopy technique that converts phase shifts in light caused by differences in refractive index into brightness variations in the image. This allows unstained, transparent specimens, which would otherwise appear nearly invisible under a standard light microscope, to be visualized with high contrast. It is particularly useful for observing living cells and their internal structures without the need for artificial staining, which can often kill or alter the cells.

The principle behind phase contrast microscopy involves splitting a beam of light into two parts. One part, the background beam, passes directly through the specimen. The other part, the object beam, passes through the specimen and is refracted or diffracted by its internal structures. The differences in refractive index between various parts of the specimen cause these two beams to travel at slightly different speeds, resulting in a phase shift between them.

Optical components within the microscope, specifically a phase plate and an annular diaphragm, are then used to manipulate these phase differences. The phase plate introduces a fixed phase shift to either the background or the object beam, while the annular diaphragm in the condenser allows only a hollow cone of light to illuminate the specimen. By carefully aligning these components, the phase shifts are converted into amplitude differences, meaning areas of the specimen with higher refractive indices appear darker, while areas with lower refractive indices appear brighter, or vice versa, depending on the specific design. This effectively translates the invisible phase variations into a visible image.