Domains and learning outcomes - C109162

• One distance learning CPD point for optometrists and dispensing opticians

• Communication

(s.2.1) After successful completion of this CPD practitioners will be able to discuss with patients the benefits and limitations of tints and filter lenses adapting language and communication approach as appropriate.

• Clinical practice

(s.5.3) After successful completion of this CPD practitioners will be able to recognise the different classifications and applications of tints and filters for resin and glass lenses and apply this to routine clinical practice.

(s.7.6) After successful completion of this CPD practitioners will be able to interpret different transmission curves when dispensing tinted spectacle lenses appropriate to individual patient needs.

The prescribing and provision of tinted lenses, whether for occupational, environmental, therapeutic or cosmetic purposes, remains an important and increasingly complex aspect of opticianry. A familiarity and understanding of the concepts, methods and regulations involved only serve to support the eye care practitioner in fulfilling this essential element of practice.  


Lens Transmittance 

When light is incident on a spectacle lens, some is reflected by the front surface, some is absorbed or scattered by the lens material, and some is reflected at the back surface. The percentage of light that passes entirely through the lens at a given wavelength is known as the luminous  transmittance for that wavelength. Due to the reflecting properties of the lens surfaces, the maximum transmittance of an uncoated standard clear lens will be in the region of 92%. The addition of a high-quality anti-reflection coating can reduce the reflectance from each surface from approximately 4% down to about 0.5% and improve the lens transmittance to around 99%. 

Practitioners will recall that the reflection factor  

  and that higher refractive indices have a much higher reflectance.  


Hue, saturation and density 

A tinted lens is defined within British Standards as one which has a ‘noticeable colour in transmission’.1 This concept of ‘colour’ however, involves a visual process that encompasses light transmittance via the optical system (cornea, pupil and crystalline lens), signal initiation in photoreceptors, transduction of post-receptor channels and decoding in the visual cortex.2 This decoding is subjective, varying for different eyes and brains, and therefore has no fixed definitive categorisation.3 The colour, or hue, has given labels, for example ‘blue’, that are perceived differently by different people and in varying light conditions. 

Furthermore, the human eye is not uniformly receptive to all wavelengths; in daylight (photopic) conditions it is most sensitive to visible light at 555nm (sodium yellow) and least sensitive to the red and blue ends of the spectrum.  

In addition to the hue, the tinted lens may be described according to the saturation (ie depth or concentration of the colour tone) and the density (transmission characteristics of the lens). Where hue is subject to personal perception and interpretation, saturation and/or density can be scientifically measured and accurately mapped. 

This mapped data, obtained using a spectrophotometer, can provide an individual and unique plot of the transmission/absorption characteristics across the spectrum in nanometre steps of all available ophthalmic lenses. These precise transmission curve charts act like a ‘fingerprint’ enabling identification and classification to British Standards requirements of the immeasurable array of clear, tinted or specialist absorber lens materials.4 Slight variation is caused by lens thickness and dioptric power; therefore, transmission curves are routinely plotted utilising a plano sample with 2mm (±0.2) centre substance and any deviation from this should be noted on the chart. 

Figure 1: Transmission of light through a spectacle lens

The grid plots in figure 2, 3 and 4 show along the horizontal (x-axis) the spectrum of light, in nanometres (nm), from short ultraviolet starting at 280nm, through visible light from 380-760nm, and into long infrared wavelengths up to 820nm. The vertical (y-axis) identifies the proportion of light (0-100%) that passes from one side of the lens through to the other side. This mapping of transmitted light therefore acts to identify both the amount of light absorbed by the lens (ABS) and the luminous transmission (LTF) at any given wavelength. 


