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OPTICAL CORRECTION
LENS COATING
The
anti-scratch, anti-reflective (AR), and hydrophobic coatings can be provided
separately, there is an increasing trend for them to be offered as a 'package'
by lens suppliers. This makes much sense, as surface treatments should be
considered an inherent feature of a high-quality lens.
Although
hard coats are still available separately, modern AR coatings are now usually
provided along with both hard and hydrophobic layers. This trend has helped to
increase sales of AR dramatically: in combination with a hard coat, the
performance of AR is improved. Opticians need no longer worry about poor
durability and scratching when dispensing AR provided by reputable supplTers
with the necessary equipment, know-how, and staff
to ensure consistent, high-quality products.
HARD
COATS AND LENS MATERIALS:
HARD-COATING EQUIPMENT
There are several different ways to apply coatings, and modern
equipment has a range of features to facilitate the process. In this
new two-part article we examine some of these machines.
APPLICATION METHODS
Hard coatings on plastic and resin lenses can be applied using the
following methods:
- Spin coating (centrifuged, usually one-sided coating)
- Dip coating (always two-sided coating)
- Absorbed hard coat (similar to tinting)
- Vacuum deposition (combined with AR coating)
- In-mould coating (used by lens casting companies).
SPINNING AND DIPPING
Plastic, which has a non-cross-linked chemical structure, is
extremely soft and it is therefore essential that a hard coat is
applied to both surfaces. (The commonest type of plastic used for
ophthalmic lenses is polycarbonate.] Resins, because of their
cross-linked chemical structure, are much more abrasion resistant. A
low-index {1.5) resin, like CR39, is relatively hard, and hence a
hard coating is optional rather than essential. However,
hard-coating is normally essential for other, higher index resins.
Hard-coating chemicals come in two basic types: those that require
thermal curing, and those suitable for curing using ultra-violet
(UV) light. There is no major difference between the two types when
used as a hard coat only, but if they are to serve as a base for
subsequent AR coating the thermal types are generally preferred.
The ability of hard coats to take a tint is not related to the
application method, but to the chemistry of the hard-coat lacquer.
Generally, tintable hard coats are softer than non-tintable hard
coats, and do not form as good a base for an AR coating.
An issue rarely mentioned is the refractive index of the hard coat,
which is becoming more important as more high-index materials are
now being coated. Hard coats can create wavelength interference
effects. These are often very visible when an AR coat is applied on
top. Typically the effects vary across the lens surface in a pattern
that is related to the dipping action. Any light that is largely
monochromatic, such as a fluorescent, tends to magnify this effect,
which can be seen as a rainbow over the lens surface.
UV-CURED HARD COATS
The UV-cured hard coats have the great advantage of fast curing,
typically less than a minute, and therefore offer benefits to a
prescription laboratory requiring rapid lens delivery. They are
particularly suited to spin-coating the back surface of the
previously hard-coated, semi-finished blanks used by many
laboratories. A new type of 'fusion' cured hard coat is now
available, which promises to combine the simplicity and speed of UV
cure with the enhanced properties normally associated with thermal
cure.
THERMALLY-CURED HARD COATS
The increased demand for AR coating and high-index materials has
resulted in an increase in the application of thermally cured hard
coats, using a dip process. Although slower than the UV spin method,
a dip technique produces greater volumes of lenses at a lower labour
cost. Dip- coating machines, previously only used by the very large
casting companies, are now available in smaller sizes, with
multitanks for different index lacquers.
OTHER HARD COATS
There are three types of hard coat, apart from the dip and spin
kinds most commonly used. One, called an absorbed hard coat,
involves immersing the lens in a special chemical, which is absorbed
into the lens surface and makes it more abrasion-resistant. Another
type, called a vacuum-deposited hard coat, involves a relatively
thick layer of silica applied within a vacuum-coating machine in the
same way as an AR coating. Applying a hard coat and an AR coat
together is quite logical as the time taken is only slightly longer
than applying an AR coat separately.
