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I Can See
Clearly Now:
Optimising Optical Transparency
A spectacle wearer’s perception of quality and comfort of vision are impacted upon by
factors beyond the prescription. With advances in lens technology and thoughtful lens
choice, it is possible to capitalise on lens related factors such as design, materials and
treatments to enhance the spectacle wearing experience and visual outcome.
This article focuses on the properties of lens materials and treatments that contribute
to the optical transparency of a lens. When dispensing, it is important to consider lens
materials and treatments as an indivisible whole: one cannot provide an optimal visual
solution without the other. Lens design is a complex topic that demands
separate attention and is outside the scope of this article.
writer Helen Venturato
To ensure good optical correction,
every spectacle lens should be perfectly
transparent and remain so over time.1
While the transparency of a lens is
challenged by deterioration due to wear,
dirt, dust and aging, the inherent optical
properties such as reflection, absorption,
dispersion, diffraction and diffusion of
light all impact on lens transparency.
The initial lens dispensing choice can
successfully control many of these
inherent optical properties,
reducing the impact on the wearer’s
quality of vision.
Each lens material has a unique set of
properties that affects the interaction with
light. Modern spectacle lens manufacture
and dispensing employs a variety of
technical solutions to optimise lens
transparency and optical performance.
As light passes through a lens material,
the different wavelengths within the
spectrum refract slightly differently
causing chromatic dispersion or
chromatic aberration. Although this
occurs in all lenses, it is considered to be
negligible in the paraxial region of the
lens because the longitudinal chromatic
aberration of an ophthalmic lens is quite
low when compared to the eye. The
wearer may perceive chromatic dispersion
when looking through the periphery
of a lens due to Transverse Chromatic
Aberration (TCA) creating multiple
offset coloured images. These appear as
coloured fringes on high contrast objects
(See Figure 1).
The value or amount of TCA generated by
a lens is determined by the formula:
TCA = P/
where P represents the deflection of rays
(in prism dioptres) and  is the Abbe Value
or constringence of the lens material.
Since P = h x F, where h is distance
between the optical centre and the point
of the lens through which gaze is being
assessed, and F is the power of the lens,
Mivision magazine, Issue 69. Reprinted with kind permission from Toma Publishing, the publishers of mivision magazine.
mivision • 53
e
ns
e
d.
e
e
as
5
6
Power of the lens (diopters)
Refractive index
1,5
1,6
1,7
1,8
12,3 %
15,7 %
Figure
1 Longitudinal
and transverse
chromatic
aberration
Figure
18: Longitudinal
and
transverse
chromatic
Total b
light
7,8
%
10,4 %
Excentricity of the
direction
b reflected
of gaze (degrees)
1,9
aberration.
18,3 %
20
10
1,00 2,00
Anti-reflective coating consists in building up on the surfaces of
the lens a number of fine layers that together interfere with the
reflected rays of light and cancel them out. To do this, light is
considered
asinalight
wave
motiondue
and
the effects of interference of
Figure
3 Reduction
transmission
to reflection
light waves taken into account.
AR
AR
3,00
4,00
5,00 6,00
7,00 8,00
9,00
10,00
Figure 29: Reduction in the intensity of light transmitted caused
by reflections from the lens surfaces.
Power of the lens (diopters)
Figure
2 The
threshold
effect ofofchromatism
on visual
Figure
19:
The effect
chromatism
onacuity
vision:
a) Threshold of perception of colour fringing
b) Threshold effect on visual acuity.
To remedy this problem of chromatism, chemists are trying to
develop materials with low chromatism and therefore, with
higher Abbe values. Unfortunately, their leeway is relatively
limited and any increase in a material’s refractive index
54 • mivision
generally leads to an increase in its chromatism. In practice,
the effect can only be partially attenuated and the wearer
Figure
4 Principles
of antireflection
Layers
Figure
35: Principle
of anti-reflective
coating.
Consider the phenomenon that occurs for an isolated layer of
coating (figure 35). The light that reaches this layer breaks down
into light reflected by the layer and refracted light that enters
the coating. The latter then reaches the lens surface and divides
in turn into reflected light and refracted light. If the thickness and
the refractive index of the layer deposited on the lens are
carefully chosen, the reflected light is cancelled out. For this to
occur, the reflected light must be superimposed and be “out-ofphase”, i.e. the crest of one wave must coincide with the trough
C
th
in
th
© Essilor International - Varilux® University
Considering that 80
the refractive index of the most commonly used
lenses is 1.6, a rule
70 of thumb is that on average, the amount of
ν =42
light lost by reflection is about 10% of the incident light.
