Photochromic Lens Testing Standards, Metrics and Methodology
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.
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.
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.
|τV0||Luminous transmittance in the faded state at (23± 2)°C|
|τV1||Luminous transmittance in the darkened state at (23± 2)°C|
|τV0 / τV1||Photochromic response|
|τSUVA||Mean UVA spectral transmittance weighted by AM2|
|τSUVB||Mean UVB spectral transmittance weighted by AM2|
|Qsign||Visual attenuation coefficient for red, green, blue and yellow incandescent and LED traffic signals|
|τsb||Solar blue light transmittance|
- Lens category in faded and darkened states
- Colourimetric parameters in CIE 1931 & CIE Lab colour spaces
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.
- EU Personal protective equipment directive, 89/686/EEC
- ISO 12312-1:2013 “Eye and face protection -- Sunglasses and related eyewear -- Part 1: Sunglasses for general use”
- EU Medical devices directive, 93/42/EEC
- ISO 8980-3:2013, “Ophthalmic optics -- Uncut finished spectacle lenses -- Part 3: Transmittance specifications and test methods”
- BS EN ISO 14889:2013, “Ophthalmic optics. Spectacle lenses. Fundamental requirements for uncut finished lenses”
- ANSI Z80.3-2015 “Ophthalmics - Nonprescription Sunglass and Fashion Eyewear Requirements”
- AS/NZS 1067.1:2016 Eye and face protection - Sunglasses and fashion spectacles - Requirements