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Photochromic Tints • • • • • • • •

The Use of Photochromic Lenses in Solar Protection Stephen Freeman BSc(Hon’s),MCOptom, FBDO(Hon’s), Cert.Ed.

Purpose of a tinted lens

Definitions

• Cosmetic • Attenuation of unwanted radiations • Modification of incident radiation profile

• Tint – having a noticeable colour • Filter – to change the intensity and/or change the spectral distribution of the radiation passing through it • Sun-glasses – spectacles (or attachments) incorporating a filter for attenuating natural solar radiation.

Purpose of sunglasses

Optical range of radiation UV

visible

IR

Solar range of radiation UVC

100

UVB UVA

280 315 380

IRA

780

Definitions Solar Radiation Effects of harmful radiation How they work Development – Glass and Plastic Fatiguing Process Availability Conclusion

1400

IRB

IRC

3000

1000000

To provide protection from harmful radiation and comfort to the eyes under all conditions of illumination. Assuming constant bright sunlight immediate protection and visual comfort may be obtained by a fixed tint filter.

wavelength in nm

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CE regulations for Tinted lenses Class

Category

0

Clear or very Indoor or light tint overcast Light tint Low sunlight Medium tint Medium Sunlight Dark tint Bright Sunlight Very dark tint Very Bright Sun

1 2 3 4

Usage

Trans %

Driving

80 - 100

No Limitations

44 - 79

Not Suitable At Night Not Suitable At Night Not Suitable At Night Not Suitable Any Driving

19 - 43 9 - 18 3-8

Transmission Graphs Untreated CR39

T%

Transmission Graphs

Solar range of radiation

Transmission Graphs Untreated CR39 UV400

T%

Untreated CR39 UV400

T% Brown 50

Traffic Signal Recognition In addition to minimum UV transmission the regulations also require the tint to not distort road traffic signal colours. Expressed as a percentage of the ratio: General level of light transmittance to that at 4 specific wavelengths Red 80 % Yellow 80% Green 60 % Blue 40%

Transmission Graphs Untreated CR39 UV400

T% Brown 50

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Ultra-Violet radiation (UV)

Ultra-Violet radiation

• UVC wavelengths shorter than 280nm are filtered out by the ozone layer. Can vary in thickness depending on latitude and altitude. Potential hazard increase due to changes in stratospheric levels

• UVB 280 – 315nm causes sunburn and therefore effects the front of the eye and lids. There is little doubt that the deeper penetration causes cataract. • UVA 315 -380 nm penetrates deeper still and although there is association with lens changes, it is potentially the harmful effect of this band to the retina (including the high energy short wavelength visible light)

Ultra-Violet radiation

Ultra-Violet radiation

• Exposure to this hazard is increased by reflection from surfaces such as snow, sand and water. • Effects may be acute (photokeratitis) or chronic following long-term exposure (skin lesions, conjunctival changes, pterygium, age related cataract, age related macula degeneration).

• Untreated CR39 will give protection up to 350nm. • 70% will have cataract by our 70’s • 80% exposure to harmful UV happens before the age of 18 (College of Optometrists August 2009) • UV damage is cumulative • Aphakics will have lost natural protection of retina, also clear lens pseudophakic eyes

Ultra-Violet radiation

Purpose of sunglasses

• Some limited exposure to certain types of solar UV does promote the production of vitamin D • On balance, protective strategies such as sun-glasses, brimmed hats, uv-absorbing contact lenses will reduce the potential hazard. • A key feature of photochromic lenses is to absorb UV radiation

This fixed tint may be too dark or too light if illumination levels vary. Photochromic tint – modification of the light attenuation characteristics under the influence of solar radiation.

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Photochromic lenses A lens made all or partially from material which reversibly changes its transmission characteristics dependant upon the radiation itself (to include lenses that have photochromic properties). BS 3521-2:1991 (BS EN ISO 13666: 1999)

Photochromic glass • Silver halide crystals dispersed in the glass melt. • Exposure to short-wave light and UVA radiation decomposes the crystals into silver (100nm in size) and the halogen gas. • The silver will absorb the light to produce a tint in the lens

Photochromic glass

History - Photography • Technology originated from photographic film processing. • The sensitive surface of ordinary film is a layer of gelatin carrying minute suspended silver halide (AgX) crystals. • Exposure to light in a camera produces an invisible change yielding a latent image reduced to metallic silver by certain developing agents. Permanent change

Photochromic glass • The greater the intensity of the source the darker the lens will become until saturation point is reached. • The halogen cannot escape from the glass, so it recombines with the silver when the source is removed

Zeiss umbramatic

The crystals are between 800 to 1500nm in size suspended in the glass, with the addition of copper added to the melt to increase the speed of the reaction. Absorption of UVA and blue light is trigger for process.

