In binoculars with roof prisms the light path is split into two paths that reflect on either side of the roof prism ridge. One half of the light reflects from roof surface 1 to roof surface 2. The other half of the light reflects from roof surface 2 to roof surface 1. If the roof faces are uncoated, the mechanism of reflection is Total Internal Reflection (TIR). In TIR, light polarized in the plane of incidence (p-polarized) and light polarized orthogonal to the plane of incidence (s-polarized) experience different phase shifts. As a consequence, linearly polarized light emerges from a roof prism elliptically polarized. Furthermore, the state of elliptical polarization of the two paths through the prism is different. When the two paths recombine on the retina (or a detector) there is interference between light from the two paths causing a distortion of the Point Spread Function and a deterioration of the image. Resolution and contrast significantly suffer. These unwanted interference effects can be suppressed by vapor depositing a special dielectric coating known as a phase-correction coating or P-coating on the roof surfaces of the roof prism. To approximately correct a roof prism for polychromatic light several phase-correction coating layers are superimposed, since every layer is wavelength and angle of incidence specific.Paul Maurer: Phase Compensation of Total Internal Reflection. In: Journal of the Optical Society of America. Band 56, Nr. 9, 1. September 1966, S. 1219–1221, doi:10.1364/JOSA.56.001219 The P-coating was developed in 1988 by Adolf Weyrauch at Carl Zeiss. Other manufacturers followed soon, and since then phase-correction coatings are used across the board in medium and high-quality roof prism binoculars. This coating suppresses the difference in phase shift between s- and p- polarization so both paths have the same polarization and no interference degrades the image. In this way, since the 1990s, roof prism binoculars have also achieved resolution values that were previously only achievable with Porro prisms.Konrad Seil: Progress in binocular design. In: SPIE Proceedings. Band 1533, 1991, S. 48–60, doi:10.1117/12.48843 The presence of a phase-correction coating can be checked on unopened binoculars using two polarization filters. Dielectric phase-correction prism coatings are applied in a vacuum chamber with maybe thirty or more different superimposed vapor coating layers deposits, making it a complex production process.
In binoculars with roof prisms the light path is split into two paths that reflect on either side of the roof prism ridge One half of the light reflects from roof surface 1 to roof surface 2. The other half of the light reflects from roof surface 2 to roof surface 1. If the roof faces are uncoated, the mechanism of reflection is Total Internal Reflection (TIR). In TIR, light polarized in the plane of incidence (p-polarized) and light polarized orthogonal to the plane of incidence (s-polarized) experience different phase shifts. As a consequence, linearly polarized light emerges from a roof prism elliptically polarized. Furthermore, the state of elliptical polarization of the two paths through the prism is different. When the two paths recombine on the retina (or a detector) there is interference between light from the two paths causing a distortion of the Point Spread Function and a deterioration of the image. Resolution and contrast significantly suffer. These unwanted interference effects can be suppressed by vapor depositing a special dielectric coating known as a phase-correction coating or P-coating on the roof surfaces of the roof prism. To approximately correct a roof prism for polychromatic light several phase-correction coating layers are superimposed, since every layer is wavelength and angle of incidence specific.Paul Maurer: Phase Compensation of Total Internal Reflection. In: Journal of the Optical Society of America. Band 56, Nr. 9, 1. September 1966, S. 1219–1221, doi:10.1364/JOSA.56.001219 The P-coating was developed in 1988 by Adolf Weyrauch at Carl Zeiss. Other manufacturers followed soon, and since then phase-correction coatings are used across the board in medium and high-quality roof prism binoculars. This coating suppresses the difference in phase shift between s- and p- polarization so both paths have the same polarization and no interference degrades the image. In this way, since the 1990s, roof prism binoculars have also achieved resolution values that were previously only achievable with Porro prisms.Konrad Seil: Progress in binocular design. In: SPIE Proceedings. Band 1533, 1991, S. 48–60, doi:10.1117/12.48843 The presence of a phase-correction coating can be checked on unopened binoculars using two polarization filters. Dielectric phase-correction prism coatings are applied in a vacuum chamber with maybe thirty or more different superimposed vapor coating layers deposits, making it a complex production process.