Anti-glare and blue light filtering get sold together and are often assumed to be the same upgrade. They are not. One of them does something you can measure. The other is sold on a claim the research does not support, while still being useful for a different reason.

Anti-glare coating

Anti-glare, also called anti-reflective or AR coating, is a set of thin layers applied to both surfaces of the lens. It cuts the light that reflects off the lens instead of passing through it.

An uncoated lens reflects somewhere around 8 percent of the light hitting it. That light goes two places. Some bounces off the front and is seen by other people, which is why uncoated glasses show a bright reflection in photographs and hide your eyes. Some bounces off the back surface into your eye, which is what produces the ring of haze around a headlight or a streetlamp at night.

The effects are noticeable in specific situations:

Driving at night is where most people see the difference. Oncoming headlights produce less scatter and the halo around them shrinks.

Screen work is easier because the reflection of the room behind you sits less strongly on top of what you are reading.

Photographs show your eyes rather than two white rectangles.

High-index lenses reflect more than standard plastic, so the stronger your prescription, the more the coating matters.

The tradeoff is that AR coating is the part of the lens that gets damaged. Scratches show more readily on a coated lens, and cleaning with a dry shirt or with anything containing alcohol degrades it. Rinse with water, use a drop of dish soap if needed, dry with a microfibre cloth.

Blue light filtering

Blue light lenses carry a coating or a tint that reduces transmission in the blue end of the visible spectrum, usually somewhere between 415 and 455 nanometres.

They are marketed on two claims and the evidence sits differently on each.

The first claim is that blue light from screens damages the retina. The research does not support this. The blue light coming off a phone or a monitor is a small fraction of what you get standing outside on an overcast day. The American Academy of Ophthalmology does not recommend blue light blocking glasses for this purpose. If you want to protect your retina from blue light, the thing that matters is sunglasses outdoors, not a coating for indoor screen use.

The second claim is that blue light lenses relieve digital eye strain. Controlled trials have generally not found a benefit over clear lenses. What does cause the symptoms is well understood: you blink less when concentrating on a screen, and holding focus at one distance for hours tires the focusing muscles. The fixes are unglamorous. Blink deliberately, look at something 20 feet away for 20 seconds every 20 minutes, and get the screen a little below eye level so your lids cover more of the eye surface.

There is one area where the evidence is more interesting, which is evening blue light exposure and sleep timing. Blue wavelengths suppress melatonin, and reducing them in the hours before bed has a plausible effect on falling asleep. Whether a lens coating is the right tool for that, rather than turning the screen brightness down or using the device's night mode, is a fair question.

So what should you actually buy

If you spend time driving at night, or your prescription is strong, or you dislike the reflections in photographs, anti-glare is worth it and you will notice it. It is the coating we would put on a pair of glasses by default.

If you want blue light filtering, we will make it, and plenty of people prefer how screens look through it. We would rather you buy it knowing what it does than buy it believing it protects your eyes from damage, because that is not what the evidence shows.

If your eyes feel tired at the end of a screen day, the first thing worth checking is whether your prescription is current and whether it is right for the distance you actually sit from the monitor. A pair set up for that working distance does more for screen fatigue than any coating.

Where to get it done

We put anti-glare and blue light coatings on prescription lenses and on plain power lenses, and we will tell you which of the two we think you need. Bring your current prescription, or your current glasses if you are not sure how old it is.

Optica Eyewear is on Shahid Bhagat Singh Road in Colaba, near INS Shikra. Open Monday to Saturday, 10am to 8pm.