From
ν =37 on
this, we can see60the importance of anti-reflective coatings
ν =3215
lenses of high refractive
indices, as the loss of light can reach
50
ν
=30
to 20% for lenses of very high indexes.
40
With anti-reflective coatings, it is possible to reduce the
30 lost by reflection to less than 1% (see below).
proportion of light
0
© Essilor International
© Essilor International - Varilux® University
4
© Essilor International - Varilux® University
3
© Essilor International
2
Therefore, the total quantity of light lost by reflection on passing
through the two surfaces of the lens is:
2. Principle of anti-reflective coatings
© Essilor International
1
n+1
In practice, Abbe values νe and νd do not differ greatly, only the
first decimal being affected. The Abbe number varies in
ophthalmic optics between 60 for the least dispersive materials
and 30 for the most dispersive. Generally speaking, the higher
the refractive index of a material, the stronger its chromatic
dispersion and therefore the lower its Abbe number (see
materials table).
© Essilor International - Varilux® University
( )
R=
2
n–1
nd : is the index for λd = 587.56 nm
(helium yellow line)
nF : is the index for λF = 486.13 nm
(hydrogen blue line)
nC : is the index for λC = 656.27 nm
(hydrogen red line)
© Essilor International
20
15
10
5
0
ne : is the index for λe = 546.07 nm
(mercury green line)
nF’ : is the index for λF’ = 479.99 nm
(cadmium blue line)
nC’ : is the index for λC’ = 643.85 nm
(cadmium red line)
© ©Essilor
EssilorInternational
International
Together
a
a with the phenomenon of refraction of light through each
Excentricity
of the the
direction
lens (which
provides
lens’s corrective effect), a phenomenon
of gaze (degrees)
is produced that reflects the light from each surface: firstly on
the front surface60of the lens, but also on the rear, after passing
55
ν =58
through the thickness
of the lens. These reflections result
in a
50
reduction in the45intensity of light transmitted by the lens. ν =42
ν
40
The higher the refractive
index of the material, the greater=37
the
ν =32
35
intensity of the30reflected light. It can be quantified for each
ν =30
surface by the coefficient
of reflection.
25
© Essilor
International
- Varilux® University
© Essilor International
- Varilux®
University
reflection from the rear surface
© Essilor International - Varilux® University
es
a
s:
g,
m
ts
e
of
e
e
°.
ot
D Anti-reflective treatments
© Essilor International
fof
is
’s
e
d
ns
n
d
is
5
n
To quantify the transverse chromatism at any point on the lens,
the equation TCA = P / ν is used, of the deflection P of the rays
at this point (expressed in prism dioptres) and the Abbe number,
Dispersion
that thisofvaries
the
of TCA
is dependent
Theamount
variation
in thepresent
refractive
index with the and
wavelength
the between regions.
ν, of the materialLight
used. The deflection P of a single vision lens
It is important to remember that the Abbe
Measurements
conducted in Europe
on
theiseccentricity
gaze
the wearer, of chromatic
light
responsibleoffor
theofphenomenon
dispersion
being, according to the Prentice approximation, equal to h x F,
value is separating
not responsible
a losscentre
of optical
andindex
Japanis(higher
the
powerlight
of the
lens and
the Abbe
) are based on the
of white
during
refraction.
Asvalue
the refractive
e
where h is the distance
the for
optical
from the
transparency,
nor
is
it
the
only
factor
of
material.
of the
E
forthe
shorter
wavelengths, there is a change wavelengths
in the degree
of green Mercury
point on the lens and F is the power of the lens, it is therefore
perception
chromatic
(= 546.07nm), Cadmium
blue
refraction of the visible light from red towardsline
blue.
the
case that TCAinvolved
= h x F /inν.the
Thus,
it can beofseen
that transverse
It should be noted that the value of Abbe
dispersion
and should
notthe
be eccentricity
considered inof the
Chromatic dispersion is an important characteristic
for and Cadmium
line (=479.99nm)
red
chromatism
depends
on
three
factors:
number of a material varies according
isolation.
