How do blue light glasses work?

The two ways a lens filters

Surface coating

A thin interference coating is applied to the lens surface, reflecting part of the short-wavelength light rather than absorbing it. This is how most near-clear "blue light" lenses work. It has two practical consequences: the lens stays close to colourless, and it often shows a faint blue or purple sheen when you tilt it under a lamp. That reflected light has to go somewhere, and some of it bounces back toward your eye or shows up as reflections others can see on video calls.

Embedded tint

The filtering material is mixed into the lens itself, absorbing rather than reflecting. This is how amber and orange lenses work. Absorption across a broad band is what makes them visibly coloured, and the colour cast is the honest visual signal that a substantial amount of light is being removed. You cannot filter a lot of one part of the visible spectrum and have the result still look neutral — that is physics, not a manufacturing shortcoming.

Why the percentage is a slippery number

"Blue light" is not one thing. The visible blue region spans roughly 400 to 500 nanometres, and it is not uniform in effect. The violet end near 400–420 nm is the part most often cited in laboratory photochemistry work. The 460–480 nm region is what melanopsin responds to, and therefore the part relevant to circadian timing. Screens emit across the range, with a substantial peak in the region that matters for the body clock.

A manufacturer can therefore quote a very high blocking figure by measuring only at 410 or 415 nm, where their coating performs best and where screens emit comparatively little. The number is not necessarily false. It is answering a question you did not ask.

What a meaningful specification looks like: a transmission curve across the full visible range, or at minimum a percentage with the wavelength band it applies to stated explicitly. Most consumer sellers publish neither. When you encounter a bare percentage, treat it as marketing rather than data.

What these lenses are not

• They are not UV protection. Ultraviolet sits below 400 nm and is a genuine ocular hazard — but essentially all modern polycarbonate lenses block it, tinted or not, and screens do not emit it.
• They are not anti-glare. That is an anti-reflective coating, a separate and rather more useful feature that reduces reflections off the lens surfaces.
• They are not a prescription. If you need correction, a filtering lens without it is a lens that still leaves you straining.

The feature that probably matters more

If your complaint is visual discomfort in a bright office or under overhead lighting, an anti-reflective coating is a more defensible purchase than a blue filter. It reduces the reflections bouncing between the lens surfaces and your eye, which is a direct and visible effect you can check for yourself by tilting a coated and uncoated lens under a lamp. It is also, tellingly, cheaper to add than most blue-filtering upgrades.

Next: anti-reflective coating versus blue light filter, or what the trials found.