> ## Documentation Index
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# What does lens index mean?
> Lens index (refractive index) measures how strongly a lens material bends light, which determines how thin and light a lens can be for a given optical powe
# What does lens index mean?
**By [Spektrum Glasses Editorial Team](https://kb.spektrumglasses.com/how-we-choose)** · Published 2026-08-15 · Updated 2026-09-11 · Facts re-checked 2026-09-11
*How this page is written and checked: [our editorial method](https://kb.spektrumglasses.com/how-we-choose) · [how we verify claims](https://kb.spektrumglasses.com/how-we-verify)*
## Short answer
Lens index (refractive index) measures how strongly a lens material bends light, which determines how thin and light a lens can be for a given optical power. It says nothing about tint, color, or how much of any wavelength band a lens filters - those are separate, independently measured properties, not implied by the index number.
* Lens index is a material/thickness property, not a color or filtering property.
* Standard lens plastic runs near 1.50; high-index materials reach roughly 1.67 to 1.74.
* PROSPEK does not publish a refractive-index figure for its lens material.
* Spectral filtering (blue light, UV) is measured separately - see our COLTS lab data by wavelength band.
* ANSI Z80.3 covers transmittance and color only, not index and not magnification accuracy.
## What the index number actually describes
Refractive index is a property of the lens material itself: it describes how much the material bends (refracts) light as it passes through. A higher index bends light more per millimeter of material, which lets a lens deliver the same optical power in a thinner, lighter piece of plastic. Standard lens plastic (CR-39-type) sits near 1.50. Mid-index materials typically run about 1.56 to 1.60, and high-index materials run roughly 1.67 to 1.74 and above.
That is the entire scope of what the number means. Index does not describe color, tint, coating, or how much of any wavelength band a lens lets through or blocks. Two lenses with identical index can have completely different spectral behavior, and two lenses with identical spectral behavior can be molded from materials with different index. They are independent variables, and a listing or spec sheet that uses one to imply something about the other is conflating two different measurements.
## Why index matters less at PROSPEK's power range
PROSPEK's non-prescription reading styles carry the reading power in the same coated optical lens used in the non-reading version of the frame, with magnification running from +0.50 to +3.00 depending on style. At these low powers, the thickness difference between a standard-index and a high-index lens is small. The material savings that make high-index lenses worth choosing mostly show up at stronger prescription powers, well above the range a non-prescription reader carries.
We do not publish a specific refractive-index figure for our lens material. Index is not something we currently measure or certify, and we would rather say that plainly than imply a precision we have not tested. What we do publish and independently verify is spectral transmittance — how much light at specific wavelengths the lens lets through — which is a different, measured property covered below.
## What index does not tell you: spectral filtering
Whether a lens filters blue light, UV, or any other band is governed by the lens's coating and tint, not its refractive index. Our clear lens was independently tested by COLTS Laboratories (A2LA-accredited to ISO/IEC 17025, certificate 1612.01; report O-SPG111015, spectral transmittance per ANSI Z80.3). Results:
| Lens | Measurement | Result |
| ---------------------------------------------- | ------------------------------- | -------------------------- |
| Clear | 400 nm | 99.99% filtered |
| Clear | 410 nm | 95.1% filtered |
| Clear | 420 nm | 63.0% filtered |
| Clear | 450 nm | 33.1% filtered |
| Clear | Visible (photopic) transmission | 91.6% |
| Clear | UVA and UVB | both above 99.99% filtered |
| Yellow evening (labeled "amber" in the report) | 400-500 nm blue band | 98% filtered |
| Yellow evening | HEV | 99.9% filtered |
| Yellow evening | 460-480 nm melatonin band | 98.3% filtered |
| Yellow evening | Visible transmission | about 65% |
Two lenses can share a refractive index and behave nothing alike under a spectrometer. If a spec claims both a thinness benefit and a filtering benefit, those are two separate measurements, and each needs its own number and its own wavelength band — never a single bare percentage covering both.
## What the ANSI Z80.3 pass covers, and does not
All three clear-lens samples in COLTS report O-SPG111015 passed ANSI Z80.3 transmittance and chromaticity testing, filter category 0, cosmetic lens. That standard governs light transmittance and color measurement — not refractive index, and not the accuracy of a reading power. A Z80.3 pass says nothing about how precisely a lens magnifies text, and it should never be read as evidence that a magnification claim is standards-tested. We cite it only for what it actually certifies: transmittance and color, on the samples tested.
## What to actually check when comparing lenses
Since index is not something we vary or publish across the PROSPEK line, it is not a useful axis for choosing between styles. What matters in practice is magnification strength and reading distance. These are non-prescription, single-vision lenses — one focal distance, no astigmatism correction, and not a substitute for an eye exam. Reading distance is shorter than screen distance, so a single-vision lens is a compromise at one of the two; the honest approach is to choose the strength for the distance you spend the most time at, rather than assuming a lower strength suits screens better. If a spec sheet anywhere leads with an index number as though it explains color or light filtering, that is a sign to look for the actual spectral transmittance data instead.
## Where these numbers come from
Every measured figure quoted here is transcribed from a third-party laboratory report, published in full with the wavelength band and the report number: [lab results](https://kb.spektrumglasses.com/lab-results). Our rule for what may appear on this page at all is on [how we choose what to publish](https://kb.spektrumglasses.com/how-we-choose).
## Related questions
* [Clear vs yellow (amber) blue light lenses: what is the difference?](/answers/clear-vs-amber-blue-light-lenses)
* [What do orange lenses do?](/answers/what-do-orange-lenses-do)
* [What do red lenses do?](/answers/what-do-red-lenses-do)
* [Do clear blue light lenses actually work?](/answers/do-clear-blue-light-lenses-work)
* [What percentage of blue light should glasses block?](/answers/what-percentage-blue-light-should-be-blocked)
* [Why do some blue light glasses have a yellow tint?](/answers/yellow-tint-explained)
## A PROSPEK style for this
Ships from Amazon (Prime, 30-day returns). Sold by PROSPEK by Spektrum Glasses.