There are several kinds of lens coatings, and some of them aren’t even a coating, but are embedded in the lens material itself. Without going to deep into the techy details, let me describe the basic types of different lens types, coatings, treatments etc:
Polarized lenses
Ask many salesman in a sport retail store, or sad to say, even your local optician, describing the process of a polarized lens seems a hard thing to do. While its basically pretty simple. Your eye needs light, without it, you wouldn’t be able to see shapes and colors and what not. This light is provided by the sun and travels in vertical light waves (up and down). Just as you know sound waves, light travels the same way. Until it hits a shiny object. This will change the light wave into a horizontal wave, which is annoying if not HARMFUL for your eyes. A polarized lens has vertical slats that will allow the regular vertical light waves to enter the lens and reach your eye. However, when light is bounced back from a shiny object, like water, snow, glass or shiny pavement, it will become horizontally, which we call GLARE. These horizontal light waves can’t fully penetrate the vertical slats of a polarized lens, therefore reducing the amount of harmful light. Okay, when i started this explanation I thought it would be shorter, but I hope you get the idea.
Anti-Reflective
Back-glare, which occurs when light bounces in the back of the lens and into the eyes, can be both distracting and damaging to the eye, especially as we now spend an increasing amount of time with digital devices, such as computers, tablets, and televisions. An Anti-Reflective coating reduces eyestrain by eliminating ambient glare from car headlights, computers, digital devices and televisions. Do however ask your opticians to put the AR coating on the inside of the lens. This coating will let more light pass through your lens, reducing the effect of the base lens. You want a sunglass lens to reduce the light that gets to your eyes so you will not need to squint in the sun.
UV Protection
Ultraviolet (UV) radiation not only damages the skin but also the structure of the eye. Some brands have big stories about their lenses having a 100% UV block and try to make you pay big bucks for that. Far as I can tell (and i was schooled by the tech division at UVEX Germany) all sunglass lenses that are being manufactured will block 100% UV, even the 5 bucks frames you buy at the gas station. Regulations made it obligatory for all lenses to offer 100% UV filtering and there is simply no factory in the world that makes lenses that won’t do that, cause no brand would buy them. I can say this with a full 100% certainty for frames bought in Europe, but since these laws have been made, i seriously doubt that there are any frames still sold in the US that do not block 100% UV. It is simply the standard, and it would cost even the lowest chinese factories more to buy NON 100% uv blocking lenses that it would to buy otherwise.
Photochromic
These lenses change tint when exposed to light. Most people know them as Transitions®, which is a popular brand that does this. UV light activates the lens technology to transform the lenses from clear to dark, and a range of tint in-between. The lenses can fully clear indoors and at night, and when UV activated block 100% of UVA and UVB rays. They darken significantly within about a minute of exposure to bright light, and take somewhat longer to clear. A range of clear and dark transmittances are available.
Hydrophobic
The word ‘hydrophobic’ means water repelling or to be accurate “hydro = water” and “phobic = fear of”. Essentially it works by making the surface of the sunglass lens unattractive to the water molecules. That means instead of the water trying to cling to the surface of the lens and flatten out, they cling to themselves forming a water ‘droplet’. The hydrophobic coating also weakens the surface tension (or bond) between the water droplet and the surface of the lens. This means the water droplet is more likely to roll off the surface of the lens. Hydrophobic means your vision is not distorted by water on the surface of the lens. However, a word of warning. The hydrophobic coating is the last layer to go onto the lens – so like any film it is easily damaged and the hydrophobic coating can become ineffective. It is important to clean your sport sunglass lenses with just water for them to be more effective and not to clean the sunglasses lenses with a harsh chemicals or detergents, which could destroy the hydrophobic coating layer on the lenses.
Scratch-Resistant
Read the words, it says scratch-resistant, not scratch-proof. No pair of lenses, even glass ones, are scratch proof. However, lenses that are treated front and back with a clear, scratch-resistant coating have a much harder surface that is more resistant to scratching, whether from dropping your glasses on the floor or occasionally cleaning them with a paper towel. Since scratch-resistant coatings are sometimes optional, make sure your optician knows that you want your eyeglass lenses to include hard coating for extra durability. Also, ask about the warranty on eyeglass lenses that are treated with scratch-resistant coating versus those without the coating.
Anti-Fog
Nothing is more frustrating than having your eyeglasses fog up when you come in from the cold. This also can be a safety issue, since it limits your ability to see until the fog clears. Anti fog treated lenses make your vision stay clear when you make the transition from a cold environment to a warm one. It may also keep your lenses from fogging up during sports and other times you are hot and perspiring. Lens fogging is caused by tiny water droplets that form by condensation on the surface of eyeglass lenses when the lenses are significantly cooler than the surrounding air temperature.
Mirror
Applied to sunglasses, a mirror coating creates a highly reflective sheen on the outside of the lens. Mirrored glasses completely conceal the eye from external view, while only a slight gray or brown tint is visible to the wearer. A full mirror can add up 1 category of light filtering.
Flash mirror
A flash mirror is an ultra thin coating that gives the lens a nice shine. Your eyes will still be visible from external view and it can add up to half a category of light filtering.