Eccentricities
gazethe
and
power
ophthalmic optics but of less consequence than
for(=643.85nm).
instrumental Measurements
line
gaze of the wearer,
the power
of the lensofand
Abbe
number
to the methodology of measurement,
optics: the human eye is itself strongly affected
chromatism.
in by
other
countries are based on
ofthe
the material. both have strong influencing relationships
Chromatism occurs in all lenses; it is always
considered
as yellow Fraunhofer
on the presence and perception of TCA.
wavelengths
of the
negligible at the centre because the longitudinal
Although, according to Guitton and
d (or D3)chromatic
Helium line (=587.56nm),
2
Abbe
– definition:
aberration
of the
lens is
lowlens
compared
ofvarious
the eye.
VolleConstringence)
, humans have an oculomotor
range
Hydrogen
blue On
line (= 486.13nm)
andvalue (or
Intrusive
reflections
of light
from
surfaceswith
canthat
be
of
To characterise
the
dispersive
power
of adegrees,
material,
a value called
other hand,
can prove
to be from
perceptible
(OMR)
of about
+/- 55
neural
Hydrogen
redwhen
line (= 656.27nm).
This
types:the
reflections
fromchromatism
the front surface,
reflections
the rear
themay
Abbe numberlimitations
or the constringence
is used
(defined by Ernst
the eye
through
the outerThey
areas result
of the measurement
lens,
becausemethod
the
to saccadic eye
movements
variation
surface
and looks
internal
reflections.
in reduced
Abbe,
a
German
physicist
and
industrialist,
1840-1905)
and
“Minimisation
of
Transverse
Chromatic
Aberration
(TCA)
of
the
lens
creates
automatically
produce
a
combination
result
in
small
differences
in
the
cited
transmission of light through the lens and cause undesirable
symbolised
by the
Greek
ν. It is atonumber
multiple
coloured
images
theseand
canunsightly
be
perceived
of head
andletter
eye movements
achieve inversely
Abbe
values
for a material and
care
reflections
thatoffset
are both
distracting
forthere;
the wearer
proportional to the
chromatic
dispersion
ofsignificance
the material and its
the wearer in the
form of coloured fringesshould
surrounding
fixation
eccentricities.
The
be takenthe
to ensure consistency
reflections
is critical
to theby
observer.
definition
varies
from
country
to country,
image oftypes
a highofcontrast
object
figure below,
18).
of
the combination
of head
and eyedepending on
ofXP
measuring
ESSILORtechniques
ANGLAISwhen
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en page
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20/05/10
10:22
Page33
The different
reflection
are (see
described
together
the wavelengths on
which theand
definitions
are based.
movements
the resultant
‘re-centring’
comparing materials.
with the
offered by anti-reflective
tosolutions
the optimisation
of coatings.
of the eye in the spectacle lens, is that
The generally accepted ‘rule of thumb’ is
80 per cent of all ocular fixations occur
visual outcomes
1. Different
typesand
of reflections
and
that the
higher the refractive index
within
of centre
and 100 countries:
per
in
Europe
and
Japan:
νe ±15º to 20ºin the
English-speaking
νd
of a lens, the lower the Abbe value of
their effect
cent
are within ±30º.
n
–
1
n
–
1
e
d
wearer comfort”
the material (See Table 1) and the ν =
νd = n – n
e
nF’ Using
– nC’ Figure 2, we see that
C
withFan eye
stronger the TCA (the greater the
a. Reflection from the front surface and
internal
rotation
of
20º,
the
power
of
the
lens
has
chromatic
dispersion).
where
where
Transparency
and durability
m
of
ic
*).
ss
–
y
ic
1. Chromatism in ophthalmic lenses
mieducation
MATERIALS & TREATMENTS
o
by
B Chromatism of the material
W
o
im
v
re
se
re
In
“m
re
u
li
e
re
h
a
p
th
w
in
It
co
to
li
m
b
F
a
AR
Figure 7 Improvement
in contrast
anti-reflective
coatings
Figure
33a: Improvement
inwith
contrast
with anti-reflective
coatings.
© Essilor International - Varilux® University
Figure 6 Double images, caused by internal reflection within the lens
Figure 31: Double images, caused by internal reflection within
the lens.
