In the third in our short series on dispensing matters,
Dr Keziah Petre outlines the optical properties of lens materials and their influence on use in spectacles. CET module C1379
When dispensing a patient with a pair of glasses there are many considerations to take into account. Will the lens type be single vision or multifocal? Will the form be spherical or aspheric? Will the lenses be tinted? What is the best lens material for this patient? The purpose of this article is to address the last of these questions by reviewing the main properties of a spectacle lens that are affected by the choice of lens material. These will be compared across the three basic categories of material: glass, plastic and polycarbonate.
REFRACTIVE INDEX
Let's first go back to basics for a reminder of the maths behind the most fundamental attribute of an ophthalmic lens, its refractive index. The mean refractive index (n) of a lens is defined as the velocity of light in a vacuum divided by the velocity of light in the lens material (which is always slower). In practice, the refractive index is measured in air, rather than a vacuum, but this difference is insignificant for spectacle lenses.
Refractive index is measured for a specific wavelength in the middle of the visible spectrum. The reason for this is that any material will refract different wavelengths by different amounts, and this will be considered more carefully in the section on chromatic aberration. Traditionally, the reference wavelength used in the UK has been the helium 'd' line, at 587.56nm, giving a refractive index of nd. Others have used the mercury 'e' line, a slightly shorter wavelength of 546.07nm giving slightly higher values of refractive index, described as ne. Table 1 shows the difference between the refractive indices of standard lens materials when they are measured with the two common reference wavelengths.
When light hits a lens surface obliquely it is slowed, and undergoes a change in direction. The higher the refractive index, the more a lens material will 'bend' the light hitting its surface. At a flat surface, the amount of bending, or refraction, is given by Snell's law:
n' sin i' = n sin i
where i and i' are the angles of incidence and refraction respectively, n is the refractive index of the lighter medium (usually air, n=1) and n' is the refractive index of the denser medium (the spectacle lens). Figure 1 shows Snell's law in action for two lens materials of refractive index 1.523
| Figure 1. Snell's law for two materials n=1.523 and 1.9 Angle of incidence (i) = 30o |
The higher the refractive index of a lens, therefore, the less of it will be needed to provide a given amount of refraction, or lens power. However, other factors make the choice of lens material more complex than simply opting for the highest refractive index available, as we shall go on to discuss.
The refractive indices of the three main types of lens material are shown in Table 1. Polycarbonate has a refractive index (nd) of 1.59. 'Standard' plastic, or CR39, has a refractive index of 1.498, and plastics are available in further indices up to a maximum of 1.74 at the present time.
'Standard', or crown, glass has an index of 1.523. High index glass is available to a current maximum of 1.9.
Glass, described as an inorganic or mineral material, has a chemical structure similar to liquid. At a normal temperature, it is so viscous that it can be described as a solid. Crown glass is made by melting quartz together with potash, soda and oxides. To increase refractive index, lead, titanium and lanthanum oxides are added. Blocks of glass are then ground to the required lens shape.
Plastics materials, also known as organic materials or resins, are made by heating monomers in glass moulds so that they form long molecular chains or polymers. In thermosetting plastics, 'cross links' are formed during polymerisation, making an interconnected network of molecules, which makes a highly stable material.
Polycarbonate is a thermoplastic material, which when polymerised forms long polymer chains independent of one another. This allows the material to be reformed and gives greater impact resistance. Polycarbonate lenses are made by injection moulding under high pressure.
Trivex material is both thermosetting (like CR39) and thermoplastic (like polycarbonate).
THICKNESS
There is a simple relation between lens thickness and refractive index: the higher the refractive index, the thinner the lens. This is shown for plastic lenses of different refractive indices in Figure 2, and will also be true for glass materials.
| Figure 2. Edge thickness in mm (y) vs lens power D (x) for four different resin materials with a constant blank size of 65 mm. Data taken from the Norville Prescription Companion |
In Figure 2 it can be seen that the difference in edge thickness between low and high index materials becomes more pronounced at higher powers. So, the higher the prescription, the greater the benefit to the patient of using a higher index lens. Figure 3 shows the increase in edge thickness as lens diameter increases for different power lenses. It can be seen that the higher the power, the more sensitive lens thickness is to the size of the lens. For high powered prescriptions, therefore, choosing as small an eyesize as possible will potentially reduce the edge thickness more than by increasing the refractive index. For example, a patient with a prescription of -12.00DS chooses a frame with a lens diameter of 65mm and opts for a lens material of index 1.67: the edge thickness will be 11.3mm. If they decided to have a 1.74 index material the edge thickness reduces to 10.2mm. If instead, they stayed with the 1.67 index material but chose a smaller frame with a lens diameter of 60mm, the edge thickness would be 9.6mm, a greater reduction in edge thickness than by increasing refractive index. A better result could, of course, be achieved by using both the 1.74 index material and the smaller eyesize, when the
| Figure 3. Edge thickness in mm (y) vs blank size in mm (x) for powers of -4.00, -8.00 and -12.00 with a 1.67 resin material. Data taken from the Norville Prescription Companion |
In negative lenses, variation in minimum centre thickness with different lens materials also affects edge thickness to a certain extent. CR39 lenses generally have a minimum thickness of 2.0mm for mechanical stability. Polycarbonate in spectacle lenses typically has a centre thickness of 1.5mm. High-index plastic and glass lenses can be surfaced down to about 1.0mm as they are more rigid materials. In summary, thinner lenses will look better, but thicker lenses will be less likely to break.
