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.
In a phase-contrast microscope, image clarity is improved via two methods: by generating constructive interference between scattered and surrounding light rays in areas containing the specimen, and by minimizing the amount of background light reaching the image plane.