The Complete Guide to Sunglass Lens Scratching and How to Avoid It
- Sunglass Science
- 3 days ago
- 13 min read
Lens scratching is the most common issue for sunglasses and one of the hardest to avoid. It makes the view less clear, it ruins the coatings, and replacements can be costly. So let's understand the science behind why lenses scratch, scratch resistance, and how to avoid this from happening to your shades.
In This Article
What Causes Lens Scratches?
The first thing to know about lens scratching, or any scratching, is the Mohs scale. Mark Rober explains it in this short video:
The Mohs scale is a way of ranking minerals by how hard they are, based on their ability to scratch one another. It was developed in 1812 by Friedrich Mohs, a German geologist.
Here’s how it works:
The scale runs from 1 to 10, with 1 being the softest (easily scratched) and 10 being the hardest (scratches all others).
Each step on the scale is defined by a specific reference mineral, from talc, which is very soft and you can scratch it with your fingernail, to diamond, the hardest natural substance that scratches everything else.
The scale is ordinal, not linear. That means the jump in hardness between some numbers is much bigger than others. For example, diamond (10) is about four times harder than corundum (9), not just a little harder.
It’s a relative test, not an absolute measurement. It’s useful in the field or lab for quick comparisons, but modern science also uses precise hardness tests (like Vickers or Knoop) that measure resistance to indentation in units. However, for practical purposes in learning how to prevent scratches, the Mohs scale will be much more beneficial.
So to understand why lenses scratch, it's helpful to know the Mohs number of the lens material, the lens coatings, and the object that scratches the lens.
It is not easy to find Mohs figures for each lens material, because the Mohs scale is for minerals, not plastics, so it doesn't translate well. But we can generalize for our purposes of understanding what would cause scratching.
Mineral glass has a Mohs hardness of 5-6.
All other uncoated lens materials are somewhere between 1-3. I am lumping together all non-glass lenses for this, because even though they vary in terms of scratch resistance, the difference is small compared to what glass has. That Glasses Guy on YouTube shows that:
Keep in mind, we are not talking here about impact resistance, weight, or clarity. Just scratch resistance.
As mentioned, the Mohs scale is ordinal, not linear. A material at 6 will scratch one at 2 every time, but the step sizes aren’t even. The jump from 5→6 isn’t the same as 2→3 or 9→10.
If you want a real multiplier, you have to use an absolute hardness test (Knoop, Vickers, Rockwell) for the specific materials and then compare those numbers. On those scales, a typical Mohs-6 material is many times harder than a Mohs-2 material (often an order of magnitude or more), but the exact ratio depends on the materials and the test method. So glass vs polycarbonate scratch resistance is probably many times more.
What The Real Enemy Is
Check out this video Otis made touting their glass lenses' scratch resistance vs plastic:
Most house keys are made of brass (Mohs 3) or Aluminum (2.75). So, it's easy to see why plastic suffers here, but glass does so well. Don't think this is true of all metals, though; some steel products can be as hard as a 6.5 Mohs.
But metal is not our main concern. The real enemy is sand, specifically quartz.
Quartz has a Mohs of 7, and is everywhere. In sand, dust, clothing fibers, etc.
This video that Otis did at the beach gives me goosebumps. I don't know what kind of rock that was, but DO NOT TRY THIS AT HOME, no matter what material your glasses are.
For glass lenses, quartz is enemy number 1, always. Keep your lenses away from rubbing against any kind of sand, dirt, or surface that has hard minerals like granite counters in a kitchen or mantle.
You can take them to the beach, but keep them off the sand. We will get to cleaning lenses soon, but do not wipe your lenses if there can be any sand on them with any type of cloth until they are washed first. Otherwise, you will drag sand across the surface of the lens.
With non-glass lenses, it is much worse. The same guidance about sand and hard minerals applies to an even greater degree, but now many more environmental and habitual risks come into play.
Want to wipe your lenses with your shirt or a tissue? With glass lenses, it's not recommended, but it probably won't be the end of the world. With non-glass, especially uncoated, doing that will destroy your lenses in short order.
Salt has a Mohs hardness of about 2-2.5. So, dealing with salt water on glass lenses is not a problem unless there are clumps that can have some sand mixed in. But non-glass lenses will struggle with saltwater.
"Dirt" is a mix of things, so its hardness depends on what minerals it’s made of. Clay and salt have a Mohs of 2-4, bad for plastics but not glass. But any quartz in there makes it much worse.
Concrete contains quartz and other hard silicates. Ceramic and porcelain, like bathroom tiles, kitchen counters, and mugs, often have a Mohs 6–7.
Jewelry like diamonds (10), sapphires/rubies (9), emeralds (7.5–8) are all much harder than glass.
For non-glass lenses, everyday items become more of a problem.