A third type, the in-mould hard coat, is only applicable to
companies involved in lens casting. Generally these coats are non-tintable,
and are applied to the front side of semi-finished lenses.
HARD-COAT ADHESION
 The
difference in chemical structure between a hard coat and the base
lens means that there can be a difference in factors like thermal
expansion coefficients. To retain good long-term adhesion, a
chemical as well as a physical bond is often used, While problems
with hard coats are rare, there can sometimes be difficulties when
an AR coating is applied on top of the hard coat. The additional
stress induced to an organic hard coat by an inorganic AR coat can
be considerable, It is vital to ensure that both the adhesion of the
AR to the hard coat, as well as the adhesion of the hard coat to the
lens, can withstand temperature, humidity and other environmental
changes over the lifetime of the product.
The major ophthalmic lens companies have developed many tests to
assess the durability of coatings, and work is now proceeding to
include some of these in international standards. Any company
involved in coating therefore needs to control closely any variable
factors, such as hard-coat lacquer deterioration, and to use
adhesion and simulated ageing tests regularly.
It
is usually an essential requirement for high-index resins and for all plastic
materials to be treated with a protective hard coat. (The difference between a
resin and a plastic is that a resin lens is 'cast' in a mould by a chemical
process, which creates molecular cross-links, whereas a plastic is chemically
preformed and then injection molded.) High-index resins are usually softer and
have lower Abbe numbers. Plastic materials, of which polycarbonate is one, are
even softer - hence the need for a hard lacquer to protect their surface from
wear and tear.
Hard
coats can be applied in a number of ways. In most cases the lens is dipped in a
polysiloxane lacquer, a type of varnish composed of a polymer that includes
silicon molecules. The resulting layer is a few microns thick, sufficient to
enhance abrasion resistance and minimize the everyday scratches wearers get when
cleaning their lenses, but not so thick as to be inflexible when the lens bends
slightly.
Multifocal
lenses are frequently supplied with a hard coat on the front surface of the
semi-finished product, and then a hard coat is spun onto the back surface at the
prescription laboratory. The front-surface hard coat can be applied in the mould
when the lens is cast.
The
durability and adhesion of hard coats is an important consideration, given the
increasing number of high-index materials currently on the market.
Laboratories
need to ensure that the appropriate hard coat is applied to each different base
material. The index of the hard coat should match that of the substrate. Most
major suppliers use 'index matching' hard coats to avoid the problems that arise
when there is incompatibility Chemical preparation is frequently critical, since
adhesion is often created by chemical bonding. The strength of this bond must be
greater if an AR coat is also to be applied.
AR
COATING:
The
change of light's speed when it enters an optical material creates a reflection;
the higher the refractive index, the greater the reflection. Typical reflection
values are 8% for 1.5 index, 10O/o for 1.6 index and 13% for 1.7 index.
It
Ts well known that AR enhances vision since, by reducing reflections, it
increases the transmission of light through the lens. However, the lower the
reflection, the more visible are dirt and defects on the lens and the more
difficult it is to keep the lens looking clean, so there must be some
compromise. Perhaps marketing claims should concentrate less on the reflection
value and more on the ease of keeping AR clean. The negative aspects of AR used
to be its susceptibility to scratches, poor adhesion and difficulty to clean,
but modern technology has surmounted these problems. Today, with modern
application methods, there is virtually no reason not to include AR.
A
hard coat forms a firm base for the AR coating, and a gradual change between the
organic plastic and the inorganic AR materials. You would be forgiven for
assuming that the hard coat underneath the AR cannot prevent scratching and is
redundant. However, the firm base offers durability and better adhesion, much
like writing on a piece of paper placed on a firm desk, rather than on a soft
cushion.
The
background reflection colour produced by AR coating is influenced by the index
of the lens material.
