Photochromic lenses adapt to light, dimming upon ultraviolet (UV) exposure to improve vision while ensuring protection from hazardous optical radiation. First launched in the 1960s, photochromic lenses were originally made using glass, with embedded silver chemicals that coalesce when exposed to UV to obstruct the incident light.
Nowadays, plastic is mainly used to construct photochromic lenses, featuring organic molecules which change conformation when exposed to light. The organic molecules change their structure upon exposure to UV wavelengths and as a result, the absorption spectrum is adjusted.
International standards are established to ensure that photochromic lenses employed in sunglasses and spectacle lenses do not adversely affect the perception of color (for instance of traffic signals) and ascertain that, in common with standard sunglasses, suitable UV protection is provided to the eye. Recommendations are also provided on limitations of the use of these lenses, for instance, in night driving.
To evaluate the compliance of a photochromic sample with international standards, the transmission of the sample needs to be measured over a spectral range of 280-780 nm before and after exposure to an AM2 simulator at 50 klx, representing the cases of the activated and un-activated photochromic. The following key metrics can be reported from these measurements:
||Luminous transmittance in the faded state at (23± 2) °C
||Luminous transmittance in the darkened state at (23± 2) °C
|τV0 / τV1
||Mean UVA spectral transmittance weighted by AM2
||Mean UVB spectral transmittance weighted by AM2
||Visual attenuation coefficient for red, green, blue and yellow incandescent and LED traffic signals
||Solar blue light transmittance
- Lens category in faded and darkened states
- Colorimetric parameters in CIE 1931 & CIE Lab color spaces
Lens transmission in faded and darkened state.
The following are essential to perform such measurements:
- Spectrophotometer with the ability to measure 280-780 nm in the presence of AM2 solar simulated light
- Monochromatic beam of sufficiently low irradiance as to not elicit photochromic effect
- Water bath to control sample temperature under simulation and in temperature studies
Where specific claims are made with respect to high or low temperature performance, measurement should be repeated at 5 °C and 35 °C, and for night driving, tested under moderate activation level. In addition, correction to air values should be considered.
This information has been sourced, reviewed and adapted from materials provided by Bentham Instruments Limited.
For more information on this source, please visit Bentham Instruments Limited.