Ophthalmic lenses are nothing more than refractive media confined within two surfaces. Their optical characteristics are determined by the geometry of these surfaces and the optical features of the media.

It is therefore important to be familiar with the properties and characteristics of the raw material the lenses are made of.

There are two main kinds of lenses in terms of the composition of the refractive media: Organic lenses: The raw material of these lenses is an organic product. It is commonly referred to as plastic, but is actually highly specialized polymers, which are used in ophthalmic optics owing to their optical and physical properties. Mineral lenses: The raw material used for these lenses is glass. They are called mineral lenses because the glass is made mainly of silicate minerals.

 

 

 

 

 

 


ORGANIC LENSES

These lenses are made of organic polymers. Their main characteristics are as follows:

• They are low-density lenses, so they are very light.

• These lenses are softer and more liable to be scratched than mineral lenses. To solve this problem, a hardcoat treatment with a film of scratch-resistant material is applied to the surface. Indo markets this scratch-resistant lacquer under the brand name Super-Durcap.

• These lenses are available with the following refractive indices:
1.49 –1.5. The first ophthalmic organic material to be sold on the market. It was discovered in the early 1940s and is a compound corresponding to a kind of plastic called CR-39®. It has a very low density - practically half that of mineral lenses - and is very light. However, lenses made with material of such a low refractive index are thick. Its Abbe number is high, so it is suitable for optical use. Classified as low-index organic material.

1.523. Known as Superfín, this organic raw material was developed exclusively by Indo and first marketed in 1992. Its refractive index allows for higher added value than index 1.49, so lenses made with this material are 25% thinner, 25% lighter and 50% more resistant than conventional organic lenses. Classified as medium-index organic material.

1.560. More recently developed than the previous material. It is lighter than index 1.523 because it has a lower density and fewer curves. However, its Abbe number is lower, so the quality is not so good. Lenses made with this material are thin and light. Classified as medium-index organic material.

1.6.More recent than index 1.56. This material, known as Ultrafín, was developed by Indo in 2001. It is ideal for medium and high prescriptions and allows for making thinner, lighter lenses. It has a very low density and good Abbe number. Classified as high-index organic material.

1.7.One of the highest-index organic materials. It was first marketed in 1999 and allows for making very thin lenses. However, its Abbe number is low, so lenses have a higher chromatic aberration. This type of material therefore accounts for a very small percentage of the market.

 
Refractive Index (nd)
Abbe No. (ν)
Density
(d/c)

Transmittance
Factor (%)

Reflection
(%)
UV Absorption
(nm)
Superfín
1.532
48
1.31
91.6
8.4
350
Superfín 1.56
1.56
38
1.22
90.7
9.3
350
Ultrafín
1.6
37
1.29
89
11
387
Indolite
1.49
59
1.32
92.1
7.9
350

 


MINERAL LENSES

Mineral lenses are basically made of silica fused with metal oxides such as titanium, barium, sodium, etc. Their main characteristics are as follows:

• They are hard and scratch-resistant.

• They are heavier than organic lenses owing to their higher density.

There are several kinds of mineral lenses on the market named after the refractive index in each case:
1.523. The first ophthalmic glass to be manufactured, known as crown glass. It was the index traditionally used in ophthalmic optics until the 1990s. It is usually is relatively light with a good Abbe number. Classified as low-index mineral.

1.6. This material, which was first marketed more recently than 1.8-index glass, has been particularly successful in recent years and tends to replace crown glass. Lenses are thinner and slightly lighter with an Abbe number somewhat lower than type 1.523. Classified as medium-index mineral.

1.7. This glass, originally know as flint glass, was first marketed more recently than 1.523-index glass. It is ideal for high prescriptions, given that lenses can be very slightly curved. However, this material has a lower Abbe number than crown glass (35-40) and images break up into colours when viewed outside the optical axis of the lens. Classified as high-index mineral.

1.8.This material appeared on the market more recently than 1.7-index glass. It requires less curvature than type 1.7, but is very dense and therefore heavy. However, its low curvature may compensate for its high density because it allows for making very thin lenses. It is ideal for very high prescriptions. Its Abbe number is low (30-35) so the chromatic aberration is higher. Classified as high-index mineral.

1.9.The most recent high-index mineral to be launched on the market. This material can be used for applications similar to those of index 1.8 and is even thinner but somewhat heavier because it is denser. Recommended for very high prescriptions.

 
Refractive Index (nd)
Abbe No. (ν)
Density
(d/c)

Transmittance
Factor (%)

Reflection
(%)
UV Absorption (nm)
V7
1.532
59.3
2.55
91.6
8.4
330
Standard 1.6
1.6
41.2
2.67
89.4
10.6
334
Indovis 1.7
1.7
34.6
3.21
86.4
13.6
342
Indovis 1.7 AS
1.7
41.6
3.21
86.4
13.6
342
Indovis 1.8
1.802
34.6
3.65
83.7
16.3
342
Indovis 1.9
1.802
31
3.99
82.6
17.4
340

 

 


POLYCARBONATE

Polycarbonate plastic (PC) has a very wide variety of uses, including optics, medicine, electronics, mechanics, etc. Examples of products made of PC are CD-ROMs, CDs, minidiscs, DVDs, watch and mobile-phone casings, aeroplane windows, astronaut and motorcycle visors, lenses, dashboards, car headlights, household appliances, artificial kidneys, etc.

Polycarbonate plastic is a polymer obtained by linear polycondensation with the group -O-CO-O (carbonic-acid esters) in a long molecular chain. The PC macromolecule is made up of long parallel chains with few bonds between them. Owing to this linear structure, the material can be softened by heat and hardened by cold as many times as required because it does not undergo any chemical transformation during this process, just a physical change. PC has a high softening temperature and still maintains its rigid form at 140ºC. Owing to the linear structure, when energy is applied to such a material, the chains slide against one another, absorb energy and make the material highly impact-resistant. Another consequence of the linear structure is that certain solvents can separate the chains and dissolve the PC.