This
is noticeable in fused bifocal lenses, where the reflection colour is different
in the area of the high-index segment. A similar effect occurs when the
refractive index of the hard coat is different to the refractive index of the
lens. In particular, where a front-side semi-finished hard coat is of a
different refractive index to a back-side hard coat, the AR reflection colours
differ between the front and the back of the lens. For this reason, many
prescription labs now prefer to use uncoated semi- finished lenses and apply the
same hard coat to both sides of the lens simultaneously, by dipping.
Where
there is a difference of refractive index between the hard coat and the base
lens, a reflection will also occur at that interface. This can cause optical
interference.
Similarly, small thickness variations caused by unevenness in the dipping
process can show up as interference fringes.
The anti-reflections coating can be simple
or multiple, they can be anti-rain and can
then be clean more easily. the cleaning with anti reflection
coatings must be performed with micronised fibers
tissues .
he anti-reflections coating let your eyes to
be seen through the lenses, and diminishes or stops
the reflection of lamps or lights while driving at
night.
Without anti-reflection coating
-- with anti-reflection coating
AR
REFLECTION COLOUR :
You
might expect AR to have no reflection colour. This is not the case. Since a
single-layer coating can only be optimized for one wavelength (colour), many
layers are required to create a low reflection without a strong background
colour. In practice, a neutral (white) colour would be very difficult to achieve
because a very large number of optical layers would be required, and the
thickness of each layer would have to be extremely precisely controlled.
Older
technology tended to reduce the reflection in the center of the visible spectrum,
leaving blue/violet and red/gold as the residual colours. Modern, multi-layer (broadband)
coatings usually leave some reflection in the central area of the spectrum,
producing a green colour, although some blue and Lila
multilayer coatings are also available. The strength of the residual
reflection colour has some practical, as well as commercial importance.
HYDROPHOBIC
LAYER
The
main purpose of a hydrophobic coating is to repel water by an electrochemical
molecular repulsion, thereby preventing water marks. The invisible hydrophobic
treatment also helps repel grease marks, making lens care easy. This in turn
means that the wearer needs to clean the lens less often, so there is less
chance of scratching.
The
hydrophobic layer is very thin, comparable to a few molecules, and therefore
does not interfere with the optics of the AR coating. Although some hydrophobic
are available as a simple dip that can be applied at the optician's workshop,
most are now an integral part of the vacuum AR process, which creates a
long-lasting clean look.
The
choice between different coatings depends on a range of technical issues and
quality standards. How does one quantify the relative abrasion resistance of
different hard coatings, and the relative durability and ease of cleaning of
different AR coatings? Certainly these properties are not easily measured, and
this is the subject of much debate at national and international standards
conferences.
The
client only needs to choose whether they should, or should not, have a hard or
AR coating, and leave the more difficult problem of which brand to the
professional's expertise. The premium brands all include the three critical
elements of hard coat, broadband AR and hydrophobic layers. hydrophobic layers.
Plasma Deposition The ion assisted deposition processes tend to compact
the coating materials after they have been deposited onto the lens surface
(and can be seen as the plastic lens equivalent to heating glass
substrates). In order to create uniformity, the ion source is often placed
near the side of the coating chamber, so that the relatively narrow ion
beam is angled across the chamber.
Obviously, the higher the power, the greater the 'compaction', however,
this is limited because excessive power can damage the lens surface. With
another technique, a plasma can be produced, with the advantage that the
plasma extends over the whole chamber, and greater energy is transferred
without damaging the lens surface. (This can be seen in the schematic
diagram.) At the same time as ion-assisted coating was being developed, so
was hydrophobic coating (the top, water/grease-repellant layer). This
coating can be applied in two ways. The simplest is to dip lenses in a
hydrophobic solution, and then evaporate the solvent.
This produces a good effect, but its lifetime is generally
not very long. The other is to use the vacuum coating equipment to apply
the hydrophobic within the coating chamber. (For companies with older
technology machinery, or to minimise production capacity, it is also
possible to transfer the lenses into a special vacuum chamber.)