(1) Stuart G. Coupland, Trevor H. Kirkham: Increased contrast sensitivity
Although
to
7.00D
withlenses
a material
of Abbeof glare, Canadian
withexceed
antireflective
coated
in the presence
Journalthis effect is most apparent in
high index materials, optical brighteners
of Ophthalmology,
1981; 16:
137-140
value
= 32 (Refractive
Index
1.67) before
(2) Trevor
H. Kirkham,
Stuart G.Chromatic
Coupland: Increased visual field
mayarea
be with
added to the lens material
visual
acuity
is affected.
antireflective coated lenses in the presence of glare, Canadian Journal of
composition
to counteract this yellowing
aberration,
therefore,
has
limited
effect,
Ophthalmology, 1981; 16: 141-144
effect.
The
except
at the
periphery
of high-powered
(3) Catherine
Eastell:
The effectiveness
of AR-Multireflection
coatings
on loss of lens transparency
night driving,
of Optometry,
1991
associated
with the apparent colour of
lenses
madeCardiff
fromCollege
materials
that are University
highly of Wales,
(4) Study conducted in the United States by an independent vision
the
material
has little impact on overall
dispersive.
It
has
no
significant
influence
research centre, 2004/2005
visual outcomes for the spectacle wearer.
on the transparency of the lens or visual
performance for the majority of wearers.
The apparent colour of the lens may also
Colour
Another aspect of lens transparency
is determined by the chromatic
composition of the light that is
transmitted. Ideally, the lens should
transmit the full visible spectrum and
appear perfectly colourless or ‘white’.
When the visible spectrum of light is not
fully transmitted, the lens takes on the
complimentary colour of the wavelength
not transmitted. For example, when the
blue wavelengths of the visible spectrum
are absorbed by the lens material,
the lens takes on a slightly yellowish
appearance. Typically, this occurs when
the material has a higher attenuation of
UV and near UV radiation ( < 400nm).
AR
© Essilor International
© Essilor International
© Essilor International - Varilux® University
AR
© Essilor International
AR
© Essilor International
Finally, it is possible to produce so-called “achromatic” coatings,
i.e. coatings with a uniform residual reflection of different colours
of the spectrum so that no specific colour can be observed… but
this often impedes their recognition and identification!
change due to the effects of aging. Plastic
materials are sensitive to the effect of light
exposure over time and have a tendency
to yellow slightly as they age. This is due
to the interaction between the chemical
structure of the lens material and
visible radiation. Due to their inherent
chemical structure, high refractive index
materials are more sensitive to this
process. Optical brighteners added to the
material substrate play a role in delaying
colour changes due to the aging process.
Similarly, antireflection lenses are less
influenced by the aging process.
Reflection
Loss of transmission of light as it
passes through the lens material due to
reflection has a significant impact on
the transparency and clarity of a lens.
Minimisation of reflections is critical to
the optimisation of visual outcomes and
wearer comfort.
Reflections are generated at the front and
back surfaces, as well as internally. These
reflections result in a reduction in the
intensity of light transmitted (See Figure
3). The surface co-efficient of reflection is
quantified by:
(insert equation here as follows)
Materials of higher refractive index
create greater intensity of reflected light
(see Table 2).
Table 2 demonstrates that 15 per cent or
more of light can be lost due to reflection.
This reduction of available light may
result in the perception of reduced
clarity and contrast sensitivity. With an
antireflection lens it is possible to reduce
the proportion of light lost by reflection to
less than 1 per cent.
mivision • 55
© Essilor International
AR
Moreover, beyond the question of aesthetics, the choice of
residual colour of an anti-reflective coating may also be based
on technical criteria, in particular as a function of absolute or
differential sensitivity of the eye to different colours. That is how
the yellow-green reflection of Crizal® coating was chosen.
© Essilor International
© Essilor International
To describe the improvement in contrast provided by anti5 coating, the visual task of a subject trying to distinguish
reflective
two object points can be analysed and, to do this, we must
examine4 the formation of images on the retina. Like any optical
device, the eye has imperfections and the image that the eye
forms of an object on the retina is not a point but a luminous
3
spot. Thus
a view of two points is seen as the juxtaposition of
two luminous spots that overlap to some extent. As long as the
distance2 separating the two points is sufficient, the image formed
on the retina allows them to be distinguished. When the points
approach each other, the two spots tend to merge and the
subject 1sees only one point.
This phenomenon may be quantified, starting with minimum and
maximum
intensities of the luminous spot, in the form of
0
contrast of 350
the400image 500
formed,600
according
700 to the
800 formula:
C = (a – b) / (a380
+ b), with “a” being the maximum780
intensity, and
(nm) (see
“b” the minimum intensity of the luminous spotWavelength
on the retina
figure). For the two points to appear separate, C must be higher
Figure
Reflection
spectrum
of anti-reflective
coating
than
a5 value
corresponding
to the
detectioncoating.
threshold.
