How do photochromic lenses work?
Photochromic lenses contain millions of tiny organic photochromic molecules that allow visible light to pass through their structure. However when activated with UV light they alter their shape forming a structure that blocks visible light. As the photochromic dye package is made up of various types of photochromic molecules which all have different characteristics these combine to deliver the activated colour and intensity.
What is the affect of temperature on photochromic lenses?
Photochromic lenses are constantly adapting to changing light conditions. On the one hand they are activated by UV light so move to their closed "light blocking" position , however thermal energy (temperature) is the fuel to open back up the molecular structure to allow light to pass through. Therefore at every moment the lens is constantly adjusting and balancing the input of UV light against thermal energy. That is why on very hot overcast days a photochromic lens will not be as dark as on a very cold sunny day. The fade back speed is also affected by the temperature, for example on very hot days the molecules are energized and quickly fade back.
What is the typical daily journey of a photochromic lens. A day in the life of a photochromic lens.
Photochromic lenses are everyday lenses that do more. They are clear indoors and outdoors at night, but in daylight automatically & continuously adjust to outdoor (UV) light to help reduce glare, eye fatigue and protect the eyes from harmful UV.
7 AM:
Indoors the lens is clear indoors allowing the maximum amount of light to reach the eye.
8 AM
On the way to work in bright sunlight the lens instantly darkens when going outdoors and can reach (category 3) activation in around 45 seconds. This rapid darkening helps to improve comfort by reducing glare.
8:30 AM
Walking to work, it's now cloudy and the lens % tint has reduced as the amount of UV light reaching the lens is also reduced.
9 AM
On reaching the office, the lens fades back reaching 80% transparency in 6 minutes. On very cold sunny days it will take a bit longer. The lenses stay clear while in the office as office lights do not contain UV light so the lenses do not activate.
5 PM
Driving home from the office, in the car the lenses remain clear as the windscreen blocks UV rays. As evening approaches, the lenses are now clear but my eyes feel less fatigued as the lenses have been working to reduce glare all day, so cumulatively they feel less tired.
How long do photochromic molecules last?
All photochromic molecules age as they are constantly adjusting, however they continue to work even if their range of activation and deactivation becomes more restricted overtime. Under normal wearing conditions the lenses will last longer than the average (3 years) life of the patients prescription. It is however possible to damage photochromic molecules by exposing them to intense heat. eg if you leave your glasses on the dashboard of a car, they could become so hot that the photochromic molecules are damaged so their life expectancy is reduced.
All photochromic lenses sold on the European & UK market have to pass ISO 14889 & ISO8980:3 which is specifically for photochromic spectacle lenses. Within this standard, all lenses are subject to an accelerated aging process which simulates 5000 hours of sunlight exposure. The lens only passes the standard if it can maintain an indoor transparency of not more than 5% difference from the original transparency.
“The absolute change in the luminous transmittance (τV’ - τV) of the lenses shall be less than or equal to 5 % absolute where τV’ is the luminous transmittance after factory process.”
How are the photochromic molecules attached to the lens?
Superchromic Primo lenses are manufactured using a spin coating process, where the photochromic dye package is placed onto the lens substrate while it is being spun at high speed. This allows the photochromic dye to be evenly covered over the surface of the lens at a consistent thickness. The lens and photochromic layer is then cured and a non removal hard coat is applied to secure and protect the photochromic layer.
Superchromic Essential lenses are manufactured using the in-mass process where the photochromic dye package is introduced to the optical monomer before casting. This provides a very stable and cost effective solution for the production of photochromic lenses.
Patient Benefits
Superchromic lenses constantly adjust to changing light conditions throughout the day. When a wearer first steps outside into daylight, the lenses darken to reduce glare, making it more comfortable to see. As the light changes, the lenses continue to adjust, ensuring optimal vision and comfort. This continuous adjustment helps reduce eye fatigue by providing the right level of tint for every lighting situation.
