Photochromic Lens Testing Standards, Metrics and Methodology

Categories: Consumer products & Textile, Safety & Compliance Testing, Performance Testing (QA), R&D, Transmittance

Faded and Darkened Photochromic Lenses

Photochromic lenses adapt to light, dimming when exposed to UV to enhance vision alongside offering protection from harmful optical radiation. First introduced in the 1960's, photochromic lenses were initially constructed from glass, with embedded silver chemicals that coalesce under UV exposure to obstruct incident light.

Photochromic lenses are now principally formed of plastic, featuring organic molecules which change conformation under light exposure. When exposed to UV wavelengths the structure of the organic molecules change, in turn adjusting the absorption spectrum.

Darkening state of photochromic lenses

International standards exist to ensure that photochromic lenses used in spectacle lenses and sunglasses do not adversely affect the perception of colour (for example of traffic signals) and ensure that, in common with standard sunglasses, suitable UV protection is offered to the eye. Recommendations are also given on limitations of the use of such lenses, for example, in night driving.


Scope of Photochromic Lens Testing Standards

Photochromic standards and legislation

EU Medical Devices Directive 93/42/EEC - Implementing EN ISO 8980-3-2013

This part of ISO 8980 specifies requirements for the transmittance properties of uncut finished spectacle lenses and mounted pairs, including attenuation of solar radiation.

This part of ISO 8980 is not applicable to

  • spectacle lenses having particular transmittance or absorption characteristics prescribed for medical reasons;
  • products where specific personal protective equipment transmittance standards apply;
  • products intended for direct observation of the sun, such as for solar-eclipse viewing.

EU Personal Protective Equipment Directive, 89/686/EEC - Implementing EN ISO 12312-1:2013

This part of ISO 12312 is applicable to all afocal (plano power) sunglasses and clip-ons for general use, including road use and driving, intended for protection against solar radiation.

Information on the use of sunglass filters is given in Annex A. Requirements for unmounted filters used as replacement or alternative filters are given in Annex B.

This part of ISO 12312 is not applicable to:

a) eyewear for protection against radiation from artificial light sources, such as those used in solaria;

b) eye protectors intended for specific sports (e.g. ski goggles or other types);

c) sunglasses that have been medically prescribed for attenuating solar radiation;

d) products intended for direct observation of the sun, such as for viewing a partial or annular solar eclipse

USA FDA CFR886-5850 - Implementing ANSI Z80.3:2015

This standard applies to all nonprescription sunglasses and fashion eyewear, normally used for casual, dress, and recreational purposes, having lenses of substantially plano power. This standard specifically excludes products covered by ANSI Z87.1, ANSI Z80.1, and those covered within the ASTM F08.57 committee. Sunglass needs for aphakics may not be met by this standard.

Consumer Protection Notice No. 13 of 2013 - Implementing AS/NZS 1067-2003

This Standard specifies minimum requirements for non-prescription eye and face protectors and associated oculars. They are designed to provide protection for the eyes and faces of persons against common occupational hazards such as flying particles and fragments, dusts, splashing materials and molten metals, harmful gases, vapours and aerosols. Requirements for optical qualities and low, medium, high and very high impact resistance are given and appendices describing appropriate test methods are included in this Standard.

Requirements for prescription-eye protectors against low and medium impact are given in AS/NZS 1337.6. Requirements for eye protectors against laser radiation are given in AS/NZS 1337, Parts 4 and 5.

The aim of this Standard is to assist in the provision of safe, efficient and comfortable vision in the occupational situation, including consideration of the need for protection against sunglare and optical radiation in the natural environment.


In order to evaluate a photochromic sample to international standards, the transmission of the sample should be measured over the spectral range 280-780nm prior to and after exposure to an AM2 simulator at 50 klx, representing the cases of the un-activated and activated photochromic. From these measurements can be reported the following key metrics.

τV0Luminous transmittance in the faded state at (23± 2)°C
τV1Luminous transmittance in the darkened state at (23± 2)°C
τV0 / τV1Photochromic response
τSUVAMean UVA spectral transmittance weighted by AM2
τSUVBMean UVB spectral transmittance weighted by AM2
QsignVisual attenuation coefficient for red, green, blue and yellow incandescent and LED traffic signals
τsbSolar blue light transmittance
  • Lens category in faded and darkened states
  • Colourimetric parameters in CIE 1931 & CIE Lab colour spaces

Photochromic lens transmission in faded and darkened state

In performing such measurements, the following are required:-

  • Spectrophotometer capable of measuring 280-780nm in presence of AM2 solar simulated light
  • Monochromatic beam of sufficiently low irradiance as to not elicit photochromic effect
  • Water bath to control temperature of sample under simulation and in temperature studies

Where specific claims are made with regards to low or high temperature performance, measurement should be repeated at 5 °C and 35 °C, and for night driving, tested under moderate activation level. Correction to air values should also be taken into account.


References

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