Hydrophobic coatings have two major inter-dependent benefits. They keep
lenses cleaner, and also the surface is 'slippier' so that scratching is
minimised, but also because the lenses require less cleaning, the wearer
is less likely to damage the coating. Most modern products are promoted as
having three coats, the hardcoat, the AR coat, and the hydrophobic coat.
COATINGS AND QUALITY STANDARDS
While manufacturers use a variety of test methods when applying lens
coatings, there is a good deal to be said for developing objective,
internationally agreed standards for the abrasion resistance of hard coats, for
the reflection value of anti-reflective (AR) coatings, and for the adhesion and
durability of all surface treatments.
Over recent years, much work has been done to create common standards for
ophthalmic lenses globally. The lens and frame businesses have always been
international in scope, and the harmonization of CEN (European) and ISO
(International) standards is, therefore, logical and of benefit to the industry
as a whole. It has been relatively straightforward to establish quality
standards for lenses (for optical power, accuracy etc). Many national standards
have been in place for some time, which have helped to decide on satisfactory
criteria and testing methods for lenses. However, the situation for lens
coatings is quite different. Very few national standards exist, and the
standards and testing methods used by individual companies have varied widely.
This is partly explained by the fact that lens coatings are relatively recent
products.
But the main obstacles to establishing standards are the practical
difficulties in quantifying such values as the hardness and durability of lens
coatings.
WHAT IS MEANT BY COATING QUALITY?
The wearer's perception of quality, particularly in relation to coatings, can
differ greatly from that of the optical industry. The spectacle lens purchaser
may think that 'abrasion resistant' means in fact 'totally scratch-proof'; that
'anti-reflection' means 'no reflection'; that 'easy clean' means 'no cleaning
required'.
While some dissatisfaction with coating performance from end-customers may
well have been justified in the past, today's sophisticated polysiloxane hard
coatings, ion- and plasma-assisted AR, and modern hydrophobic layers, combine to
give long-lasting lenses with good visual performance. The question is
determining test methods that meet the standards required in the market place.
WHAT ARE THE CRITICAL QUALITIES OF A COATING?
While coatings are applied to provide additional benefits of hardness and
reduced reflection, it is important that they should not be used at the expense
of the fundamental optical properties - clarity and optical power - of the lens.
So, firstly, coated lenses should meet the standards of performance of uncoated
lenses.
The prime consideration of a hard coating is to provide abrasion resistance,
a quantity that is difficult to measure. Perhaps even more important is the
strength of adhesion between the coating and the base lens, and particularly the
durability of this adhesion over time and under conditions of varying
temperature and humidity.
For AR coating standards, the criteria mentioned above must be met. There is
also a need to quantify the reflection value, something that is more difficult
to achieve than might be expected. Regarding adhesion, there are difficulties
that derive from the different chemical structures of AR layers and plastic lens
materials. AR has an inorganic structure; plastics and most hard coats are
organic, which means that adhesion can be affected by temperature, humidity and
UV light, so the process must ensure a strong, long-lasting bond between the two
components.
COATING QUALITIES THAT ARE EASILY MEASURED
One of the criteria for coatings is that they should not interfere with the
optical properties of the lens. This is easy to achieve by requiring that coated
lenses satisfy the same standards as uncoated lenses. Such CEN and ISO standards
already exist.
It is known that the impact resistance of a lens may be affected by coating.
However, except in situations where impact-resistance is important, such as for
sports, or in the USA, where impact-resistance is a legal requirement, this does
not pose a serious problem.
