Zen-Ray: Charles Optics Talk

Roof  binoculars employ Schmidt-Pechan prisms to achieve image erection in a more compact platform.  Unlike Porro prism system, which can transmit light by lossless total internal reflection, Schmidt-Pechan prism relies on reflective coating on the non-TIR surface to minimize the loss.  Aluminum metallic coating has been commonly used in entry level binoculars. The next major step-up is silver prism coating, which allows no more than 94% reflectivity.  To achieve almost lossless light path, dielectric prism coating is used by Zen-Ray’s ZEN ED2 and the newest ZRS HD binoculars.  A flawlessly executed dielectric coating, like the ones used in ZEN ED2/ZRS HD, offers many benefits over conventional metallic coatings.

First of all, dielectric coating presents a flat response curve with the wavelength, therefore, allows a more neutral color display. By looking at the graph above, one can see that the silver prism coating shows rapid fall-off of tranmission at short wavelength.  As a result, the image often presents warmish color tint, in addition to the brightness degradation.

With close to 99% light reflection over the full visible spectrum, dielectric prism allows at least 5-7% brightness improvement over the best metallic coating.   This extra illumination improvement is critical to enhancing the overall user experience under extreme low light condition.