Fingernails are about 2.2-2.5 Mohs. Paper towels are 2-2.5 (but the real issue with those are contamination with dust and dirt they pick up). Most fabrics (shirts, microfiber cloths if clean)
made of cotton, polyester, nylon fibers are roughly 2–3 Mohs.
Here's a chart:
Material / Item | Mohs Hardness | Risk to Plastic Lenses | Risk to Glass Lenses | Notes |
Fingernail | 2–2.5 | ✅ Safe | ✅ Safe | Won’t scratch either. |
Microfiber cloth (clean) | ~2 | ✅ Safe | ✅ Safe | Safest option — traps particles. |
Shirt / cotton / soft fabric | 2–3 | ⚠️ Low (if clean) | ✅ Safe | Safe if clean, but dangerous if dusty. |
Paper towel / tissue | 2–2.5 (cellulose) | ⚠️ Moderate (with pressure) | ✅ Safe (if clean) | Main risk: picks up dust/silica. |
Wood | 2–2.5 | ⚠️ Moderate | ✅ Safe | Dust in wood surface can scratch. |
Plastic objects (pens, etc.) | 2–3 | ⚠️ Moderate | ✅ Safe | Same hardness as lenses, may leave marks but not deep scratches. |
Dirt / soil | Mixed (2–7) | ❌ High | ❌ High | Contains silica/quartz (Mohs 7). |
Sand (quartz) | 6–7 | ❌ Severe | ❌ Severe | Biggest everyday risk — scratches both instantly. |
Metals (coins, keys, etc.) | 3–6 (varies) | ❌ Severe | ⚠️ Low–Moderate | Keys often have hardened steel tips (~5.5–6). Can scratch plastics easily, glass occasionally. |
Ceramics (tiles, mugs, sinks) | ~6–7 | ❌ Severe | ❌ Severe | Similar hardness to quartz; can scratch both. |
Diamond jewelry | 10 | ❌ Severe | ❌ Severe | The hardest thing you’ll encounter — will cut glass easily. |
In short: Plastic lenses (Mohs 3–4) are vulnerable to almost everything except very soft items (fingernails, cloth). They need scratch-resistant coatings.
Glass lenses (Mohs 6–7) are naturally resistant but not immune. Silica, sand, ceramics, and diamonds can scratch them.
The number one real-world threat: sand.
Anti-Scratch Coatings Explained
The “hard coats” used on plastic lenses are usually siloxane or polysiloxane-based (a type of cured silica resin).
They’re not as hard as pure glass, but they bump the lens surface up to about 5–6 on the Mohs scale, depending on the formula and curing method (dip coat vs. spin coat vs. UV cure).
Maui Jim states that their glass lenses do not have anti-scratch coatings, while their non-glass lenses do. Why is that?
Because the glass is already harder than the coating itself. Coatings can actually reduce durability. A scratch-resistant coating on glass could introduce adhesion issues, micro-crazing, or peeling over time. Since the coating would be softer than the glass, any scratch would still cut through it, and now you’d also have the risk of damaging the coating. The final polish step gives glass a hard, smooth surface that resists scratching better than any applied coating could improve upon.
Anti-scratch coatings only make sense for plastics. For glass, they’re redundant (and possibly counterproductive), which is why brands like Bajio, Costa, Maui Jim, etc., never add them to their glass lens lines.
How premium brands apply anti-scratch vs. budget sunglasses also makes a huge difference.
Premium brands start with better base materials (e.g., Trivex, high-index, or glass), which already have stronger surface hardness. The hard coat is tuned to match the lens refractive index, so optics aren’t degraded.
Lenses are fully submerged in a liquid polysiloxane (silica-based) resin. Excess drains off uniformly, then the coating is UV or heat-cured into a dense, glass-like surface. This ensures even thickness across the entire lens. A proper scratch coat is engineered to be compatible with AR and hydrophobic layers (so it doesn’t crack or delaminate).
Premium coats are harder (Mohs ~6–7 equivalent) and thicker (~3–5 microns). That makes them much more resistant to micro-abrasions and swirl marks. Every lens batch is tested for haze, adhesion, and abrasion resistance (e.g., steel wool or Bayer abrasion test). Failed lenses don’t leave the lab.
Cheaper brands like you will find for $20 on Amazon, Alibaba, fast-fashion brands, and almost anyone who uses TAC lenses use a different method. Instead of precision dipping, many low-cost lenses get a quick spray coat or a thin dip. Coatings may not be tuned to the lens’ refractive index, which can lead to glare, haze, or slight distortion.
The coating is often <1 micron thick, so it wears off quickly.
Inexpensive coatings may be air-dried instead of UV/heat-cured. This makes them softer and more prone to peeling or crazing.
You can sometimes see rainbow-like streaks or thin spots under light, a sign the coating is inconsistent. Little to no abrasion testing is done. If the coating fails in weeks or months, the brand expects the consumer to buy another pair.