Figure
37: Reflection
spectrum
of eye’s
anti-reflective
MATERIALS & T
Reflection (%)
A) Improvement
in contrast
6
© Essilor International - Varilux® University
visible effect on the colour of the reflection. This is why, in
prescription laboratories, both lenses in a pair of spectacles are
usually anti-reflective-coated in the same production run. On the
other hand, for mass production lenses, strict control is
necessary to ensure that lenses manufactured at different times
and with different equipment, match up when mounted in the
same frame. That is why, in every production run, control lenses
are included to ensure that the specified reflection and
colorimetry of anti-reflective coatings are adhered to.
© Essilor International - Varilux® University
30: Alteration of the visual contrast caused by reflection
from the rear surface of the lens.
ont
Crizal®-type
coating,
with a yellow-green
reflection.
conditions
(such
as driving
at night), residual
and may
be considerably
Note that controlling the colour of the residual reflection is a
reduced
by
applying
an
anti-reflective
coating
difficult technical exercise because the slightest variation into
the the two
surfaces
of the
lens.
refractive
index
or the thickness of the layers has an immediate
© Essilor International
appearance
of the lenses.
These
benefits are
always
fully
Medium
1,0 à 1,8
%
96,0 not
à 97,5
%
understood by eyecare professionals themselves, and even less,
Standard
1,8 à 2,5
%
94,5 àhere
96,0 %in detail,
therefore,
by the general
public.
It is shown
supported by the results of experimental studies, the two most
significant visual benefits: improvement in visual contrast and
reduction of the effects of glare.
the
ver
of
ect
the
ally
to
has
ive
tly
.
ror
33
mieducation
In order to understand how reflections are
reduced and light transmission improved,
we must first understand the nature of
antireflection lenses.
Antireflection lenses consist of a number
of layers built up or ‘stacked’ on the
surface of a lens. These layers act together
to interfere with the reflected light. Light
must be considered as a wave motion, and
the effects of interference of light waves
are taken into account.
Consider the destructive interference
that is achieved by an isolated layer (See
Figure 4). The light that reaches this layer
breaks down into light reflected by the
layer and refracted light that enters the
layer. The latter then enters the material
substrate and divides further into reflected
light and refracted light. If the thickness
and refractive index of the first layer are
carefully chosen, the reflected light is
‘cancelled out’. For this to happen, the
reflected light must be superimposed and
‘out of phase’, that is, the crest of one wave
must coincide with the trough of another
wave. This acts to suppress the reflected
light. Any light that is not reflected is
added to transmitted light, thus markedly
improving light transmission through the
lens. Calculations show that in order to
cancel out the reflected light, the thin layer
must have:
•a refractive index (n’) equal to the square
root of the index of the material; and
•a thickness of /4, with  being the
wavelength of light to be suppressed.3
With a single layer, it is possible to
suppress the reflection for a given
wavelength of light, but it is impossible to
suppress reflections for every wavelength
in the visible spectrum. Generally,
reflection reduction is targeted to the
part of the spectrum to which the eye
is most sensitive, that is, green-yellow
light (=555nm). As the intensity of
reflected light is higher in the blue and
red wavelengths, the residual reflection or
‘bloom’ of the lens will be purple.
By using several layers to enable multiple
suppressions of reflected waves, residual
reflections are eliminated across the whole
visible spectrum. Each of these layers
produces a reflected light wave, and these
various light waves are out of phase with
each other. A complicated calculation is
used to determine how to obtain almost
complete suppression of reflected light.
While the single layer gives residual
reflection in the order of two per cent per
surface, the multiple layers result in less
than one per cent reflection. The multiple
56 • mivision
layers also have the effect of reducing the
chromatic effect (the colour of residual
reflection) to a very low intensity.
The effectiveness of an antireflection lens
is not directly proportional to the number
of layers, but rather the way they are
‘stacked’ and how the light reflected from
each layer interacts. While the number of
layers with an ‘antireflection stack’ varies
between manufactures, it is generally
between three and eight layers.
