Why Scope Lenses Look Purple, Green or Gold

Bare glass loses light at every surface

Every time light passes from air into glass, or from glass back into air, some of it bounces off instead of going through. Wikipedia's article on anti-reflective coatings gives the number for ordinary glass: about 4 percent per surface. That does not sound like much until you count surfaces.

A riflescope is a stack of lenses: objective, erector group, eyepiece and often more. Each element has two surfaces, and many are cemented groups with extra interfaces. Lose 4 percent at each of a dozen or more air-to-glass surfaces and a meaningful share of the light never reaches your eye.

The lost light does not simply vanish. Some of it bounces around inside the tube and lands on your eye as haze or flare, which washes out contrast. That is why a scope with uncoated or poorly coated glass looks dim and flat, especially at dawn and dusk.

How a coating cancels a reflection

An anti-reflection coating is a very thin layer of material deposited on the lens surface. The classic version, as Wikipedia describes it, is a single layer of magnesium fluoride made exactly one quarter of a wavelength thick.

The trick is interference. Light reflects from the top of the coating and from the boundary between the coating and the glass. Because the second reflection travels an extra half-wavelength, its waves arrive out of step with the first reflection, and the two cancel. Less reflected light means more transmitted light.

Wikipedia gives the result: magnesium fluoride on crown glass cuts reflection from about 4 percent to about 1 percent per surface. The binoculars article on Wikipedia puts the classic magnesium fluoride figure at about 1.5 percent and notes that effective modern multi-layer coatings reflect 0.25 percent or less.

The history is older than most shooters assume. Wikipedia credits Lord Rayleigh with noticing in 1886 that old, tarnished glass transmitted more light than new glass, Harold Dennis Taylor of Cooke with a chemical coating method in 1904, and Olexander Smakula at Carl Zeiss with inventing interference coatings in 1935. Those coatings stayed a German military secret for several years, and became generally available in the 1940s.

Why you see color at all

Here is the key point: a coating that is exactly one quarter-wave thick can only be exactly right for one wavelength of light at one angle. Wikipedia's optical coating article says so directly: the reflections only exactly cancel for one wavelength at one angle.

So a single-layer coating tuned for the middle of the visible spectrum, which is greenish-yellow light where the eye is most sensitive, cancels that part well and cancels the blue and red ends less well. The small amount of blue and red that still reflects mixes into a violet or magenta tint. That is the classic purple look of a single-coated lens.

Multi-layer coatings stack several layers of different materials and thicknesses so that different layers handle different parts of the spectrum. Wikipedia notes that two or more well-chosen layers can push reflection below 0.5 percent across the visible range. The leftover reflection is weaker and can land in a different color: green, amber, blue or a mix that shifts as you tilt the lens. Wikipedia's anti-reflective coating article notes that color effects often appear at oblique angles, which is why the tint changes as you rotate the scope under a light.

What the coating labels mean

Spec sheets use a standard set of terms. Wikipedia's binoculars article defines them, and they apply the same way to riflescopes:

"Fully multi-coated" is the label to look for on a hunting or general-purpose scope. Many mid-budget scopes now carry it. Ammunition Depot's listing for the Sig Buckmasters 3-9x40 lists fully multicoated lenses, and the listing for the Burris Droptine 3-9x40 lists a fully multi-coated objective with Burris's HiLume multicoating.

The label tells you where coatings are, not how good they are. Two "fully multi-coated" scopes can perform very differently because the number of layers, their design and the quality of the glass underneath all vary. That is one reason the same spec line appears on scopes at very different price points.

The ruby lens problem

Some low-cost binoculars and scopes have a front lens with a strong, mirror-like red or orange finish, often sold as "ruby" coating. It looks technical. It deserves a skeptical look.

Think about what the color means. You see red on the front of the lens because red light is being reflected back toward you. Light reflected away from the lens is light that does not reach your eye. A coating that reflects a lot of red will tend to make the image dimmer and shift its color toward blue-green, which is the opposite of what a hunter in fading light wants.

There are legitimate optical coatings that reflect specific colors on purpose. Wikipedia's optical coating article describes coatings designed as filters that reflect a narrow band of wavelengths. But a strongly reflective red front element on a general-purpose scope or binocular is usually a sign that looking flashy took priority over transmitting light. If you see one, check what the coating actually claims to do before you buy.

Red dots are a special case

If you have noticed that many red dot lenses have a slight blue or amber tint, that is deliberate. Wikipedia's red dot sight article explains that a standard red dot uses a red LED and a mirror with a partially silvered multi-layer dielectric coating designed to reflect just the red part of the spectrum while letting most other light pass through.

That coating is how the dot reaches your eye: it reflects the LED's red light back toward you. The side effect is that some red light from the scene is also reflected away, so the view through the window can look slightly tinted. Better designs keep that tint small.

Holographic sights work differently. Wikipedia notes that because the reticle is a hologram recorded in the window, the window does not need that partially reflective coating, which is one reason holographic windows can look clearer. The site's red dot buyer's guide covers both technologies.

Compare current options in Ammunition Depot's red dots category.

Caring for coated lenses

Coatings are durable, but they are thin and can be scratched. A few habits keep them in good shape:

1. Blow off grit first with a lens blower or soft brush before wiping anything. Dragging grit across a lens is how coatings get scratched.
2. Dampen before wiping. EOTech's HWS manual says to always dampen glass surfaces before cleaning and never to clean the window with a dry cloth. That is good practice for any coated lens.
3. Use lens tissue or a clean microfiber cloth and a cleaner the optic maker approves. Avoid household glass cleaners unless the manual says they are fine.
4. Keep caps on when the optic is stored or in a case.

For more optics to compare, browse Ammunition Depot's rifle scopes or Optics Planet's riflescopes category.

Editorial note: This is an independent editorial overview built from manufacturer specifications, retailer listings and published sources. We have not performed hands-on testing of the products discussed, and nothing here is a performance guarantee. Verify specifications, compatibility and the law where you live.

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