An optical technique used with roof prism binoculars to increase color fidelity.
Due to a roof prism’s optical design, the light entering a binocular’s image-erecting roof prism is split in two. The two halves travel through the prism independently and are rejoined before entering the eyepiece. Because the two light paths are slightly different lengths, one half of the light takes a little longer to travel through the prism than the other. When the two halves of the image are rejoined, the longer light path half is slightly out of phase with the light that took the shorter route. This can reinforce some colors of light and cancel out others, affecting the color balance and fidelity.
Phase correcting coatings are optical coatings that are applied to one surface of the shorter light path half of the prism. The coating slightly slows down the short light path half of the incoming light that passes through that surface, causing it to once again be in phase with the light that traveled the longer path when they halves are rejoined.
With phase-corrected prisms, no colors are reinforced or cancelled, giving a more accurate color reproduction. The effect is particularly visible when looking sunward at a back-lit or silhouetted bird, where more color and detail can clearly be seen in the shadowed areas of the bird.