Research
Spectral Transmittance and Wavelength-Selective Optical Filters
Updated 19 October 2026 / Sleepaxa Private Limited, NeuroCalm FLX+ editorial team
In short
Spectral transmittance is the fraction of light a lens passes at each wavelength; luminous transmittance is the single overall figure obtained by weighting that curve for daytime vision. Together with the test method, instrument, sample conditions and uncertainty, these define what a wavelength-selective filter does. A curve is only evidence when it is a measured trace with its provenance; an illustration shaped to published band averages, like the one on this site, is not.
Claims about tinted and filtering lenses are easy to make and hard to compare. Two lenses can look the same colour and pass very different amounts of light in the bands that matter; two products sold under one name can have different curves. This page sets out the quantities, standards and reporting conventions that make transmittance data interpretable, and it states plainly which data for NeuroCalm FLX+ exist today and which do not.
Spectral transmittance chart. Illustrative curve shaped to the band-averaged values in granted Indian Patent 587746 (FIG. 3, Table 2); not a spectrophotometer trace. Wavelength 380 to 700 nanometres on the horizontal axis, transmittance 0 to 100 percent on the vertical axis. 460 to 490 nm attenuated, 83 ± 2 percent (granted claim 1), mean 17 percent; 520 to 560 nm green comfort band, 23 to 25 percent transmission peak (granted claim 1), mean 24 percent; 585 to 600 nm attenuated, 78 ± 2 percent (granted claim 1), mean 22 percent. Use the left and right arrow keys to move the wavelength cursor by one nanometre, Shift with an arrow for ten, Home and End for the ends of the range, Enter or Space to select or deselect the band under the cursor, Escape to clear.
| Band | Role | Mean transmittance (percent) |
|---|---|---|
| 460 to 490 nm | Attenuated, 83 ± 2 percent (granted claim 1) | 17 |
| 520 to 560 nm | Green comfort band, 23 to 25 percent transmission peak (granted claim 1) | 24 |
| 585 to 600 nm | Attenuated, 78 ± 2 percent (granted claim 1) | 22 |
The quantities that describe a filter
- Spectral transmittance T(lambda): transmitted divided by incident spectral radiant flux at each wavelength, usually reported from 280 or 380 nm to 780 nm in 1 to 5 nm steps.
- Luminous transmittance (Tv): the spectral curve weighted by the photopic sensitivity of the eye and by a standard daylight illuminant, giving one percentage that describes overall darkness.
- Band-averaged transmittance or attenuation: the mean over a stated band, for example 460 to 490 nm. Attenuation is 100 percent minus transmittance. Band edges must be stated.
- Optical density: the negative base-10 logarithm of transmittance, used for very dark filters.
- Alpha-opic quantities: the curve weighted by photoreceptor functions defined in CIE S 026, for example melanopic transmittance. These describe light, not biological response.
- Blue-light hazard weighting under IEC 62471 and the solar UV weighting used for UV-blocking claims are further weightings of the same curve.
Filter categories and driving suitability
The sunglasses standard also requires a minimum luminous transmittance of 75 percent for lenses used for driving at night or in twilight, and tests whether traffic-signal colours remain recognisable through the filter. A category 2 lens, such as the patentee's test table reports for NeuroCalm FLX+ (Tv 21.72 percent), therefore cannot be a night-driving lens regardless of its spectral shape.
| Category | Luminous transmittance | Typical use |
|---|---|---|
| 0 | above 80 percent to 100 percent | Clear or very light tint |
| 1 | above 43 percent to 80 percent | Light tint, overcast conditions |
| 2 | above 18 percent to 43 percent | Medium tint, average sunlight |
| 3 | above 8 percent to 18 percent | Dark tint, bright sunlight |
| 4 | above 3 percent to 8 percent | Very dark; not for driving |
How transmittance is measured
Spectacle-lens transmittance is measured with a spectrophotometer under the conditions of ISO 8980-3: a defined measurement area near the optical centre, normal incidence, a stated wavelength range and step, and a reference measurement without the lens. A complete report states the instrument and its calibration, the lens sample (material, thickness, any coatings, prescription if not plano), the number of samples and batches, the date and operator or laboratory, and the measurement uncertainty. Band averages should be computed from the full-resolution trace, not read from a graph.
Reports that omit the laboratory, the sample description or the uncertainty are patentee or vendor self-reports. They can be accurate, but they cannot be checked, and they should be labelled as such.
Sources: [1]
Illustrative visualisation versus measured transmittance on this site
The chapter 05 explorer on the homepage and the chart on the technology page show an illustrative curve. It is a piecewise interpolation through the band-averaged values the granted specification of Indian Patent 587746 reports: about 1 percent below 400 nm, 17 percent at 460 to 490 nm, a 23 to 25 percent peak at 520 to 560 nm, 22 percent at 585 to 600 nm, rising to about 47 percent in the red. The shape between those anchors is drawn, not measured, and the anchors themselves are the patentee's figures.