20/20 08/01
ANTI-REFLECTION AND HYDROPHOBIC COATING METHODS BY DR PETER WILKINSON
he major lens and sunglass manufacturers, which coat large numbers of lenses
of the same type, size and edge shape, often use specially designed equipment to
achieve low costs per lens. In prescription labs, the requirement is to apply
coatings quickly, to a wide variety of lens shapes, sizes, refractive indices
and materials. Economies of scale dictate that anti-reflection (AR) equipment is
relatively large, with many machines coating over 100 lenses per batch. In
recent years, attempts have been made to provide equipment more suitable for the
average independent laboratory. The Saris MC and Leybold CCS are examples.
Satis Vacuum MC Lab 360 (photo courtesy of Satis Vacuum)
THE METHODS
The techniques and equipment used for applying AR coats to lenses do not
differ greatly. Often the only differences are the methods of cleaning and
preparation, the temperature within the coating machine and, in some cases, the
use of a thin adhesion layer applied prior to the AR coating.
After manual and ultrasonic cleaning, the lenses are placed in holders. Plastic
lenses are heated to remove excess moisture. Inside the coating chamber a vacuum
is created. With old technology, evaporation methods used crucibles of coating
materials heated by electricity or electron beam heaters. The material
evaporated and traveled through the vacuum to deposit on the lens.
Current technology incorporates ion-assisted evaporation. This adds energy to
the coating molecules so that a harder, more adhesive coating is produced. This
increased hardness is obviously an advantage; however, the coating must not be
too rigid in comparison to the base lens. Modern AR coatings are therefore more
compatible with hard-coated lenses. The higher energy also permits the
application of hydrophobic top coats. With good process control, far superior
coatings can be produced using ion- assisted methods.
More sophisticated again is 'plasma-ion-assist'. This is an even more energetic
form of ion-assisted coating. The main difference is that the coating materials
are not heated. Instead of using evaporation, a metal is bombarded with
high-energy gas molecules; the resulting released metal molecules combine with
oxygen to form an oxide AR coating.
Uncut lenses are not necessarily circular, particularly where the prescription
involves strong astigmatism. This can cause a problem, as coating molecules get
through the gap between the edge of the lens and the holder and affect the back
side of the lens. In some processes, the back of the lens needs to be masked
while the front side is coated. For this reason some AR labs choose to coat both
sides, and use edge-holding methods and automatic turn-over mechanisms so that
both sides of the lenses may be coated without turning them manually.
An Applied Vision plasma AR machine (photo courtesy of Applied Vision)
SPUTTER COATING
In a sputter machine a gas is injected into the chamber and an electric
charge 'excites' the gas molecules.
These hit the coating material - a block of metal - with high energy so that the
material is dispersed onto the lens surface. This method is easy to control and
does not require expensive quartz electronic monitoring. Sputter coaters are
smaller and cheaper, more suited to the lower volumes of lenses handled by small
prescription labs or retailers.
SOL-GEL
A new technology involves applying AR using a dip method similar to those
used for hard coating.
Available from Couget under the brand name Kelar, this method does not permit
the production of broad- band coatings, due to the difficulty of creating
multilayer coatings (as provided by vacuum techniques). Nevertheless, the fact
that it only requires the same equipment as dip hard coating and is considerably
less expensive may make it attractive in some markets.
HYDROPHOBIC COATING
Hydrophobic layers are important as they help to keep AR clean and reduce
scratching. They are usually applied immediately after AR coating in the same
vacuum chamber. For older-technology machines without the necessary equipment,
it is possible to purchase a dedicated hydrophobic vacuum chamber. As an
alternative to using a vacuum process, it is also possible to place the lenses
in a tank (as for dip coating), and then to stabilize this hydrophobic coating
by oven drying.
THE FUTURE OF COATING
A new and potentially interesting, but as yet incomplete, development in
ophthalmic coating is based on technology already used in non-ophthalmic
applications. Techniques similar to those for coating large sheets of window
glass are being developed, including combined hard, AR and hydrophobic coating
by vapor. This might permit 'conveyor-belt' methods, where lenses are coated
singly. The consequences would almost certainly be reduced costs per lens after
the initial development and capital costs.
20/20 12/01
Satisloh
official site
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