Scratch coatings are often marketed in a way that makes people think their lenses are “scratch-proof.” In reality, no lens or coating is scratch-proof, not even glass. What these coatings do is reduce the rate and severity of scratches, not eliminate them entirely.
They buy you time, not invincibility. A good scratch-resistant coating makes a lens far more durable in daily life (resisting swirls from cleaning cloths, micro-abrasions from dust, and casual contact with soft objects). But hard abrasives like sand, cement, or metal keys will still cut through. They won’t stand up to being dropped lens-first on a sidewalk or rubbed with grit.
Scratch coats themselves are a surface treatment. With years of cleaning and handling, they can thin or develop micro-cracks. Once compromised, scratches can form more easily. This is true even of premium coats.
Keep in mind, a scratch coating doesn’t make a lens tougher against cracks, chips, or deep gouges. It only hardens the surface against abrasion.
Hydrophobic and oleophobic coatings (water and oil-repellent) don’t add scratch resistance but indirectly help prevent scratches by keeping lenses cleaner. By reducing smudges and making cleaning safer (less pressure, less wiping), there is less chance of scratches. These do wear off over time.
Anti-reflective layers are softer than glass or hard coats and scratch more easily. Many scratches people complain about are actually on the AR, not the lens itself.
Mirror Coatings and Encapsulated Mirrors
Mirror coatings themselves are very soft compared to lens materials. They’re made of ultra-thin metallic or dielectric layers (aluminum, titanium dioxide, chromium, silicon oxides, etc.), often only a few hundred nanometers thick.
On the Mohs scale, metallic mirror layers (Al, Cr, Ti) are generally 2–3 Mohs. Dielectric layers (SiO₂, TiO₂, MgF₂, etc.) can vary, but often ~4–6 Mohs in bulk form.
But because mirror coatings are so thin and brittle, their effective scratch resistance is much lower than the raw mineral values. In practice, most mirrors scratch like they’re in the 2–4 Mohs range.
That’s why even a tiny grain of sand or dirt can damage a mirror-coated lens almost instantly.
When the mirror gets scratched, it is more noticeable than a lens without a mirror for two reasons. The first is cosmetic. The second is that it will mess with how light gets through the lens in that spot relative to the rest of the mirrored lens, and it will change the tint profile there. It will usually be tolerable for small scratches, but can get annoying for large scratches or accumulated ones.
This is definitely a factor to consider when selecting lenses from a brand like Maui Jim or Serengeti, as they apply their mirrors on the exterior of their lenses and they do not resist scratching like the glass lenses underneath. Keep in mind though, that a mirror on a glass lens will still be functionally more scratch-resistant than on a non-glass lens because the substrate is so much harder.
Some brands use encapsulated mirror coatings, meaning the reflective oxide layers are applied under the front layer of glass, rather than sitting exposed on the outermost surface. Brands like Costa, Bajio, Otis, and Revo claim to use this structure for their glass lenses.
The mirror coating itself doesn’t become harder or more scratch-resistant when it’s encapsulated. Its Mohs hardness doesn’t change; it’s still a very soft, fragile, thin film. What changes is where it sits: instead of being exposed to the environment, it’s sealed under glass. That means everyday hazards (sand, dirt, microfiber grit, sunscreen, sweat) never directly contact the mirror layer. This allows for better cosmetic longevity and less annoying distortion, even though the lens as a whole is still only as scratch-resistant as its outer glass.
By the way, the reason Oakley lenses scratch so much is a mix of all the reasons discussed so far. They use very scratch-prone polycarbonate lenses with very brittle metal oxide mirrors with no anti-scratch coatings on top (except on prescription lenses).
Myths About Fixing Scratched Lenses
There’s a lot of DIY advice floating around online about “fixing scratches” on sunglasses or eyeglasses. Most of it recommends household products like toothpaste, baking soda paste, or even car wax. These are all dumb.
Toothpaste and baking soda work by grinding away material, which is fine for polishing teeth or sinks, but terrible for lenses. On plastic lenses, they actually remove part of the hard coat or AR layer, leaving the lens more exposed and easier to scratch. On glass lenses, they can cause fine hazing that permanently lowers clarity.
DIY abrasives strip these coatings off unevenly, which can create rainbow patches, peeling, or cloudy spots.
It is part of the lens structure missing. Toothpaste or anything else doesn't replace that in any way. It will not "buff out". Even if it could, it would alter the curvature of the lens and its corrective power (0 is also a corrective power). If it’s done unevenly (and that means up to 0,01 mm), you’ll get a laughing mirror effect (distorts a person's reflection to create an altered image) when looking through the lens. To do it without changing the corrective power of the lens would require computer-guided CNC equipment, and even then, you’d be left with an uncoated lens that would scratch again very easily.