The effectiveness of an antireflection lens
is measured by its ‘reflection spectrum’,
a graph which shows the intensity
of reflected light as a function of the
wavelength (See Figure 5). The area
under the curve represents the quantity
of reflected light remaining. The residual
colour of antireflection lens is defined
by the part of the light spectrum that it
reflects. Depending on the type of layers,
residual reflection may be different
colours. In Figure 5, (which represents the
reflective spectrum of the surface of a lens
of index 1.5):
•T he white line represents a standard lens
without antireflection properties, where
we see that all wavelengths are reflected
in a uniform manner at four per cent
•T he blue curve represents the reflection
of a single antireflection layer, where
we see the intensity of reflected light is
higher in the blue and red, giving a purple
colour to the bloom;
•T he yellow curve represents the reflection
of a multilayer coating (in this case a
Crizal type coating), with a low intensity
yellow-green residual reflection.
Figure 5 also demonstrates that light
is reflected across the entire spectrum,
including the wavelengths within the UV
range. This means that, along with back
surface glare from visible light, UV is also
reflected off the rear surface of a lens into
the wearer’s eye. Hitherto, antireflection
lenses have reduced the amount of back
surface visible light reflected into the
eye, without reducing the UV reflected
off this surface. However, recent product
introductions globally have addressed this
issue, extending the attenuation beyond
380nm. This new benefit does not alter
the transparency or visual performance of
the lens, but has obvious repercussion on
ocular health.
It should be noted that controlling the
colour of the residual reflection is a
difficult technical exercise because
the slightest variation in the refractive
index or thickness of the layers has an
immediate visible effect on the colour of
the reflection. Strict quality controls in
the manufacturing process are required to
ensure consistency of the colorimetry and
specified reflection.
One phenomenon occasionally associated
with antireflection lenses is interference
fringes (or Newton’s rings). These
fringes may appear to compromise lens
transparency. Interference fringes may
occur when there is a significant difference
between the refractive index of the lens
material and the adjacent layer (often
the anti-abrasion layer), or where the
interfacing layer thickness varies. In this
case, interference fringes may appear
under monochromatic light such as
fluorescent tubes. While this may disturb
the aesthetic appearance of the lenses, it is
not able to be seen by the wearer and does
not impact visual quality.
This issue may be overcome by
manufacturers by either:
•‘Index matching’: reducing the
difference in refractive index between
the layer and material substrate; or
•‘Quarter wave layering’: the introduction
of an additional layer to suppress the
wave reflected by the material substrate.
Ultimately, the interference created by the
antireflection stack and the subsequent
reduction of reflections improves the
transmission of light and aesthetic
appearance of the lens. Probably the
most obvious and best known reflection
from a spectacle lens is the front surface
reflection. This is the reflection seen
by an observer situated in front of the
spectacle wearer and appears as a mirror
image of the source of ambient light. It
does not affect the spectacle lens wearer.
The reduction of front surface reflection
is largely an aesthetic issue, and is one
of the most common reasons stated for
dispensing an antireflection lens.
Although often overlooked in preference
for the cosmetic benefits, antireflection
lenses have two significant visual benefits:
improvements in visual contrast and
reduction of the effects of glare. Both of
these factors have a significant impact
on the wearer’s perception of transparency
and clarity.
The double internal reflections within
a spectacle lens produce a significant
impact on optical performance. After
refraction at the first surface of the lens,
the light beam reaches the second surface
where, in addition to refraction, a second
reflection of light occurs. The reflected
light is then again reflected off the front
surface of the lens and after refraction off
the second surface, gives rise to a second
image of lower intensity slightly displaced
from the main image. This results in the
perception of a double or ‘ghost’ image.
These images are usually most noticeable
in low light levels such as driving at night
(See Figure 6).
Reflections from the back surface of the
lens may also give a wear the perception
of loss of transparency of the lens.
Typically, back surface reflections occur
when a light source is offset behind
the wearer. This is visually annoying
particularly in low light conditions.
This unwanted reflected light may
superimpose over the object being
viewed, causing a reduction in contrast
and quality of vision. This is often
perceived as glare.
To describe the improvement in
contrast provided by antireflection
lenses, the visual task of a subject trying
to distinguish two object points can be
analysed. To do this, we must examine
the formation of images on the retina.
Like any optical device, the eye has
imperfections and the image that the eye
forms of an object on the retina is not a
point but a luminous spot. Thus a view
of two points is seen as the juxtaposition
of two luminous spots that overlap to
some extent. As long as the distance
separating the two points is sufficient, the
image formed on the retina allows them
to be distinguished. When the points
approach one another, the two spots tend
Table 1 – Material Refractive Index, Abbe Value
Table 2 Percentage Light Reflection and Transmission by Index
to merge and the subject sees only
one point.
