Carl Zeiss Vision’s new photochromic lenses – PhotoFusion – uses a new patented technology that responds quickly to changes in ultraviolet irradiation resulting in a rapid change between clear to dark, then back again.
Since the introduction of photochromic plastics in 1983, the technology has continued to improve. Each successive generation of photochromic lens materials has provided wearers with greater performance and value.
PhotoFusion lenses utilise a patented chemistry that offers faster reaction speeds, greater protection from harmful radiation, and longer-lasting performance.
Evolution of Photochromic Technology
Photochromic or ‘self-tinting’ spectacle lenses have a tint that automatically increases or decreases in transmittance in response to ambient light levels. Photochromic lenses change from a colourless or ground state indoors to a darkened or activated state outdoors when exposed to the ultraviolet (UV) radiation in sunlight.
The novel naphthopyran molecules used in PhotoFusion lenses respond more rapidly to changes in ultraviolet irradiation compared to other photochromic molecules…
Plastic photochromic lenses work using leuco dyes, which contain special molecules that change reversibly between two forms-or isomers-depending upon the relative balance between thermal and ultraviolet radiation. UV radiation drives the molecules to their activated state. Photochromic lenses provide the convenience of clear vision indoors with effective sun protection outdoors.
Since photochromic plastic lenses were first introduced in 1983 by American Optical, a division of Carl Zeiss Vision, ongoing innovations in these remarkable materials has improved the performance of the lenses with each new generation.
Modern photochromic plastics offer greater thermal stability with less temperature dependence. These lenses also now offer greater photochromic ‘swing’ between the ground (clear) and activated (dark) states, achieving higher transparency indoors and greater absorption outdoors.
PhotoFusion lenses represent the latest advancement in photochromic plastics by Carl Zeiss Vision, offering improved overall photochromic performance combined with several enhancements that wearers will appreciate:
- Faster response outdoors and indoors;
- Greater protection from harmful radiation; and
- Truer, longer lasting colour.
Faster Response Outdoors and Indoors
PhotoFusion lenses employ a patented photochromic technology based upon the chemistry of indeno-fused naphthopyran molecules.* These molecules are photo-reactive chromophores that absorb visible light when exposed to the UV radiation in sunlight, resulting in a self-tinting lens material.
The novel naphthopyran molecules used in PhotoFusion lenses respond more rapidly to changes in ultraviolet irradiation compared to other photochromic molecules, changing quickly between the ground (clear) state and activated (dark) state.
The efficiency of the photochromic change with respect to the amount of ultraviolet radiation absorbed by the material is referred to as the quantum yield. Because of their high quantum yield, PhotoFusion lenses activate with less ultraviolet radiation, allowing them to darken more quickly (Figure 1).
When darkening, the average luminous transmittance of PhotoFusion lenses decreases to 20 per cent in bright sunlight in about 41 seconds. The lenses eventually decrease to an average transmittance of 11 per cent and PhotoFusion lenses satisfy the ISO 8980-3 requirements for dark (Category 3) lenses.
A major drawback of photochromic lenses has been the length of time required for the lenses to fade back to clear when going from outdoors to indoors. Photochromic molecules are suspended in a three-dimensional polymer network. In order for each molecule to change form easily between the ground state and activated state, the molecule must have sufficient space to move. If the size of the free volume surrounding each molecule within the polymer matrix is not large enough, the photochromic change is slowed, particularly when fading back.
The free volume surrounding the naphthopyran molecules in PhotoFusion lenses has been optimised with a larger, more open matrix in order to allow the molecules to change form more quickly from the activated state to the ground state (Figure 2).
The lower accumulation of energy required as a result of a high quantum yield accelerates the fading process further. When fading, the average luminous transmittance of PhotoFusion lenses increases to 80 per cent indoors in about six minutes. The lenses approach the transparency of clear glass in just minutes. Existing lenses from Carl Zeiss Vision, however, may take up to twice as long to fade back to this point.
Unlike previous photochromic lenses from Carl Zeiss Vision, PhotoFusion lenses absorb hazardous blue light radiation up to 400 nm as well as absorb all UVA and UVB radiation.
Eyeglass wearers are exposed to illumination levels that can vary during the day from as high as 100,000 lux in direct sunlight to less than 10 lux in a dimly lit room, representing a dynamic range of 10,000 to 1. Excessive levels of illumination, including bright sunlight and sources of glare in the visual field, can cause ocular discomfort, squinting, and eyestrain. Visual acuity is also reduced under extremely high illumination.1 Excessive glare can impair visual function completely. Additionally, prolonged exposure to bright sunlight can later delay the start of dark adaptation and slow the dark adaptation process, impairing visual performance at night.2
Although the pupil of the light-adapted eye can mediate retinal illumination to some extent, a tinted lens with the proper transmittance offers superior control of excess brightness.