Figure 2: Transmission curve for CR39; Figure 3: Transmission curve for Trivex; Figure 4: Transmission curve for Polycarbonate


The transmission data is especially useful for identifying the cut-off point for UV transmission particularly with tinted lenses where the widened pupil may make the wearer more vulnerable to its harmful effects. It cannot be automatically assumed that a dark lens, transmitting low levels of visible light, will also be effective at absorbing harmful short-wave energy.4 

Conversely, UV absorption does not necessitate a visibly coloured lens. The cut off for standard clear crown glass is around 300nm, in comparison to some of the newer higher index plastics that may, due to their manufacturing processes, naturally absorb 380nm or even more. In the UK, tinted lenses are routinely identified using LTF format as an established standard.4 This use of LTF allows practitioners and manufacturers to compare tints more easily using a single figure and common language. 

Tints can be applied, to some extent, to most spectacle lens materials including spectacle crown and hi-index glass, plastics resins, and polymers such as CR39, Trivex, Polycarbonate and Hi-index, and to newer inventions such as liquid crystal. The materials used, and the end results required, dictate the method employed and the processes needed. 


Resin lenses   

The acrylic resin diethylene glycol diallyl carbonate (DAC), marketed by PPG under the trademark CR39, has a ready capability to absorb organic pigment. This ‘tintability’ results from the dipolar attraction between the carbonate carbonyl groups of the polymer (O-CO-O) and the solubilising groups that constitute the active part of the tinting solutions, for example hydroxyl or sulfonic acid species of the azo and anthraquinone dyes.5

The water-soluble liquid and powder dip dyes designed and developed for use with CR39 offer a safe, quick, and easy method of in-house tinting using compact and moderately affordable equipment. Lenses are submerged in the diluted dye mix, pre-heated to approximately 93°C, for a calculated time to achieve the desired saturation, the longer the time, the greater the absorption. This take-up rate is affected by the chemical form of the lens, its age and thickness and the type of any coatings applied. 

Hence, a clear CR39 prescription lens from two different manufacturers may well result in variable spectral accuracy after time together in the same pot. The artistry of an experienced technician is then called upon to fine tune the dying process to produce two matching lenses. Gradient, double gradient1 and multi-colour tints can be achieved by repeatedly dipping only a part of the lenses into the dye, or dyes. Again, a fine eye is needed to align the lenses carefully and produce a bespoke matching pair for the consumer. 

Although modern specialist tint manufacturers promise fast, stable and consistent custom tinting in a substantially infinite range of colours, these heat-treated lenses will eventually fade with continued exposure to UV light. As some dye components are less chemically stable than others, the hue can gradually show changes over time for example grey lenses can, over a period of years, take on a reddish hue as the green dye constituent breaks down.6  

In contrast to dye-loving CR39, the thermoplastics material Polycarbonate can only have its hard coat layer tinted with a maximum absorption of around 20%. To achieve anything beyond this it is necessary to start with a pre-dyed substrate and adjust the final result, if necessary, up to a maximum variable of 20%. Trivex (Trilogy/PNX) is also naturally resistant to tinting and availability is limited to a set range of pre-dyed semi-finished stock. Although the use of a pre-tinted base material prevents the issue of fading, it also restricts the range of obtainable options and rules out the possibility of graduated designs. Similarly, the popular mid-index material, Tribrid (1.6), is also tintable to around 15% absorption. 



It is common knowledge in the optical dispensing field that the higher the index, the denser the material, and subsequently the harder the rear surface hard coat needs to be. Therefore, hi-index plastics have historically also only been tintable to around 20% depending on the tintability of this hard coat micro-layer. However, with ongoing research and continued development of high index lens materials, and the advance of specially formulated organic dyes and lens primers, dip dyes that promise faster and darker results with hi-index lenses are now commercially available. 

Contemporary liquid crystal adaptive ‘omnifocal’ lens designs incorporate a layer of liquid crystal sandwiched between two sheets of voltage-controlled indium-tin oxide coated glass. A sensor on the frame controlled by the wearer to activate a mask consisting of mosaic style colour cells and alter the transmission properties of the lenses according to the ambient conditions. Whereas traditional fixed tints are designed for filtering a specific pre-determined narrow range of wavelengths, the adaptive lens offers an adjustable solution, including tuneable polarisation, suitable for multiple environments.7 This development in spectacle lens technology is truly a game-changer. 


