Phase contrast converts invisible phase shifts (refractive index differences) into visible brightness variations, allowing clear visualization of transparent, unstained specimens like live cells without the need for fixation or damaging stains.

The objectives themselves can usually be physically interchanged (if parfocal), but phase contrast objectives **require a matching phase annulus** in the condenser to function correctly. Without the correct annulus, they will not produce the phase contrast effect.

Yes, Phase Contrast Microscopy is absolutely essential in IVF laboratories for assessing sperm motility, viability, and evaluating oocytes/embryos with high precision and minimal invasiveness.

The illumination system is typically LED and does not require color filters for basic Phase Contrast use. The phase contrast effect is achieved using the phase ring (annulus) in the condenser and the corresponding phase plate in the objective lens.

An infinity-corrected system allows light rays leaving the objective to be parallel. This design reduces optical aberrations and allows accessories (like DIC prisms or filter cubes) to be added into the light path without affecting image quality or magnification, offering greater flexibility.