The phenomenon may be quantified,
starting with minimum and maximum
intensities of the luminous spot, in the
form of contrast of the image formed,
according to the formula: c=(a-b)/(a+b)
- a being the maximum intensity of the
luminous spot; and
- b being the minimum intensity of the
luminous spot.
For the two points to appear separate, C
must be higher than a value corresponding
to the eye’s detection threshold (See Figure
7). When driving at night, the headlights
of a car following behind the driver may
reflect off the rear surface of a spectacle
lens, creating a perception of glare. This
distracting glare creates a luminous spot
of uniform intensity on the retina. In the
case of a driver trying to clearly distinguish
two approaching cyclists at night, this
glare created off the back surface of the
lens is added to the intensity of the two
points observed for the cyclists. The result
is a net decrease in contrast that becomes:
C'=(a'-b')/(a'+b'). If the decrease in contrast
is sufficient, it may cause the driver to
no longer be able to distinguish the two
separate cyclists or even to lose sight of the
cyclists completely. By reducing reflections
of light from the rear surface of the lens,
antireflection lenses can minimise or even
eliminate this effect altogether.
Studies4 have shown that in the presence
of a disruptive light source antireflection
lenses can considerably improve contrast
sensitivity. In the same way, it has been
established that, under predetermined
conditions of glare, a spectacle wearer’s
field of vision is considerably wider with
antireflection lenses when compared to
uncoated lenses5. It has also been shown
that improving the transmission of light
and reducing unwanted reflections,
with an antireflection lens, results in a
reduction of 2 to 5 seconds in recovery
time to normal vision after being dazzled6.
When recommending or dispensing
a spectacle lens, it is important to be
aware that these factors associated with
antireflection lenses have a significant
impact on
perceived quality
of vision, optical
performance and
visual fatigue,
particularly in
situations such as
driving at dusk
and night.
Although the challenges to lens
transparency of ‘wear’ factors such as dirt,
dust and scratches may seem to be beyond
the capacity of lens choice, options do exist
to improve lens performance in this area.
Surface irregularities such a scratches
and dirt cause diffraction (changes to
the direction of light), which can be
distracting to the wearer. Scratch resistant
coatings aim to make the surface of the
lens harder in order to resist the effect of
fine particles. These coatings are highly
flexible to withstand the challenge of
large particles.
Lenses may also offer oleophobic (oil and
smudge resistance), or anti-static features
that ensure the surface remains cleaner
for longer. In cases where lens fog creates
an unacceptable loss of lens transparency,
coating technology is now available to
resist vapour droplet formation on the lens
surface ensuring consistency of clarity.
Lens transparency is affected by a
number of factors including the lens
material and treatments. Combined
with good lens design and frame choice,
the appropriate material and treatment
choice will significantly enhance
the optical performance of a pair of
spectacles. Optimal patient outcomes
and spectacle wearer satisfaction relies
on the consideration of all of the facets
of lens dispensing.
Helen Venturato is the Professional Services
Manager for Australia and New Zealand at Essilor
Australia. She has practiced optometry throughout
Australia since 1990. The material source was The
Ophthalmic Optics Files: Materials and Treatments
written by Dominique Meslin for the Essilor
Academy Europe and is available at
www.essiloracademy.eu
To earn your CPD points from this article,
answer the assessment available at:
www.mivision.com.au/optimisingoptical-transparency
References
1. Meslin D. Ophthalmic Optics Files: Materials and
Treatments, Essilor Academy Europe, 2010.
2. Guitton D and Volle M. Gaze Control in Humans:
Eye-Head Coordination During Orienting Movements To
Targets Within And Beyond The Oculomotor Range, JN
Physiol 1987 Sept; 58 (3): 427-459.
3. Efron N. Optometry A-Z, Butterworth Heinemann, 2007.
4. Coupland,SG and Kirkham TH. Increased Contrast
Sensitivity With Antireflective Coated Lenses In The
Presence Of Glare, CJO 1981 16: 137-140
5. Kirkham TH and Coupland SG. Increased Visual Field
Area With Antireflective Coated Lenses In The Presence
Of Glare, 1981 16: 141-144
6. Eastell C. The Effectiveness Of AR- Multireflection
Coatings on Night Driving, Cardiff College of Optometry,
University of Wales, 1991.
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