Glass lenses 

Crown glass is made of a mixture of silica, soda and lime. To produce solid tints, metallic oxides are added to the constituents of the glass before the fusion of the mixture, see table 1.8. 

Due to the precision process of glass manufacture, this method of solid tinting allows for perfect reproducibility and has the added advantage of long-term colour stability. However, because the oxides are evenly distributed throughout the lens blank, it suffers from a thickness dependency that produces results of variable density in higher prescriptions whereby thicker areas of the lens are darker. Thus, a minus-power tinted glass lens is darker at its edge than at its centre, and a plus-power lens is darker at its centre than at its edge.  

This predicament of uneven tints in high-powered lenses can be addressed by instead applying a thin film of metal oxide to the rear surface of the glass using a vacuum process. This process results in a stable, uniform coating where the transmittance does not vary with either lens power or thickness. The coloured layer is added to the rear surface of the lens because it is soft, and a second protective layer is often applied to improve the durability. 

However, it can still be scratched with heavy handling and worn away over time. Also, unwanted interference effects can occur whereby light reflected at the back surface may appear coloured; often red, blue or purple. This bloom may be irritating to the wearer, or simply cosmetically unacceptable. Consistent colour accuracy can also be achieved by bonding a wafer-thin tinted film sandwiched within the lens. This laminating process offers the manufacturer the opportunity to combine multiple features, such as contrast enhancement, photochromic and polarising, within a single lens.9 

The  application of tints and filters to spectacle lenses serves many purposes; they can simply serve a fashion purpose, make vision more comfortable, improve visibility and contrast, and protect the eyes from the effects of harmful radiation. Acknowledging that possibly the most common use of tinted lenses is for driving has attracted the attention of designers of both ophthalmic prescription lenses and those in commercial sunglass production and led them to develop specialist driving ranges. Recognising the specific, yet multifaceted, needs of this specific group of users has increased the availability of lens designs, coatings and filters aimed directly at addressing the complex issues they face. 


Driving and Sunspecs lenses 

Driver road safety and driving performance involves numerous aspects of visual function for example, central and peripheral visual acuity, contrast sensitivity, stereoacuity, visual tracking and colour recognition.10 

However, visual sensory impairments and deficits in the processing of visual information are common and the dynamic and visually cluttered driving environment presents significant additional challenges for the driver. Bright sunlight, as a primary example, can occur on most UK roads and is additionally problematic when the sun is low to the horizon or rebounding from a wet road surface thus rendering an overhead visor completely useless. This ‘intense illumination may cause glare, gaze diversion, incomplete attention to the full visual field, reductions in speed-sensation, motion blindness, or dazzle with temporary lost vision.’11 In this circumstance of being dazzled by bright sunlight, the Highway Code (Rule 237) states that the driver must ‘slow down and, if necessary, stop’.12 

However, as humans have an instinctive aversion reaction to this blinding bright light they will readily reach for sunspecs when faced with such conditions. These sunglass filters manufactured for general use (including road use and driving), intended for protecting the eyes from solar radiation, are assigned to one of five density categories based on the luminous transmittance at a given reference point. These categories are organised according to the values given in table 213 where Ʈv is the value of luminous transmittance and D65 denotes an incandescent CIE (International Commission on Illumination) standard illuminant source simulating average daylight (white) light with a colour temperature at around 6,500K as determined by the standards for testing14 and standards for CIE illuminants.15  


Category zero filters have a transmission range from 100% (clear) to 81% and, as such, have no regulatory restrictions upon their use. Described as clear to very light, these tints are suitable for use indoors, for visual comfort and for cosmetic purposes. With a transmission value between 80% and 43%, category one filters provide a light tint suitable for low sunlight. Category two filters (43%-18%) are suitable for medium sunlight, and category three (18%-8%) for brighter sunlight conditions. 

