How is blue light filtering measured?
By Spektrum Glasses Editorial Team · Published 2026-08-07 · Updated 2026-08-07 · Facts re-checked 2026-08-07 How this page is written and checked: our editorial method · how we verify claimsShort answer
Blue light filtering is measured on the lens with a spectrophotometer, not on the eye. The instrument records spectral transmittance: the percentage of light passing through at each wavelength, typically every 1 to 5 nm. Filtering equals 100% minus transmittance at a stated wavelength, or averaged across a stated band. A percentage with no band is uninterpretable.- Filtering is derived from a transmittance curve, not from a single number.
- A percentage is only meaningful with the wavelength or band attached.
- PROSPEK clear lens: 99.99% filtered at 400 nm, 33.1% at 450 nm.
- Heavier filtering across 400-500 nm requires visible tint; that is physics, not branding.
- Accurate lens measurement says nothing about what a wearer will experience.
What the measurement actually is
Blue light filtering is measured on the lens, in a laboratory, using a spectrophotometer. The instrument sends a known quantity of light at the lens one narrow wavelength slice at a time and records how much comes out the other side. The output is a spectral transmittance curve: percent transmitted at each wavelength, usually sampled every 1 to 5 nm across the ultraviolet and visible range. Filtering at any wavelength is simply 100% minus transmittance at that wavelength. Everything a brand says about blue light is derived from that curve. A “blocks X%” headline is one figure pulled out of several hundred data points, and which figure you get depends entirely on where in the curve you pull from. That is why the curve, not the headline, is the primary evidence. For non-prescription eyewear the transmittance method commonly used is ANSI Z80.3, which defines how transmittance is computed for fashion and sunglass lenses. The method matters less than who ran it. A laboratory accredited to ISO/IEC 17025 by a body such as A2LA has had its procedures, equipment calibration and competence independently audited, and its reports carry a traceable number. The PROSPEK clear lens data below comes from COLTS Laboratories report O-SPG111015, A2LA certificate 1612.01.Why a percentage without a band is meaningless
Blue light is conventionally the 400-500 nm region of the visible spectrum. Transmittance can change dramatically across it, which is exactly why a single unqualified number is a warning sign. The measured PROSPEK clear lens shows how steep that drop can be:
Every number in that table is accurate and comes from the same report. Quoting only the first row as “blocks 99.99% of blue light” would be technically sourced and practically misleading, because the same lens filters 33.1% at 450 nm, deeper into the band. Both facts belong together.
This is a physical limit of near-clear lenses generally, not a quirk of one product. The PROSPEK clear lens has 91.6% visible (photopic) transmission, meaning it looks essentially clear. The ZENOX clear lens averages about 52% across the blue band, with 100% of UV filtered. A clear lens can be very strong at the short-wavelength edge of the band and modest in the middle of it. Any honest figure therefore takes one of two forms: a percentage at a stated wavelength, or an average across a stated band.
Three legitimate ways to compute the number
There are at least three defensible ways to turn a transmittance curve into one percentage, and they produce different answers from identical measurements.- Single wavelength. Filtering at one point, for example 99.99% at 400 nm. Precise, verifiable, and easy to cherry-pick.
- Band average. The mean across a stated range, for example the PROSPEK amber evening lens at 97.9% across 400-500 nm and 98.3% across the 460-480 nm melatonin-sensitive band. Harder to game, because the range is declared.
- Weighted calculation. Each wavelength is weighted by a photobiological hazard function before averaging, so the result depends on both the lens and the assumed light source. Weighted figures are usually far lower than band averages. Leung, Li and Kee (PLOS ONE, 2017) measured commercially available blue-light-filtering lenses and found they reduced the calculated blue-light hazard by roughly 10-24%.
Tint is the constraint measurement keeps exposing
Filtering works by absorbing or reflecting light. Removing most of the 400-500 nm energy removes most of what the eye reads as blue, which necessarily changes how colors appear. Measurement makes that tradeoff explicit rather than rhetorical.
The orange and red lenses are not suitable for driving or for any task where accurate color judgment matters. Neither is any lens filtering nearly all of 380-500 nm. That limitation is inherent to the measured curve.
One honesty note about evidence quality: the clear and amber figures come from the A2LA-accredited COLTS report. The orange and red figures were measured in 2026 by our lens manufacturer’s own optical laboratory. Both are real measurements, but manufacturer-run testing is a weaker form of evidence than accredited third-party testing, and we would rather say so than blur the two.
How to read a lab report critically
If a brand publishes a number, these are the things worth asking for. Absence of any of them is informative in itself.- The laboratory name, report number, and accreditation status, so the report can be requested and checked.
- The method, for example ANSI Z80.3, and confirmation that a production lens was tested rather than a raw material coupon.
- The wavelength or band attached to every percentage, with the band’s numeric limits stated.
- Photopic (visible) transmission, which reveals how tinted the lens actually is.
- Ideally the curve itself, or at minimum several sampled wavelengths across 400-500 nm rather than only the strongest one.