One of the properties of spectacle lenses is the ability to selectively absorb electromagnetic waves (UV, Visible, Infrared) depending on whether the lens is transparent or coloured. These systems can therefore be viewed as filters. But what are the phenomena that explain this ability to absorb electromagnetic waves? After an explanation of the absorption properties of lenses, wearer protection and comfort aspects will be addressed including an overview of the standards that guarantee the quality of the lenses. This article exclusively describes lenses made from organic polymers, which represent the largest proportion of ophthalmic lenses made.
Moreover, filters using the principle of selective reflection of light (e.g. mirrors) will not be treated in this presentation.
1. Light – molecules interaction
A few notions on spectroscopy
The organic polymers used in spectacle lenses are an assembly of molecules, elementary units constituted by a core and electrons. The interaction of these molecules with light results primarily in an excitation of the electrons that move from a fundamental electronic state S0 to an excited state S1 known as «electronic transition» (fig. 1). The difference between these two energy levels can be recorded using a spectrometer which gives a graphic representation known as the absorption or transmission spectrum, depending on the mathematic model chosen. This spectra is a characteristic fingerprint of a molecule or chain of given molecules. All matter absorbs light but in distinct zones of the solar spectrum.
As the assembly of molecules comprising a polymer increases, so does the electron density, which is intrinsically linked to the nature of the atoms and the bonds between them, leading to a further shift the transmission spectra toward longer wavelengths.
Fig. 1: Theoretical model of light absorption. (a) Electronic transitions caused within a molecule taking electrons from a fundamental state S0 to an excited state S1. Recorded in absorptionspectrum (b) or transmission spectrum (c)
2.1. Lenses without apparent colour
In the case of clear lenses, the intrinsic composition of the polymer is generally sufficient to cut most UV. Even when this is not the case, it is possible to add supplementary molecules, called UV absorbers, to obtain total protection (fig. 2).
2.2. Coloured lenses
In order to obtain protection in the supplementary visible, typified by sun lenses, it is necessary to incorporate colorants into the polymer. These colorants, by their very high electron density (effect of conjugation), shift the absorption spectrum into the visible, resulting in a coloured lens.
A coloured lens must provide efficient protection in the ultraviolet region as well as, of course, in the visible.
Depending on the result sought, lenses can protect from various distinct sections in the solar emission spectrum (fig. 3) : UV, blue light, non-blue light visible, and near infrared. The international standard on Spectral Bands (ISO 20473) states that visible light begins at 380 nm, but does not suggest any specific name for the 380 to 400 nm spectral range (High Energy Visible). In the visible region, the benefit brought by colorants is a reduction in light intensity, avoiding the phenomenon of glare.
Coloured lenses are classified into 5 categories (fig. 4) depending on their transmission in the visible, which is to say their ability, to a lesser or greater extent, to filter light intensity. They are characterized in terms of τv (see formula), which characterises the level of transmission of the lens perceived by a standard observer under a standardized light representing the solar spectrum.
τ(l) : Spectral transmission factor of the filter
V(l) : Relative spectral light efficiency of vision (photopic)
SD65(l) : Spectral distribution of radiation from the D65 standardised light
Lenses in category 3 and higher are known as sun lenses. They provide optimal protection for the eye in the UV and visible.
Although the level of transmission is the principal factor in ensuring protection, it is important to offer several hues, for reasons of wearer preference or to achieve a specific filter (we will come back to this in the section on comfort). It is important to know that a coloured lens contains a mixture of colorants. Typically, there is a minimum of 3 so-called primary colours (specific colorants with broad absorption bands), generally a yellow that absorbs blue light, a red colorant that absorbs green and yellow and a blue colorant that absorbs red. By mixing these primary colours in the required proportions, one obtains by effect of subtraction the perceived colour of the lens (brown, grey, green, etc.) (fig. 5).