Lighting engineers typically recommend no more than 1,000 lux of uniform illumination for comfortable vision in office environments. This level will also provide maximum visual acuity. Typical daylight, on the other hand, can produce 10,000 lux or more of illumination, representing a ratio of 10 to 1.
In order to reduce this daylight illuminance to a more desirable level, a spectacle lens would need to transmit incident light at a ratio of 1 to 10, or a luminous transmittance of 10 per cent.3 Because the wearer is exposed to a wide range of light levels throughout the day, PhotoFusion lenses, self-adjust to the ideal level of absorption in order to maximise visual comfort and performance.
PhotoFusion lenses are capable of reaching near-sunglass density under intense sunlight and offer this protection from harmful radiation and excess brightness in two colours – neutral gray hue for lifelike colour perception and warm brown hue.
Truer, Longer Lasting Colour
The various chromophores utilised in leuco dyes each produce a distinct absorption spectrum when exposed to UV radiation, depending upon their particular molecular structure. In practice, a single chromophore may not be available with the desired absorption spectrum. In order to obtain a specific hue, the use of two or more chromophores is often required in leuco dyes in order to create a combined absorption spectrum that achieves – or nearly achieves – the desired colour. Because different chromophores may respond differently to temperature, however, variations from the desired colour are common in either hot weather or cold weather.
The patented chromophores used in PhotoFusion lenses have wide absorption spectra that allow for better colour control. PhotoFusion lenses therefore exhibit excellent colour stability with less variation in colour compared to existing lenses from Carl Zeiss Vision. Further, the colour of each chromophore has been carefully tuned to natural sunlight. As the lenses change colour outdoors, PhotoFusion lenses maintain either a neutral gray hue – with no bluish undertone – or a warm brown hue – with no greenish undertone (Figure 3).
Like all organic dyes, photochromic leuco dyes are susceptible to degradation by oxygen and free radicals after continual exposure to UV radiation and environmental elements. Over time, photochromic plastics will suffer a reduction in photochromic swing between the ground state and the activated state due to photo-oxidation precipitated by UV radiation. Additionally, photochromic plastics are subject to discolouration as the individual chromophores deteriorate, losing transparency indoors and, in some cases, assuming a less desirable colour.
The discolouration and loss in reversibility of the photochromic change is referred to as fatigue. Photochromic fatigue can be reduced by adding a UV stabiliser or by providing a barrier to oxygen and other chemicals. This prolongs the expected period of adequate photochromic performance. PhotoFusion lenses employ hindered amine light stabilisers within the polymer matrix, which are chemical additives that act to inhibit the degradation of the polymer. These stabilisers slow down the photochemically-initiated degradation reactions, working in a manner similar to antioxidants.
PhotoFusion lenses exhibit minimal loss in transmittance in the ground (clear) state after prolonged exposure to UV radiation compared to existing lenses from Carl Zeiss Vision, which may suffer from more noticeable fatigue after extended use.
PhotoFusion lenses have been engineered with significant improvements over existing photochromic lenses from Carl Zeiss Vision. The patented photochromic molecules in PhotoFusion lenses react more quickly to ambient light levels, both outdoors and indoors; offer even greater protection against harmful radiation, up to 400 nm; and maintain excellent colour stability, with less loss in performance over time. PhotoFusion self-tinting lenses from Carl Zeiss Vision, in either gray or brown, offer wearers a state-of-the-art combination of performance and convenience.
Darryl Meister s the Technical Marketing Manager for Carl Zeiss Vision and has been an active member of the Optical Industry since 1990. He was one of the youngest opticians ever to earn the American Board of Optometry’s Master certification, contributed to many important industry initiatives, written for several optical publications and lectured extensively.
1. Hecht S., Hendley, C., and Ross, S. “The effect of exposure to sunlight on night vision.” Am J Ophthalmol, 1948; Vol. 31, No. 12, pp 1573-1580.
2. Peckham, R. and Harley, R. “Reduction in visual acuity due to excessive sunlight.” Arch Ophthalmol, 1950; Vol. 44, No. 4, pp 624-627.
3. Richards O. “Sunglasses for Eye Protection.” Am J Optom Arch Am Acad Optom, 1971; Vol. 48, No. 3, pp 200-203.
* US Patents 5,910,516; 6,723,859; and 7,521,004. Other patents pending.