Lens form also drives lens thickness. In spherical designs, using higher refractive index materials requires the use of more curved lens forms for all prescriptions to eliminate off-axis aberrations such as oblique astigmatism. These steeper lens forms are thicker and heavier than flatter forms. Using aspheric lens forms allows elimination of off-axis aberrations and the use of flatter lens forms, reducing the centre thickness of positive lenses and the edge thickness of negative lenses.
DENSITY
Lenses of higher refractive index are in general denser, or have a higher specific gravity. Density is the weight of 1cm3 of the material in grams.
For example, 1 cubic centimetre of water weighs 1.0g, so its density is 1.0gcm-3. The same volume of 1.9 index glass, however, weighs 4.0g, so its density is 4.0 gcm-3. Specific gravity is the ratio of the density of the material to that of water, and has no units. Density and specific gravity are essentially identical at normal temperatures and pressures.
Values for the density of different lens materials are shown in Table 1. It can be seen that density increases with increasing refractive index.
Glass is much denser than plastic. Early high-index glass lens materials were particularly dense as the increased refractive index was produced by the addition of lead to the lens. More recent glass lenses increase refractive index using titanium oxides, and while the density is increased, it is not by so much as for the lead-based lenses. The thermoplastic materials (polycarbonate and Trivex) have the lowest density of the lens groups.
WEIGHT
The final weight of a lens depends on all the factors discussed so far: not only the density of the material, but also the amount of the material present as described by the thickness, which is driven by the refractive index.
| Figure 4. Weight saving of plastic material compared to CR39 in % (y) vs material by nd (x). 60mm round lens, -10.00 DS. Data taken from the Norville Prescription Companion. |
Although lenses of higher refractive index are thinner, since they are denser, they have the potential to be heavier. Does the reduction in volume of a lens from its increased refractive index compensate for the increased density of the material? Figure 4 shows how the weight of plastic lenses varies with material as compared to the weight of a CR39 lens, while Figure 5 shows the same information for glass materials compared to crown glass. It should be pointed out initially that a plastic lens will always be lighter than a glass lens of a similar refractive index by about a half.
Increasing refractive index in plastic lenses results in lenses that are lighter than CR39. The benefit of the increase in refractive index and decreased thickness outweighs the increased density. This is especially true for the lowest density materials (polycarbonate and Trivex) and the highest index materials (1.74 index).
| Figure 5. Weight saving of glass material compared to crown glass in % (y) vs material by nd (x). 60mm round lens, -10.00 DS. Data taken from the Norville Prescription Companion |
In conclusion, for a high powered negative lens, a 1.9 index glass lens will always be the thinnest option. However, the lightest lens will be a 1.74 index plastic followed by a polycarbonate or Trivex material. For positive lenses, the lens form will have a significant impact on centre thickness, but within a lens form, the rules of thumb above will still apply.
REFLECTANCE
Not all light hitting a spectacle lens passes through the lens to reach the eye. Some light is reflected at each surface of the lens. For a lens of refractive index n in air, the reflection from a surface (<03C3>) is given by:
<03C3> = [(n-1)/(n+1)]2 x 100
Reflectance (<03C3>) therefore has a percentage value of between 0 and 100, where 0 per cent indicates that no light is reflected at the surface, and 100 per cent indicates that all the incident light is reflected. From this equation it can be calculated that an uncoated CR39 lens loses 4.3 per cent of incident light in reflections at each surface, and so has a transmission of 91.4 per cent. In contrast, a 1.7 index lens loses 6.7 per cent at each surface and has a transmission of 86.6 per cent. The practical upshot of this calculation is that reflectance increases with higher index lens materials and it is more important for these to be anti-reflection coated. Indeed, most high-index materials are supplied coated as standard. Appropriate multi-layer anti-reflection coatings improve the transmission of lenses to over 99 per cent (see article 2 in this series).
ULTRAVIOLET ABSORPTION
When light hits a lens, some is lost by reflectance as described above, and some is absorbed by the lens rather than transmitted. Absorption is dependent on the wavelength of the light. Manipulation of the absorption characteristics of a lens within the visible spectrum is obviously crucial when considering tinted lenses (see article 1 in this series). When considering untinted lenses, the absorption or transmission of light in the ultraviolet region of the spectrum is also important.
