September 23, 2010
By J. Guthrie
By J. Guthrie
Modern lens coatings perform far beyond the original anti-reflective coatings invented in 1935 by a Carl Zeiss engineer. They reduce reflection to almost zero, transmit an extraordinary amount of light, and fine tune that light so that color transmission is near perfect as well.
Though not nearly as glamorous as ballistic compensating reticles or new side-focus knobs, a scope's lens coatings are essential components of that optic's ability to transmit the available light to a shooter's eye, tune the color of that light, and protect expensive lenses from the elements. With the exception of the quality of an optic's glass, coatings might just be the most important part of a bright, clean, and clear image.
In 1935, a Ukrainian physicist named Alexander Smakula invented anti-reflective lens coatings while working for Carl Zeiss. His single-coating treatment dramatically improved the performance of a wide range of optical instruments.
Smakula understood that light, like many forms of energy in nature, traveled through the air as a wave, and when that wave met glass, a certain portion of the wave was bounced back as a reflection. The reflection not only distorted the image but robbed the viewer of a fraction of the overall light being transmitted. This happened every time the light entered and left a lens. If the glass surface were coated with the right material to the right thickness, the reflective wave could be tuned, in a sense, so that the incoming and outgoing waves canceled out one another. A clearer image with less distortion and more light transmitted through the lens were the results of Smakula's early experiments with anti-reflective coatings, and the world was suddenly a better place--especially when viewed through a binocular or riflescope.
More than 70 years later, lens coatings are standard on just about every piece of glass that comes between the human eye and a distant object. A host of exotic materials and more exotic means of applying those materials to glass have evolved, as have the number of coatings that are stacked on a lens. Scott Smith has a graduate degree in optical engineering and manages Leupold's R&D and design engineering departments. His work centers on delivering the best possible image to the eye, and much of that optical sorcery is accomplished with lens coatings.
"If you take a single lens made of glass, it has two surfaces," Smith said. "If there is no coating on that lens, you lose four percent of light transmission per surface--or eight percent per lens. In a riflescope with 10 elements, you would have at least 20 surfaces with four percent loss per surface. So you can see how this adds up very quickly and why it is crucial to use anti-reflection coatings."
It is lens coatings that turned both riflescopes and binoculars into practical tools for hunting and shooting. Before coatings, light transmission severely limited the practical use of both.
With enough lenses between the eye and a target, the target could essentially be reflected away to nothing. Magnesium fluoride (MgF) is a simple, widely used single-coating material that greatly reduces the amount of light reflected by a lens. The compound's refractive index--the measure of how much the speed of light is reduced in a given medium--falls between the refractive index of air and glass, giving it the physical properties it needs to reduce reflections by nearly 50 percent, reducing light loss to just two percent per lens surface.
"Leupold has different coatings spanning from very simple MgF to Multicoat and Multicoat 4 to index-matched broadband AR to XTended Twilight with DiamondCoat 2," Smith said. "As you move from MgF up the scale, you tend to get better light transmission across the visible spectrum, which appears to your eye as a brighter, crisper, and more accurate optic."
Adding multiple coatings allows lens makers to fine tune portions of the visible light spectrum, but they have to keep in mind the refractive index of each layer and keep a running total of the combined index of the layers or the equation quickly unhinges. They control the refractive index through layer thickness--measured in nanometers--of that particular layer.
Using certain types of glass with certain coatings could produce unnatural tints, at least to the human eye. Though pure, perfect colors are usually more important to camera lens manufacturers, optic makers can adjust the coating to increase or decrease the amount of light in the specific area of the light spectrum to correct unnatural tinting. The end user sees not only a lot of light, but the correct color of light as well. Some scope makers have introduced coatings that increase the contrast between certain colors, like the brown of animal fur and the green of tree leaves, to give hunters searching for game a small edge.
Leupold's more advanced coatings alternate layers of metal oxides, like aluminum oxide, titanium oxide, and silicon oxide. Each oxide manipulates a specific wavelength of light. The layers work in concert, producing a better coating, and less reflectance is achieved. Really good coatings can reduce the amount of reflected light loss to less than 0.5 percent per lens surface--an amazing accomplishment. No scope, no matter how good, can transmit 100 percent of the light through all its elements--it would simply defy the laws of physics. There is also a small amount of light absorption that occurs with any glass, although this is usually on the order of 0.1 percent per lens or less.
The catch, of course, is that as the coatings get more high-tech and complicated, the cost of putting those coatings on a lens increases exponentially. Compounds can be deposited directly onto a prepared lens or embedded into a microscopically thin film that is affixed to the lens; it just depends on the compounds, the lens, and where and how it is to be used.
"There are three or four different coating processes used for applying optical coatings," Smith said. "Evaporative coatings, e-beam assist, ion-assisted deposition, and ion-beam sputtering are some of the processes we use at Leupold. IAD and IBS produce the highest quality and most dense coatings. These are also more expensive, as the equipment to deposit them is more expensive."
After the beams have finished assisting and the ions quit sputtering, the resulting coatings not only reduce reflectivity and produce a bright, clear image, but they are extremely hard and durable. The armored lens is less susceptible to abrasions and scratches that, over time, would rob its ability to transmit light in the manner engineers like Smith worked so hard t
o accomplish. This quality is so important, the U.S. military has sought to quantify it and requires optics to meet or beat an abrasion test. As an example, Leupold's DiamondCoat 2 coating withstands the requirement of 500 rubs of steel wool without scratching and meets the military's standard of a hard, tough lens.
Some optics makers have coatings that can repel water and resist fogging, while other coatings on specialized equipment work to improve light from parts of the spectrum unnoticed by the human eye. The cost for these sometimes-important features can be measured in their effect on light transmission.
Unfortunately, simply tipping up an optic and catching a green or red or yellow hue off the objective lenses will tell you very little about the quality or technology contained within. In the most general of generalities, the more coatings an optic has, the better. Multi-layer coatings are superior to single-layer coatings, while fully coated, index-matched coatings are the best, although quantifying coating quality would sorely test the skills of even the most enlightened optics enthusiast. And since every human eye sees light differently, about the only way to really get a feel for a scope's lens-coating quality are side-by-side comparisons.
Look through an optic into shadows, at areas of bright light and at various colors--the brighter the image, the better the coating. View an object or target with alternating black and white bands and you can quickly see the difference in resolution or dispersion (color separation) if the black/white edge appears smeared or purple.
Unless you are an engineer like Smith, lens coatings are a little less than exciting. But they have elevated the common riflescope from a sunny-day novelty to a tool that can be used from first to last light.