Once coatings are compromised, they can’t be reapplied at home.
Proper Cleaning Methods
If there is sand on a lens and you wipe it with a cloth, you are wiping sandpaper on your lenses. So please, if your lenses are dirty with actual dirt and not just grease, clean them properly.
Step 1: Rinse under lukewarm water. This removes the sand, dirt, dust etc, without rubbing them against the lens.
Step 2: Use lens cleaner or mild dish soap. I use Palmolive for what it's worth. This helps get rid of the grease.
Step 3: Wipe with a clean microfiber cloth. Not a towel or paper towel. Not a shirt. Microfiber clothes usually come with any glasses or sunglasses you bu,y or you can get them fairly cheap from Amazon. I use these.
Dry wiping your lenses is a bad idea, at least if you do it too much. Lenses pick up microscopic particles of dust, dirt, and sand throughout the day. Many of these are made of silica or quartz (Mohs 6–7), which is as hard or harder than glass. When you wipe a dry lens, those particles get dragged across the surface.
At first, you won’t notice much. They are just “micro-scratches.” Over time, these create haze, glare, and reduced clarity, especially when light hits the lens at night or from the side.
Scratch-resistant, AR, and hydrophobic layers are thin. Dry wiping slowly polishes them off, leaving lenses more vulnerable.
Microfiber cloths are the safest way to clean lenses, but only if they’re clean themselves. Over time, a cloth picks up dust, oil, and microscopic grit. If you keep using it without washing, that cloth basically turns into sandpaper in disguise.
Just wash them with mild liquid detergent in warm water (no fabric softeners or bleach, they clog fibers).
Storage & Handling
I'm not going to drag out this way-too-long article too much, so here are the basics of storage.
As Rob Tavakoli from SportRx says, "On your face, or in the case, so you don't have to replace".
Always use a case (hard cases are best).
Never put lenses face down.
Avoid leaving sunglasses loose in bags or car dashboards.
Heat damage can soften coatings and make them more prone to scratches.
I'll do another post elaborating on some of these points.
What To Do If Your Lenses Get Scratched
No matter how careful you are, scratches happen. The important thing is knowing the difference between scratches you can live with or when it’s time to replace the lenses.
Cosmetic scratches are just small, light marks that don’t affect clarity. These are usually only visible in certain lighting angles and don’t alter how you see through the lenses. Don't worry about them.
Vision-impacting scratches are deeper scratches that scatter light, cause glare, or create distortions in your vision. On mirrored lenses, even small scratches can be distracting and very noticeable.
The rule of thumb is that if a scratch interferes with your ability to see comfortably, it’s time for a fix.
Warranty replacements will not generally get you far. Most premium brands cover manufacturing defects but not everyday wear and tear. They know the lenses will likely get scratched. Still, it’s worth checking what they can do for you because sometimes brands will offer a discounted replacement if the damage is beyond repair.
Top-tier sunglass makers like Maui Jim and Costa have formal replacement lens services. You send in your scratched sunglasses, and they’ll install brand-new factory lenses for a fraction of the cost of a new pair. This option preserves the integrity and optical quality of your sunglasses. You know you’re getting the same tech you originally paid for.
For brands that don’t have an official replacement program, you can look into aftermarket lens makers. Quality varies widely. Some aftermarket lenses can be decent, but they rarely match the optics of premium originals. Make sure to still get lenses with at least an anti-reflective coating. Some options are Tajima Direct, Fuse, and Revant.
Final Tips
Even the toughest sunglasses need care. Developing a few small habits can make a big difference in keeping sunglasses like new.
The most important one is to rinse your lenses before wiping them. Dust and sand are harder than most lens materials and can act like sandpaper if rubbed across the surface.
Another good habit is to always keep a case handy. Dropping your sunglasses in a bag, on a car seat, or worse, in a pocket with keys or coins is one of the fastest ways to scratch lenses.
Remember that no coating or material makes a lens invincible; even glass can be scratched by quartz sand.
The best mindset is to treat sunglasses as precision optics, not just fashion accessories. With a little extra care, you’ll prevent scratches on sunglasses and get years of clear, comfortable vision.
Conclusion - The Complete Guide to Sunglass Lens Scratching and How to Avoid It
Lens scratching is the most common and often most frustrating issue for sunglasses, but understanding the science behind it makes prevention much easier. From knowing the hardness of your lens material and coatings to recognizing the real threats like sand, dirt, and hard minerals, small habits go a long way in protecting your optics. Proper cleaning, safe storage, and thoughtful handling can extend the life of your sunglasses, while replacement lens programs from premium brands offer a reliable path if scratches do occur. Remember, no lens is invincible, but treating your sunglasses as precision optics rather than just accessories will keep them looking and performing their best for years to come.
