Coatings Flashcards
Phase-correction (P) coatings:
Developed by Carl Zeiss (who else?) and introduced as “P-coating” in 1988, phase-correction coating is second in importance only to anti-reflection coating in roof prism instruments. The problem (nonexistent in Porro prisms) is that light waves reflecting off opposite roof surfaces become elliptically polarized so as to be one-half wavelength out of phase with each other. This results in destructive interference and a subsequent deterioration of image quality. The P-coatings correct the problem by eliminating the destructive phase shifts.
Reflection coatings:
These mirror-like coatings—which often owe their effectiveness to constructive interference—are used more often in sports optics than one might think. Examples include: most laser rangefinders and the few riflescopes that employ beamsplitters; red-dot sights where a wavelength-specific coating is used to reflect the image of the dot back to the shooter’s eye; and, as previously discussed, in roof prism instruments with Pechan prisms.
Hydrophobic (water repellent) coatings:
The archetype for water-repellent coating is Bushnell’s Rainguard coating that sheds water and resists external fogging. I extensively tested Rainguard coating in cold climates where inadvertently breathing on a scope’s eyepiece lens would have obscured one’s view of the target. The results were that, even when I intentionally breathed on both the objective and eyepiece lenses causing them to either fog or frost over, I could still see targets well enough to shoot.
Abrasion-resistant coatings:
A persistent shortcoming with some anti-reflection coatings is that they tend to be soft and, therefore, scratch easily. Thankfully, today’s “tough” coatings, though still not universally used, are greatly improving the durability of outdoor optics ranging from eyeglasses to riflescopes. The toughest coating, by far, that I have tested is on the T-Plated external lens surfaces of Burris’ Black Diamond 30 mm Titanium riflescopes. I couldn’t scratch it, even with the cutting edge of a razor-sharp pocketknife. The latter is not recommended.
Coated optics (C):
Means that one or more surfaces of one or more lenses have been coated.
Fully coated (FC):
Means that all air-to-glass surfaces have receive at least a single layer of anti-reflection coating, which is good.
Multicoated (MC):
Means that one or more surfaces of one or more lenses have received an AR coating consisting of two or more layers. When used by reputable manufacturers, this designation usually implies that one or both of the exterior lens surfaces are multicoated and that the interior surfaces probably have single-layer coatings.
Fully multicoated (FMC):
Means that all air-to-glass surfaces should have received multi-layer anti-reflection coatings, which is best.
Ruby coatings:
Unfortunately, not all AR coatings of a given type are created equal, and some may even be bogus. Lovely as they are to behold, I am very skeptical regarding the value of the so-called “ruby” coatings, which reflect a dazzling amount of red light, making viewed objects appear ghastly green. When leading manufacturers, such as Carl Zeiss, Leica, Nikon and Swarovski, start using ruby or other offbeat coatings, I’ll start believing in them.
The Light Thieves:
The spoilers that have bedeviled optics users since the invention of Galileo’s first telescope in 1610 are absorption and reflections, which dramatically reduce the amount of usable light that reaches the viewer’s eyes. Each optical element (individual lens, prism or mirror) inevitably absorbs some of the light that passes through it. Far more significant, however, is the fact that a small percentage of the light is reflected from each air-to-glass surface. For uncoated optics, this “reflective loss” varies between 4 percent and 6 percent per surface, which doesn’t seem too bad until you realize that modern optical instruments have anywhere from 10 to 16 such surfaces. The net result can be a light loss of as much as 50 percent, which is particularly troublesome under low-light conditions.
Flare:
Defined as “non-image-forming light, concentrated or diffuse, that is transmitted through the optical system.” The result is a veiling glare or haziness that obscures image details and reduces contrast. In extreme cases, it may even cause ghost images. An extreme example would be if you were trying to glass game on the shady side of a low ridge with bright sunlight streaming over the top and into the instrument’s objective lens.
Single-Layer Anti-Reflection Coatings:
This single-layer anti-reflection coatings reduced the reflective light loss from between 4 percent to 6 percent for uncoated surfaces to about 1.5 to 2 percent for coated surfaces, thus, increasing overall light transmission for fully coated instruments of about 70 percent, which, considering the accompanying reduction in image-degrading flare, was a remarkable improvement.
Multi-Layer Anti-Reflection Coatings:
Today’s best multi-layer coatings can reduce reflective light loss to as little as two-tenths of one percent at each air-to-glass surface.
Evaluating AR coatings:
The only tool needed is a small flashlight or, lacking that, an overhead light. The trick is to shine the light into the instrument’s objective lens so that when looking along the beam you can see images of the light reflecting off the various air-to-glass surfaces within the instrument. (Note: Reflection will be coming from both the near and far sides of lenses and prisms.) Now, based on the above information, regarding color, you’ll get some idea concerning the types of coatings used and, more importantly, whether some surfaces are uncoated.
Coating Colors:
Many believe that the quality of AR coatings can be determined by the color of light reflected from the surfaces. Perhaps, but to do so with any certainty requires considerable expertise. The color seen is not that of the coating material itself, which is colorless, but the reflective color or combined reflective colors of the wavelengths of light for which the coating is least effective.