All three of these, categories 1, 2 and 3, with an LTF of lower than 75% are unsuitable for driving at night or in poor visibility conditions. Drivers should not, at any time, use filters with a luminous transmittance of less than 8% therefore, according to the regulation, category 4 sunglasses are considered entirely unsuitable for road use and driving in normal conditions. Category 4 filters may in fact only be recommended for use in extremely high luminance conditions, such as those experienced in desert or snowfields in full sunlight.16 


Figure 5: Transmission curve showing maximum transmittance in the green

The hue of a lens is dependent on the wavelengths it absorbs and transmits. A lens that blocks the red and blue ends of the spectrum will have a maximum transmittance in the green where the human eye is more sensitive as can be seen in the transmission curve in figure 5.17  

In reality, as can be seen from figures 6, 7 and 8, a green sunglass lens would be constructed with a more discerning wavelength curve to offer some contrast enhancement combined with low levels of colour distortion suitable for general sun protection and driving. Brown sunglass lenses selectively absorb the blue end of the spectrum, decreasing the effects of blue haze and increasing the subjective impression of contrast. This can be preferable for many users. 


Figure 6: Resin brown Sunspecs Transmission Curve; Figure 7: Resin green Sunspecs Transmission Curve


Figure 8: Resin grey Sunspecs Transmission Curve; Figure 9: Vista-Mesh Transmission Curve


However, a grey lens is perhaps the most popular because its transmittance, which is more evenly distributed across the spectrum, can appear most neutral. As all wavelength specific filters alter the visible spectrum that is transmitted to the eye, and subsequently compromise colour contrast, it is important to remember that, in addition to the requirements for filters to conform to the minimum luminance transmission values, it is also essential to ensure that the lens colour conforms to regulations for acceptable traffic signal recognition.18 

Age-related changes and ocular pathology, such as onset of cataract for example, also exacerbate visual challenges faced by drivers and there is an endless raft of tints designed to mitigate them. In addition to visual acuity, symptoms of principle concern include decreased contrast sensitivity, increased sensitivity to moderate glare and distorted colour perception, especially in bright conditions. 

Blue light scattering within the eye increases the likelihood of veiling glare. Filtering out UV and blue light reduces this glare and relieves some of the associated visual discomfort.4 For outdoor, daytime conditions yellow/orange/amber filters have shown to provide subjective improvement, relieving symptoms of discomfort irrespective of the type of ocular disease. 

These contrast enhancing filters optimise the available light; thus, improving visibility and enhancing definition. They are valued for these benefits in shooting sports and those involving tracking moving objects, such as tennis. Importantly, the caveat for scotopic driving conditions remains. According to the Eyecare Trust, there is no evidence that tinted lenses, such as widely available amber night driving glasses for example, can improve vision on the road for drivers at night; indeed tinted lenses and tinted windscreens can make matters considerably worse.19 The Highway Code warns drivers not to use any form of tinted glasses, visors or lenses at night.20 

The provision of tints and filters for relieving the symptoms for patients with low vision has been long debated and remains a controversial subject. Research outcomes presenting mixed results have failed to produce a definitive scientific protocol for tinted lens dispensing recommendations based on either task or eye condition. Consequently, practitioners remain reliant on marketing literature, subjective reports, clinic-based observations and trial and error when providing advice about the suitability and benefits of colour filters for patients with low vision. It is essential to recognise that ocular pathology does not have a homogenous effect on all sufferers, and it is therefore important for eye care professionals to make clinical decisions and seek individual remedies based on qualitative information and subjective preferences.21  

Full UV protection is an important consideration and is now often catered for with even clear plastics lens materials. However, dispensing opticians should pay particular attention to UV cut-off on the transmission curves when dispensing young people, those who spend considerable time outdoors, anyone with pterygium, pingueculae, early-stage cataracts or aphakia, people taking medication that increases their photosensitivity, and those whose jobs make them vulnerable. 