Ultraviolet (UV) light has short wavelengths of 100-380nm, and so is found beyond the blue end of the visible electromagnetic spectrum. UV light can be split into three categories. UVC (100-280nm) is mostly absorbed by the ozone layer of the earth's atmosphere. UVB (280-315nm) causes acute ocular problems such as snow blindness or arc eye, and also causes sunburn. UVA (315-380nm) exposure is associated with chronic visual problems such as the development of cataract.
Patients who require particular protection from UV include pseudophakes and aphakes, who no longer have a lens to protect the posterior chamber from UV exposure. Patients who are exposed to UV light at high levels or for prolonged periods will also require protection. This is particularly true in environments such as snow, sand or water, where reflections can substantially increase UV exposure. UV protection should therefore be considered for anyone who works, or spends a lot of time, outside.
To provide full UV protection, a lens should absorb all wavelengths up to 380-400nm. The wavelength at which a lens stops absorbing, and starts transmitting light, is called its cut-off value. UV cut-off values for standard lens materials are shown in Table 1. Crown glass provides poor protection against UVA as its cut-off is only 300nm. Higher-index glasses are better, with cut-off values of 330-350nm. Plastic materials have better UV protection than glass. For an uncoated CR39 lens, the cut off is 350nm, and for higher index lenses the cut off can be around 380nm.
Plastic lenses can also be coated to improve their UV protection. A UV400 filter will be clear, whereas higher levels of protection will result in a yellow or orange colour as the short wavelengths of the visible spectrum are blocked. Thermoplastic materials offer the best UV protection in their standard form, with rays not entering the material of below 385-395nm.
IMPACT RESISTANCE
Thermoplastic materials (polycarbonate and Trivex) have the greatest impact resistance of all standard lens materials. If impact resistance is a priority, due to a patient's hobbies, or perhaps for dispensing a child, then these materials should be used. Considering other lens materials, plastic lenses are more impact resistant than equivalent glass lenses.
For general purpose lenses supplied in Europe, BS EN ISO 14889 (1997) specifies that a lens must not break while a static 100N load is applied through a 22mm steel ball for a minimum of 10 seconds.
The impact resistance of general-purpose spectacle lenses will be lower than in protective safety eyewear made of the same material. In protective eyewear the lenses are generally thicker, and the entire appliance, not just the lenses, is tested (BS EN 166, 1996). For example, safety spectacles with polycarbonate lenses that fulfil the regulations for low-energy impact (grade F) are often made with a minimum centre thickness of 3mm. A general-purpose polycarbonate spectacle lens, however, will have a centre thickness of around 1.5mm. Impact resistance is also affected by lens coatings. Scratch resistant coatings decrease the impact resistance of polycarbonate lenses substantially, but even a coated polycarbonate lens is several times more impact resistant than a thermosetting plastic lens.
DURABILITY
To be durable, a lens material should be resistant to chemical attack and have a hard surface.
Most lens materials are fairly inert to other chemicals, but polycarbonate is attacked by hydrocarbon and ketone liquids. In optical practice, this means that polycarbonate lenses should not be cleaned with acetone. It also means that polycarbonate lenses would not be suitable for someone who works with, or comes into contact with, petrol or benzene.
In terms of surface abrasion, glass has a hard surface and does not scratch easily. Plastic materials are softer but can have scratch resistant coatings applied to improve their durability. These improve surface hardness, but not to the same level as glass lenses. Thermoplastic lenses are particularly soft and will require hard coating. A coated polycarbonate lens has the same scratch resistance as a CR39 lens. The problem with applying hard coatings to a soft thermosetting or thermoplastic lens substrate is that a coating which is too hard and inflexible will crack under stress. The coefficient of thermal expansion of the lens and the coating is often different too, causing stress in the coating at extreme temperatures.
'TINTABILITY'
Tints themselves were covered in part one of this series. If a patient requires a specific tint, however, this may influence the choice of lens substrate. Glass lenses can be vacuum coated or given solid tints, which may be fixed or photochromic. They can also be 'equitinted' or laminated. CR39 is easily tinted. Polycarbonate, Trivex and higher index resins are more difficult to tint. Polycarbonate, for example, can currently be tinted to a light transmission factor of 50 per cent in any colour, but only up to 20 per cent in grey.
CHROMATIC ABERRATION
The mean refractive index, as we saw earlier, depends on the wavelength of the light used for measurement. All materials will refract short wavelengths of light more than long wavelengths ('blue bends best' helps me to remember this). Newton showed this chromatic dispersion in his classic experiment where white light incident on a prism is split into its constituent wavelengths (Figure 6).
| Figure 6. Refraction of light by a prism, showing dispersion of white light into its constituent wavelengths |
An ideal lens material would have a constant refractive index right across the visible spectrum, rather than varying with the wavelength.
Dispersion is the amount of difference between refractive indices at the red and blue ends of the spectrum when compared to the mean refractive index in the yellow green region of the visible spectrum. Constringence is the reciprocal of dispersion, indicating how similar the refraction of light is at opposite ends of the visible spectrum. The constringence value (Vd), also known as the Abb