2.3. Photochromic lenses
Photochromic lenses are non-permanent coloured filters containing so-called «photochromic» colorants. The latter consist of molecules that are reversible upon exposure to UV light [1, 2] (fig. 6). Their hue is obtained using the same principle of colour subtraction by mixing primary colours, as described above. This property of reversibility offers the advantage of having, depending on one’s activities, only one pair of spectacles : clear inside and coloured outside when in the presence of UV. The reversibility of clear and coloured states is governed by a thermodynamic equilibrium and therefore the system is highly influenced by the external temperature, according to the law of Arrhenius for given UV radiation. The disadvantage is the difficulty to achieve solar category transmissions at high temperatures.
Beyond the protection offered by clear, sun or photochromic lenses, comfort aspects should be taken into account to ensure wearer acceptance. Although the primary function of the filter is protection, the wearer experience can be made more pleasant, particularly by eliminating uncomfortable parasite reflections during glare in certain conditions (polarized lenses). On the other hand, wearing a coloured filter, due to its transmission curve in the visible, can alter one’s perception of certain natural exterior colours. Second, it is, therefore interesting, to look at this question in a little more detail (physiological lenses).
3.1. Polarized lenses
In certain circumstances in everyday life, where light is reflected from a flat, smooth surface (e.g. water or glass), visual comfort and contrast can be significantly reduced by the phenomena of natural polarization of reflected sunlight. The reflection of the sun’s rays from a smooth horizontal surface produces an intense light that can be very uncomfortable, even blinding.
Light from the sun is composed of a large number of rays directed at random in every direction. As shown on figure 7, flat surfaces, such as water, act not only as a mirror but also act as a reflecting polarizer, in which light rays with horizontal polarization are selectively reflected compared to vertical polarizations. At a certain critical angle, known as «Brewster’s angle» light rays with horizontal polarization are totally reflected. The value of this angle varies according to the index of the medium : 53° for water (n=1.33) to 56° for glass (n=1.50). In a wide range of angles around Brewster’s angle, reflection of horizontally polarized light is significantly higher than its vertical complement.
Although traditional sun lenses help to reduce glare caused by the sun and its reflections by reducing the overall transmission of visible light, they do not offer any improvement to visual contrast and only limited comfort to the glare caused by polarized light. Polarized lenses contain a specific filter (polarizer) which blocks uncomfortable horizontally polarized light (fig. 8). The use of polarized lenses thus offers the wearer increased comfort by reducing visual fatigue linked to the possible glare as well as an improvement in the vision of contrast in the world around him.
3.2. Physiological lenses
When choosing between two pairs of spectacles fitted with coloured lenses of a given hue (for example brown), why does one feel immediately more at ease with one pair compared to the other?
Six years of research in collaboration with the National Natural History Museum in Paris  have enabled us to show that some filters lead to greater distortion of the view of a given coloured scene, merely due to the shape of the transmission spectrum concerned (fig. 9). Due to the utilization of broad band colorants, one observes specific peaks and valleys in the visible spectra of an ophthalmic lens. As such, the lens has many zones of contraction or dilation that lead to distortion in certain spectral zones of observation (that is to say the ability to distinguish contrasts of green in a country landscape, for example). Although the brain’s chromatic adaptation compensates in large part for the distortion caused by the filter, the result is always a feeling of comfort, to a lesser or greater extent, or of «natural vision» of colours for observers. It has also been shown that the darker the lens (in the sun range) the more it tends to increase this phenomenon compared to clear shades.
In a given range of hues, thanks to the use of special software, selection of these filters has given rise to the optimised lenses sold by Essilor.
Another, more elaborate approach  took account of the Model of Colour View drawn up by the International Lighting Commission (CIECAM02) in calculation of ability to restore colours . Models of colour view are predictive models of the appearance of colours that take into account the coloured surface being explored, its environment and its lighting, as well as the human visual system and its adaptation to the environment. A new method of representation is adopted, allowing one to observe the ability of a coloured filter to respect colours according to the chromatic circle NCS : Natural Color system  (fig. 10).
This approach, which is complementary to the first, enables better appreciation of the quality of a filter.
By opening the chromatic circle along a horizontal axis, the distortion caused by a given filter is highlighted compared to the reference axis 0 (fig. 11). If one calculates the sum of absolute values between the base line and the profile of differences of hues divided by the number of samples in the NCS atlas, one obtains a quantitative value that can be used to judge the result of a given filter F (fig. 12). The smaller the average difference, the better the view of colour through the tinted filter. This method is used to grade filters between themselves in terms of how colours appear.