At risk occupations include dentists (UV protection to 550/600nm and with orange/red tint) and some workers in industrial environments such as welders, for example, need UV protection to 600nm and a very low transmission eye shield. At the opposite end of the spectrum, with long waves of 780nm-1mm, accidental exposure to infra-red radiation must also be avoided. Risks of injury include cataract, scotoma, red eye, swelling and haemorrhage. Although there are some sources in everyday domestic life, excessive radiation is more likely in hazardous occupations including those working with molten glass and in the steel industry. Specialist safety protection such as matt finish barrier screens and face shields should be used to prevent employees coming into contact with light hazards.22 

Although there is scant published evidence to provide a reliable scientific base for the use of coloured lenses, the potential applications for sports vision dispensing remain extensive. Richer contrast continues to be a key factor, with enhanced depth perception and improved visual tracking among the possible benefits. As seen in table 3, filters in the middle of the spectrum are a good place to start, however those at either end should not be ignored. A highly photosensitive athlete, for example, may express a preference for a violet or blue filter and someone keen to enhance their clay pigeon shooting might choose a deep red tint. 

Vocational environments are always an integral part of any dispensing discussion, and no less so when considering tints. The office setting, for example, may subject the worker to eye strain, tension headaches, glare, and visual discomfort under fluorescent lights and from prolonged computer use with poor desk ergonomics. Large banks of fluorescent lamps, without specialist diffuser casings, can be responsible for emitting UVB rays (290-315nm) and the harsh lighting conditions can cause distracting internal reflections, reducing contrast and creating in-focus ghost images for spectacle wearers. The intensity of this ‘stray light’ can be reduced with lightly tinted lenses, the recommended colours are pink, rose and pale grey. These tints can also be favourable for those with photophobia and blepharospasm. 

Another solution here would be the Vista-Mesh lens, with its unique mesh filter construction acting as a ‘light comb’ to improve the quality and form of light delivered to the eye. Vista-Mesh has a light brown hue and luminous transmittance of 87%  (figure 9) and is thought to also be beneficial for a number of other conditions including migraine, photosensitive epilepsy and motion sickness.4  

In the same way that an individual’s hue perception is subjective, so too is any apparent benefit. However, that is not to belittle genuine visual enhancement to that of a placebo, far from it, but to assert that the enrichment is very much in the eye (and visual cortex) of the beholder. One size will most certainly not fit all and, as with all elements of dispensing, the skill of the optician lies in taking their wealth of product knowledge and mapping it with the diverse needs of each patient. When approached with a measure of caution, blended with an understanding of the regulations and equipped with a comprehensive filter sample set the optician can transform patients’ view on the world; potentially extending their independence, enhancing their working life and even improving their sporting expertise.  


  • Michelle Derbyshire MA FBDO SMC(TECH) has 35 years’ experience in the optical profession; as a technician, dispensing optician, senior manager, director and consultant; combining professional practice with education management and project and programme leadership. She now works as a freelance consultant utilising her diverse experience, and Master’s degree in management studies, to offer services including research, data extraction, copy writing, governance advice and programme validation support. Derbyshire is also a theory paper marker for ABDO and works part-time for the NHS in the hospital eye care service. 
  • Transmission curves adapted from Norville Prescription Companion 2018. 

