Some high-quality binoculars carry the designation "phase-coated". This refers to a special coating on the roof prism, which is also referred to as a P-coating. When a beam of light meets the roof prism, it is reflected several times in the shape of a roof edge.
In addition, the reflection divides the beam into two segments. This procedure causes a shift of the phases of the light waves. The waves of the rays of light are not equal, but displaced.
This would minimize the resolution and contrast. To prevent this, special coatings are applied which correct this phenomenon.