Due to its chemical composition, PC has a low density, high refractive index and low Abbe number. Despite its high impact resistance, this material is easily scratched and must be protected with hardening lacquers, depending on the intended application. Owing to its low crystallinity, it is highly transparent and can be used to make windows, visors, lenses, etc. UV-absorbing additives should be used to protect this material from UV rays and avoid premature ageing. These additives also make it possible to increase the UV cut-off wavelength (at which the transmittance factor is 1%).

Ophthalmic lenses made of PC were launched on the market in the late 1970s and were at first mainly used for safety lenses. The technological development of the PC process in recent years has made it possible to achieve quality standards comparable with thermostable materials such as CR-39®. The table shows the main characteristics of PC lenses compared with CR-39® reference lenses.

A PC lens with a centre thickness of 1.5 mm is over 50 times more impact-resistant than a CR-39® lens of similar characteristics. However, a PC lens is much less abrasion-resistant than a CR-39® lens and always requires an additional protective coating. This coating also protects the PC lens from the action of chemicals. PC lenses belong to the group of high-index lenses.

PC lenses can be made thinner than CR-39® lenses with the same prescription. However, the low Abbe number of PC lenses may produce more visible chromatic aberrations. PC lenses are less dense than those made of CR-39® and absorb all UV radiation up to 380 nm. Bearing in mind all these properties, PC lenses are recommended for children, sports enthusiasts and those who want to improve their aesthetic appearance by wearing thinner lenses.

 


SPECIAL RAW MATERIALS

Certain special glass and polymer materials are designed to meet specific requirements, such as photochromic and polarizing materials.

Photochromic
Photochromic optical lenses are made of materials that darken when exposed to UV rays. They provide protection outdoors from the sun’s rays, but revert to very low absorption levels when worn indoors.

The photochromic effect is obtained by means of chemical substances that modify the lens colour and get darker when they absorb UV light. Silver salts are usually used for glass lenses. Organic compounds are added to organic lens materials to reversibly modify their structure and change their colour.

Activation and Deactivation
The activation reaction begins when UV light enters into contact with the lens. The lens gradually gets darker until a maximum degree is reached: the lens is activated. The deactivation process occurs when the influence of the UV rays is removed, i.e. the colour gradually disappears until the original tone is obtained. Reversibility is often lost over time. This is called “fatigue”. However, photochromic lenses have been developed in recent years that are not prone to fatigue.

Activation affects the transmittance factor, but not reflection. In other words, the substrate reflects the same percentage of light, but greatly increases absorption in detriment of transmittance.

Spectral-transmittance curves vary considerably, depending on whether the lenses are activated or deactivated.

The graph of the deactivated lens shows that the transmittance factor is very high throughout the entire spectrum and is practically a straight line after 430 nm. This means that approximately 90% of the light that reaches the lens is transmitted in equal quantities for all colours, which means that the light coming from the lens is white.

In the graph of the activated lens, the transmittance factor is much lower and varies depending on the colour: on average, it is around 31% and, given that it absorbs much more blue than red, the lens looks brown.


There are two ways of applying the photochromic effect:
01. In-mass photochromism
In this case, the photochromic substances are included within the substrate itself.

This provides two advantages:
The photochromic properties do not disappear after severe superficial abrasion.
Greater resistance to ageing. This is the case of mineral Indocromic lenses and organic Superfín Indocromic lenses.

The only relatively significant disadvantage involves lenses with high prescriptions, particularly negative ones, in that different thicknesses produce different degrees of darkness because the thinner lens zones contain less material.

02. Surface photochromism
There are two ways to obtain this:
• By means of a treatment that infuses the photochromic material to a certain depth in the surface of the organic material.

• By coating one or both surfaces of the mineral lens with a thin layer of photochromic material by means of polymerization. This is the case of Polcromic.

The advantage of this kind of photochromism is that a uniform colour can be obtained, regardless of the prescription.

The disadvantages of the process are that the effect ages more quickly because it involves less photochromic material, and the photochromic effect can be removed in any areas subject to severe abrasion. Furthermore, a lens will not darken if the inner, concave side is exposed to UV rays, given that the photochromic coating is only on the outside of the lens.

Indo markets polymerized surface photochromic lenses under the brand name Polcromic.

Within its range of organic lenses, Indo uses in-mass photochromism for its Superfín lenses, marketed under the brand name Indocromic Superfín. This product also always has a Super-Durcap hardcoat to guard against any damage to the material.

 



Polarizing
Sunlight is a mixture of electromagnetic waves that propagate by vibrating on all planes perpendicular to the direction in which they travel. Under certain circumstances and when it goes through certain materials, light is polarized, i.e. it vibrates on only one plane in space.


Light polarization. a) Nonpolarized light. b) Partially polarized light. c) Fully polarized light.

If light that is polarized in a given direction is passed through a polarizing material at 90º to the light-polarization plane, the light completely disappears.

In 1808, the French scientist, Étienne-Louis Malus, observed that light falling obliquely on a reflective surface such as glass or water is polarized to a certain degree when it emerges.

Sir David Brewster subsequently discovered that the degree of polarization is greatest when light falls at a specific angle of incidence whose tangent is equal to the refraction index of the material. This angle is called Brewster’s angle.

Polarization makes it possible to eliminate harsh glare on water and other shiny, non-metallic surfaces such as snow.

Given that light reflected on these surfaces is polarized to a certain degree, the intensity of the reflections can be greatly reduced by wearing glasses with a polarizing filter whose polarization direction is perpendicular to the light.

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