  References 

  1. British Standards Institute. BS EN ISO 1366:2019 Ophthalmic optics – Spectacle lenses – Vocabulary [Internet]. London:BSI; 2019  
  2. Ao M, Li X, Qiu W, et al. The impact of age-related cataracts on colour perception, postoperative recovery and related spectra derived from test of hue perception. BMC Ophthalmol 2019;19:56. https://doi.org/10.1186/s12886-019-1057-6 
  3. Logvinenko AD, Beattie LL. Partial hue-matching. Journal of Vision. 2011 Jul 5;11(8):6-. https://doi.org/10.1167/11.8.6 
  4. Norville. Ophthalmic Lens Technical Reference & Transmission Data (Prescription Companion) May 2018 E edition 11. https://www.norville.co.uk/user/lens_catalogues/pdfs/Companion%2025518.pdf [Accessed 5th April 2024] 
  5. Kohan G, inventor; Kohan, George, assignee. No-mess ophthalmic lens tinting process. United States patent US 5,743,920. 1998 Apr 28. https://patentimages.storage.googleapis.com/0c/24/32/a225fe24b2569c/US5743920.pdf 
  6. Chou BR, Cullen AP. Spectral Transmittance of Selected Tinted Ophthalmic Lenses. Canadian Journal of Optometry 1983;45(4):192-199 https://openjournals.uwaterloo.ca/index.php/cjo/article/download/4322/5184  
  7. Black M, Kupershmidt V, inventors; Reliant Laser Corp, assignee. Liquid crystal sunglasses with selectively color adjustable lenses. United States patent US 5,114,218. 1992 May 19. https://patentimages.storage.googleapis.com/38/d6/07/ad4760da78c82c/US5114218.pdf 
  8. Keirl A. C35770: Essential course in dispensing – part 10. Optician 2008 https://www.opticianonline.net/cpd-archive/10 
  9. Keirl A. Complete course in Dispensing Part 7 – Coatings. Optician 2010 https://www.opticianonline.net/content/features/complete-course-in-dispensing-part-7-coatings 
  10. Owsley C, McGwin Jr G. Vision and driving. Vision research. 2010 Nov 23;50(23):2348-61. https://www.sciencedirect.com/science/article/pii/S0042698910002531 
  11. Redelmeier DA, Raza S. Life-threatening motor vehicle crashes in bright sunlight. Medicine. 2017 Jan 1;96(1):e5710. https://journals.lww.com/md-journal/fulltext/2017/01060/life_threatening_motor_vehicle_crashes_in_bright.28.aspx 
  12. Department for Transport (DoT). The Highway Code Rule 237 [Internet]. London:DoT [Updated 2023 September 17] https://thehighway-code.co.uk/highway/code/226-to-237/driving-in-adverse-weather-conditions.html [Accessed 31st May 2024] 
  13. British Standards Institute. BS EN ISO 12312-1;2022 Eye and face protection – sunglasses and related eyewear [Internet]. London:BSI; 2022 
  14. British Standards Institute. EN ISO 186526-2:2020 Eye and face protection – Test methods – Part 2: Physical optical properties Clause 7 [Internet]. London:BSI; 2020 
  15. British Standards Institute. BS EN ISO/CIE 11664-2:2022 Colorimetry. CIE standard illuminants [Internet]. London:BSI; 2022 
  16. British Standards Institute. BS EN ISO 12312-1:2022 Eye and face protection — Sunglasses and related eyewear — Part 1: Sunglasses for general use Appendix A Para A7 [Internet]. London:BSI; 2022 
  17. Stephens GL, and Meister DJ. Spectacle Lens Tints and Coatings. Ento Key: Fastest Otolaryngology & Ophthalmology Insight Engine [online].[Citace 24.10. 2022] Dostupné na: https://entokey. com/spectacle-lens-tints-and-coatings. 2016. 
  18. British Standards Institute. ISO 16508:1999 Road Traffic Lights – Photometric properties of 200mm round signals [Internet]. London:BSI; 1999 
  19. The Eyecare Trust. Night Driving – The Facts [Internet]. Bath: The Eyecare Trust https://www.eyecaretrust.org.uk/view.php?item_id=52 
  20. Department for Transport (DoT). The Highway Code Rule 94 [Internet]. London:DoT [Updated 2023 September 17] https://thehighway-code.co.uk/highway/code/89-to-102/rules-for-drivers-and-motorcyclists.html [Accessed 26th May 2024] 
  21. Eperjesi F, Fowler CW, Evans BJ. Do tinted lenses or filters improve visual performance in low vision? A review of the literature. Ophthalmic and Physiological Optics. 2002 Jan 9;22(1):68-77. https://publications.aston.ac.uk/id/eprint/4530/1/Tints_review.pdf 
  22. Flores C. Light and Infrared Radiation https://ehs.lbl.gov/resource/documents/radiation-protection/non-ionizing-radiation/light-and-infrared-radiation/ [Accessed 7th June 2018].