Most blue-light glasses come with a claim and nothing else. "Blocks 100% of blue light." "Filters harmful rays." A number, printed on a box, with no report behind it.
The honest version is less dramatic and more useful: a transmittance report. It's a page of percentages and a curve, and almost nobody reads it — including, we suspect, a lot of the brands selling against it.
Here's how to actually read one.
Three things matter. Not one number.
A real lens transmittance report has three components worth understanding:
- Transmittance (%) — how much light passes through the lens at a given point. 0% means fully blocked. 100% means the lens has no effect at all.
- Wavelength (nm) — where on the light spectrum you're measuring. Blue-violet light, the range most blue-light glasses target, sits roughly between 400 and 450 nanometres.
- The curve — not a single number, but how transmittance changes across the whole range. This is the part almost nobody shows you.
Most marketing claims give you one number at one wavelength. A real report gives you the curve.
Why "100% blocked at 400nm" doesn't mean what you think
This is a popular claim, and it's often technically true. Many lenses genuinely do block all light right at 400 nanometres.
What that claim leaves out: 400nm is the very edge of the visible spectrum, bordering ultraviolet. It's an easy point to block completely — and a single easy point to advertise. The real question isn't what happens at 400nm. It's what happens across the next 50 nanometres, where most of the blue-violet range actually sits.
A lens that blocks 100% at exactly 400nm and almost nothing by 410nm isn't doing much. A lens that maintains meaningful filtration across the full 400–450nm range is doing the actual job. You can't tell the difference between these two lenses from a single number. You can tell immediately from a curve.
What a real curve looks like
A properly tested lens report — to standards like ISO 12312-1:2022 or ANSI Z80.3 — plots transmittance at regular intervals, usually every 5 nanometres, across the visible spectrum and beyond.
Read left to right, you're watching how much light gets through as wavelength increases. A well-engineered blue-light lens should show low transmittance at the shortest, harshest wavelengths, then rise — gradually, not suddenly — as it moves toward wavelengths where colour accuracy and natural light matter more.
A rising curve isn't a flaw. It's the lens easing off exactly where it should, instead of staying aggressive across the whole spectrum and turning every screen the same flat yellow.
Our own numbers, shown plainly
This is our lens, tested independently to ISO 12312-1:2022 and ANSI Z80.3-2018.
At 400nm, transmittance measures 0.00% — fully blocked. By 420nm, it's climbed to roughly 22%. By 450nm, it's at 93.83%. Visible light overall transmits at 90.35%. UVA and UVB transmittance both measure 0.00%.
Average that climb from 400nm to 450nm across the raw 5-nanometre data points in the report, and you get filtration of roughly 55% — which is where the number on our Technology page comes from. Not a single wavelength. Not a cherry-picked moment. An average across the whole range we claim to filter, calculated the same way we just showed you.
Read it yourself
You don't need to take our word for any of this. If a brand can't show you a curve — not a number, a curve — across a real wavelength range, that's worth noticing.
We'd rather you understood the report than just trusted the number.