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History Commercially available since 1964 Corning – Photogray (1968) Zeiss Umbramatic (1970) darkening to around 45% LTF Chance Pilkington – Reactolite (to 21% LTF)

Development By the early 80’s: Photogray Extra, Photobrown Extra Reactolite Rapide Brown/Grey Umbramatic,

History Mid 70’s major improvement in speed of reaction, different colours, pre-tinted forms Available, high refractive index, bifocals (fused and solid) and Progressives

Development Typically achieving 90% to 30% transmission in just 20 seconds exposure. Recovering to 65% in 5 minutes

Zeiss umbramatic

Photochromic glass

Zeiss umbramatic

Performance depends upon • Amount of light • Type of radiation • Temperature (LTF measured at 230C, 350C and 50C BS EN1836:2005) • Thickness of lens • Number of previous cycles (Variation should not exceed 5% in lightest, and 20% in darkest state after 500 activations

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But they are glass • Weight • Breakage (can be toughened) • Solid tint

Equitint (2mm lamina on 1.7/1.6 index glass)

Photochromic plastics materials • Safer • Lighter (in weight) • Available from 1970’s but speed of reaction much less and fatigued relatively quickly it took a number of years before a plastics photochromic became available that could compare favourably in performance.

Zeiss umbramatic equitint

Transitions • Launched in 1990 • Spiro-oxazines or spiro-pyrans and fulgides added to CR39 lens material. • Exposure to UV causes a portion of each molecule to rotate • Radiation is absorbed in this process • Removal of exciters causes molecules to flip back to original orientation

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Photochromic plastics materials • Dye imbibed into lens surface 100 -150 microns using chemical bath • Lens form and thickness therefore has no effect on appearance • Temperature has greater effect compared to glass (LTF measured at 230C, 350C and 50C BS EN1836:2005)

Fatiguing process • Speed of darkening/fading slows. • Decrease in transmission in both lightest and darkest state. • CE regulations state that transmission should not be less than 80% for night driving. • Indoor (or night-time) transmission can be improved with MAR finishes.

Fatiguing process DVLA Medical Advisory Panel (June 2007).. ‘Some photochromics even when fully bleached may only transmit of 80% of the light#The Panel recommends drivers should avoid using photochromic lenses at night.’ Focus - DVLA Newsletter May 2009 www.dvla.gov.uk

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Pride of Bilbao

Ouzo Yacht

Maritime Accident Investigation Branch (MAIB)

Maritime and Coast Agency (MCA)

Report (June 2007) of the loss of yacht Ouzo in August 2006: ‘Lookout’s spectacles were photochromic lenses which only transmitted 80% when tested after 30 minutes of darknessJ..may have been contributory factor’ ‘inferior to 5 others tested’ (Trans V (MAR), Zeiss, Rodenstock and Sunsensors) www.maib.gov.uk

Maritime Guidance Note MGN 357(2007) all ship operators, masters/skippers, officers and lookouts Recommendations: ‘Photochromic lenses should not be worn for lookout duties at night’ www.mcga.gov.uk

Transitions Original launch 1990, Transitions II 1992 Transitions III 1997, Transitions IV 2001 Transitions V 2005, Transitions VI 2008 All improvements on reaction time, unreacted and reacted transmissions, temperature performance, fatiguing effects etc.

Transitions VI • • • • •

Clearest LTF 93% Darkest LTF 20% after 2 minutes 25% LTF after 1 minute Fade back to 40% after 30 seconds 5 minutes to fade back fully

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Transitions VI

Transitions VI

Photochromic plastics materials

% of lenses dispensed with variable tint

• Available in huge range of lens forms • Available as CR39, Trivex (1.53), Spectralite (1.54), Polycarbonate, 1.6 and 1.67 index plastics Alternative production methods • Rodenstock Colormatic (1.54 and 1.67) • Signet Armolite (Kodak) Sunsensors 1.56 • Hoya Suntech 2 on 1.6 and 1.67

Combination tints Can be combined with polarising filters to produce very effective sunglasses such as: Serengeti (Corning glass photochromic lens now also available on trivex base) Drivewear (CR39 Transistions - 35%LTF to 12%LTF at 230C also available in polycarbonate)

June 2007 7% June 2008 9 % June 2009 12% Source – Opticians Index July 2009

Conclusion • Modern photochromic lenses provide 91%100% UVA and 100% UVB Protection • Appropriate advice regarding driving should be given. • MAR enhancements should be considered if spectacles used for general purpose.

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Conclusion • Photochromic lenses will darken behind windscreen but only 15% change. • Temperature change will cause up to 25% difference in transmission. • Improvements in performance (and huge range of lens forms) makes plastics photochromics something to consider for all spectacle lens wearers

Thankyou • Information sourced from manufacturers websites • Ophthalmic Lenses Availability (ABDO 2009)

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