The main benchmark defining requirements in terms of transmission properties for corrective lenses is international standard ISO 8980-3  which has existed for several years.
In the case of sun lenses, several standards co-exist, amongst which mention may be made of the benchmarks in America ANSI Z80.3, Australia AS1067 and Europe EN 1836 . At ISO, two new documents are at draft stage, that is ISO CD 12311 for sun lens testing methods and ISO CD 12312, which is a general standard for sun lenses.
Criteria to define and specify the transmission properties of lenses are numerous and are mostly contained in the above-mentioned standards, with terminology and requirements that may vary from one to the other.
The following presentation is limited to listing the assessment criteria defined in standard ISO 8980-3 and to introducing the principle of their definition.
The method consists of giving a category to the lens which is a function of its visual transmission factor τv, and of assessing the efficiency of its transmission properties through two main functions of the products: wearer protection and comfort.
4.1. Wearer protection in the Standards
In terms of wearer protection, the analysis covers several criteria so as to study compatibility of the filter with certain life situations, particularly exposure to natural light and driving :
- UV Absorption : Maximum transmission rates defined respectively for the UVA and UVB spectral ranges are associated with the category of the lens.
- Absorption of blue light : This is an additional criteria for protection against UV ; however, no minimum requirement is specified. This amount, which corresponds to the filter’s level of transmission in the spectral range 380-500nm, can be considered when a particular amount of absorption is claimed in blue light.
- Compatibility with driving : A multi-criteria analysis must be carried out to eliminate the risk of an incompatibility of the filter with car driving :
- Recognition of traffic lights (Quotients of relative visual attenuation) :
The check consists, based on the transmission spectrum, of defining attenuation caused by the filter of light emitted by traffic lights. The standard defines a maximum acceptable rate of attenuation for each of the signals, respectively red, yellow, green and blue.
- Compatibility for driving in tunnels :
The standard defines a minimal transmission value τmin in the yellow light region (typically 500-650nm) indexed on the visual transmission factor.
- Transmission in infrared : The standard does not contain a requirement in terms of lens transmission in infrared.
- Resistance to the sun’s rays : To ensure the stability of transmission properties over time, the standard includes a test that consists of exposing the lens to radiation representative of an intense solar exposure with the same spectral emission distribution. ISO 8980-3 then defines a maximum value for variation in terms of transmission, which is specific to each lens category.
We note that this type of photo-ageing testing specific to photochromic lenses is intended to ensure no significant deterioration in the photochromic efficiency in darkening of these filters over time.
- Photochromic filters : The standard specifies a minimum value in the ratio of visual transmission factors τv in the clear state and darkened state, corresponding respectively to the maximum transmission of the lens and after a 15 minute exposure under intense UV-visible light. Note that the efficiency of the filter’s photochromic function will be all the greater where this ratio is high. The latter must in no circumstances be less than 1.25.
4.2 Wearer comfort in the Standards
These criteria concern additional functionalities specific to certain lenses for which minimum requirements are defined in the standard ISO 8980-3 :
- Polarizing filters : The standard defines 2 additional criteria for polarizing filters, one relating to the filter efficiency the other to orientation of the axis of polarization :
- The efficiency level is defined by a ratio of transmission factors in the visible measured parallel τ// and perpendicular τ⊥ to the polarization plane. Here again, the efficiency of the function will be greater where the value of this ratio is higher, and typically must not be less than 8.
- Orientation of the filter’s polarization axis must be checked to ensure that the angular difference between the actual axis and the polarization reference axis does not exceed ± 3°.
The filtering properties of spectacle lenses have been addressed by describing the principle of absorption of a lens. Protection and comfort aspects have been looked at non-exhaustively, ending with standards involved. At a later stage it will be possible to extend the scope of the study of the filtering properties of lenses to specific applications : occupational lenses, specific colours, narrow band filters in an extremely precise section of the spectrum, for a given functionality, for example.