Vortex Optics Viper Binoculars Compact XR Lens Coatings: Vortex XR proprietary anti-reflection lens coatings increase light transmission with multiple coatings on all air-to-glass surfaces. XD Lens Elements: Extra-low dispersion (XD) glass enhances resolution, color and contrast. Fully Multi-Coated Lenses: Increase light transmission with multiple anti-reflective coatings on all air-to-glass surfaces. Phase Correction: Used on roof prism models only-coating enhances resolution and contrast. Roof Prisms: Valued for greater durability and a more compact size.