A verified measured dataset would replace the illustration. The site already carries a machine-readable schema for such a file (instrument, laboratory, date, standard edition, sample description, per-wavelength values and uncertainty) and a loader that rejects any file not marked as measured. Until a report meeting those conditions is supplied, every percentage shown remains a claimed value.
| Wavelength region | Approximate transmittance | Basis |
|---|---|---|
| Below 400 nm | about 1 percent | Claim 3: at least 99 percent UV blocking |
| 400 to 450 nm | about 6 to 7 percent | Specification FIG. 3 bins |
| 460 to 490 nm | about 17 percent | Claim 1: 83 ± 2 percent attenuation |
| 520 to 560 nm | 23 to 25 percent peak | Claim 1: green comfort band |
| 585 to 600 nm | about 22 percent | Claim 1: 78 ± 2 percent attenuation |
| 610 to 700 nm | rising to about 47 percent | Specification FIG. 3 bins |
Comparing filters honestly
- Compare curves measured under the same standard, on named samples, with the same wavelength step.
- Do not assume that every lens sold as FL-41 has the same curve; laboratories tint to different depths and the name is not a specification.
- State band edges whenever quoting band attenuation; 83 percent at 460 to 490 nm is a different quantity from blue-light transmittance over 380 to 500 nm.
- Keep luminous transmittance next to any band figure, so the reader knows how dark the lens is overall.
- Treat published measurements of other products, such as the 86 to 93 percent luminous transmittance measured for commercial blue-light lenses, as examples of transparent reporting, not as comparators for a lens that has no equivalent independent data.
Evidence: established versus emerging
Established
- The definitions of spectral and luminous transmittance, filter categories and the measurement conditions in the cited standards.
- The luminous-transmittance requirement for night and twilight driving.
| Level | Status |
|---|---|
| Measured laboratory transmittance of NeuroCalm FLX+ (independent) | Not published. |
| Patentee-reported figures | Band averages in the granted specification; EN ISO 12312-1 table in the FER reply (laboratory unnamed). |
| Illustrative visualisation | Interpolation through the patentee's band averages; labelled on every chart. |
Questions and evidence-based answers
How do optical filters affect spectral transmittance?
A filter multiplies the incoming spectrum by its own transmittance curve. Absorptive tints lower broad regions; dyes and thin-film interference coatings can target narrower bands. The result is read as a transmittance curve and summarised by luminous transmittance and band averages.
What should a transmittance report for a lens include?
The standard used (for spectacle lenses, ISO 8980-3), instrument and calibration, sample description, number of samples, wavelength range and step, the full curve, luminous transmittance, measurement uncertainty, laboratory and date.
Sources: [1]
What should clinicians consider when evaluating light-filtering lenses?
Whether a measured curve with provenance exists, how dark the lens is overall and whether that suits the wearer's tasks and driving, whether the claimed band edges match the stated rationale, and whether any clinical claim is backed by a trial of that specific lens rather than of another product.
Related technology
The NeuroCalm FLX+ technology page lists the patentee's reported figures, the filter category and the use limits that follow from them.
References
- ISO 8980-3, Ophthalmic optics, Uncut finished spectacle lenses, Part 3: Transmittance specifications and test methods. International Organization for Standardization. (Cited by designation; licensed text not opened.)
- ISO 12312-1, Eye and face protection, Sunglasses and related eyewear, Part 1: Sunglasses for general use (filter categories 0 to 4 by luminous transmittance; driving suitability). International Organization for Standardization. (Cited by designation; licensed text not opened.)
- CIE S 026:2018 CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light. International Commission on Illumination, 2018. (Cited by designation; licensed text not opened.)
- Transmittance (spectral transmittance, optical density, luminous transmittance). Wikipedia.
- Notch filters: stop band, centre wavelength, FWHM, optical density, angle of incidence. EKSMA Optics product documentation.
- Leung TW, Li RW, Kee CS. Blue-Light Filtering Spectacle Lenses: Optical and Clinical Performances. PLoS One, 2017. DOI 10.1371/journal.pone.0169114.
- Published granted specification IN587746 B1: An optical lens with dual-band selective light attenuation (10 claims). Intellectual Property Office, India, 2026.
- Reply to the First Examination Report with EN ISO 12312-1 test table. Sleepaxa Private Limited (applicant submission through its patent agent), 2025.
- Katz BJ, Digre KB. Diagnosis, pathophysiology, and treatment of photophobia. Survey of Ophthalmology, 2016. PMID 26875996. DOI 10.1016/j.survophthal.2016.02.001.
About this page
Author: Sleepaxa Private Limited, NeuroCalm FLX+ editorial team. Published: 19 October 2026. Last updated: 19 October 2026.
Review status: Referenced educational draft prepared from the cited peer-reviewed sources and official patent documents. Independent clinical review by a named professional is pending; no part of this page has been reviewed by a physician.
Editorial policy: Every scientific statement cites a source that was opened or retrieved by identifier. Statements about NeuroCalm FLX+ are limited to official patent documents and are labelled as the patentee's figures. No cure, treatment or prevention claims are made for any product. Corrections: connect@sleepaxa.com.
Medical disclaimer: This page is educational and is not medical advice, diagnosis or treatment. It does not replace an examination by an eye-care professional or physician. NeuroCalm FLX+ is an optical lens, not a medical device, and no statement here claims that it treats, cures or prevents any condition.