 Vision is a complex activity. For the majority of patients though, vision is very
simple; open your eyes, put on your glasses and the world is crisp and clear.
Even a majority of eyecare professionals, when asked about the most important attribute of glasses, will say “clear vision.” Yet, clear vision is a combination of many effects.
Visual performance is the combination of three things: visual acuity, contrast sensitivity, color or texture. In single-vision lenses, proper selection of
the front surface (base curve)
and use of aspheric surfaces
diminish aberration and provide clarity. In progressive lenses it is more complex.
A progressive lens provides
clear vision by managing the
vertical integration of distance
to near along with the horizontal effects of the periphery.
Providing adequate zones of
visual acuity at all visual distances while controlling aberration to provide optimal contrast sensitivity, are the keys to
the best vision in progressive
lenses.
VISUAL ACUITY
Visual acuity is the most common clinical measurement of
vision. Visual acuity measures
the resolution (i.e., smallest
recognizable object) of the
eye. What is commonly called
“20/20” refers to a specific letter size that can be
recognized from 20 feet.
It’s a way of quantifying
a patient’s vision, is used
to assess whether someone sees normally or
well enough for driving
or is legally blind.
Each line of the Snellen
chart uses letters with
precise size and spacing
to provide a consistent,
clinical measurement of
visual acuity. Line 8 of
the Snellen chart is the
20/20 line.
However, there is another aspect of vision that
impacts visual quality. The
crispness of the edge of
the ‘E’ is also important, as is the ability to see the ‘E’ in less than optimal lighting conditions. In other words, the ability to perceive contrast is another key
component of visual quality.
CONTRAST
Contrast refers to the difference in color or dark and light between parts of
an image. The greater the contrast, the easier objects are seen. In a visual
acuity test, the bulb that projects the letters has a specific brightness. The
screen has a particular color and surface smoothness and the exam room
lights are dimmed. In this way, the test conditions are controlled to provide
optimal contrast (black letters on an illuminated white background). This
ensures that acuity can be best tested. However, even though contrast control is required for good acuity testing, the patient’s sensitivity to contrast
is not measured.
CONTRAST SENSITIVITY
In “real life,” vision is used to view images with all levels of contrast (i.e., few
of the images patients view will have black objects on an illuminated white
background). isual acuity alone does not measure the quality of a patient’s
vision. Contrast varies, so a patient who achieves “20/20” vision during an
optimal assessment of acuity may still complain of poor night vision or unsatisfactory vision in general.
 Contrast sensitivity is the visual ability to see objects that may not be outlined clearly or that do not stand out from their background. For example,
it’s the ability to see a shade of gray on a white background. As we age,
cataracts, cloudy media, diabetic retinopathy and macula conditions can
reduce contrast sensitivity.
From Wikipedia: a person with low contrast sensitivity may have such vision
difficulties as, trouble seeing traffic lights or cars at night, not being able to see
spots on clothes, counters or dishes, missing facial gestures, not seeing whether
a flame is burning on a stove, needing a great deal of light to read, experiencing
tired eyes while watching television.
Therefore, we must consider acuity and contrast sensitivity when
choosing the best lenses for a particular patient—particularly when prescribing and fitting progressive addition lenses (PALs). Due to their
complex surfaces, PALs usually contain higher order aberrations that
diminish image contrast. If a lens can
better deliver the prescription as
intended—while maintaining image
contrast—then the lens will provide better real-life vision to the wearer and will
be preferred. By controlling aberrations—and maintaining image contrast—a
PAL can improve the patient’s ability to see visual detail and perceive color. Such
a PAL would deliver “real world” vision (i.e., the ability to perceive detail even in
non-optimal lighting conditions). Improving contrast sensitivity will help distinguish objects from their backgrounds, see textures (like the weave in a carpet),
see visual details (like freckles on the face of a child) and see colors in all their
richness and vividness (like the intensity and depth in a bouquet of flowers).
RECOGNITION
Lastly, knowledge of the object, its name and its color is the final part of
vision. Previous knowledge allows the individual to recognize the letters in
the Snellen chart for testing or a street sign’s words to know when to turn.
Put visual acuity, contrast sensitivity and recognition together and you have
each of the key components for vision.
HOW DOES CONTRAST SENSITIVITY AFFECT VISION?
How important is contrast sensitivity to visual quality? Consider the left image.
The acuity is good in this picture, the lines are sharp and most individuals
would readily identify this image as a flower with distinct petals and leaves.
However, when increased contrast is added the image “comes alive.” With
sufficient contrast, the veins of the leaves, folds of the petals and the inner
parts of the flower become visible.
A patient with decreased contrast sensitivity may be able to see the flowers on the left, appreciate the rich yellow color, accept it as normal but miss
the detail. In reality portions of the image are literally “invisible” to those
with decreased contrast sensitivity.
Contrast sensitivity is the quality, which allows the eye to perceive subtle
changes in color—such as the pattern on a leaf, granules of snow or the various shades of the moon. In each case, the image can be ‘seen’ with
decreased contrast perception, but contain details that are only visible when
contrast sensitivity is high.
CONDITIONS THAT AFFECT CONTRAST SENSITIVITY
The eye’s ability to perceive contrast is known as the Contrast Sensitivity
Function (CSF). Several factors may inhibit Contrast Sensitivity Function.
They are lighting, the visual system and the health of ocular structures,
including aging.
Lighting — As the level of illumination decreases, differences in shade/contrast (as well as differences in hue/color) become harder to perceive. In
mesopic (dim light) and scotopic (near darkness) conditions, the human eye
is less able to perceive contrast. This is due in part to the function of the rods
and cones in the retina. In dim light, only the rods are active and rods do
not perceive color. In reduced light, cone sensitivity is also reduced and subtle differences in color are lost. So, in low light or night conditions, the best
vision requires all the light that one can get.
Although many believe that a light tint (especially yellow) will improve
night vision, in reality any tint—or reduction in the amount of light reaching the retina—will decrease the patient’s night-vision.
Visual System — In addition to lighting, other viewing conditions impact
the ability to perceive visual details and images. Imperfections in the visual
system, like fog, a car windshield, eyeglass or contact lenses and the eye itself,
all introduce potential imperfections to the visual system that impact the level
of visual detail that reaches the eye from the image viewed.
Aging — Aging also has a detrimental effect on the eye’s ability to perceive
contrast due to the reduction of light reaching the retina. The retina of a 60
year old receives only 1/3 the light it did when the individual was 20 years old, due to changes in the ocular structure. The greatest age-related impact
on contrast sensitivity is likely a reduction in neuron function.
The Eye Itself — The eye itself adds a variety of aberrations that can affect
contrast, especially higher order aberrations that negatively impact the ability to transmit contrast. Every eye contains a certain amount of internal aberration. These aberrations are contained in the optical elements of the eye,
the cornea, crystalline lens and even the vitreous humor. Aging brings with it
changes to the clarity of all the objects and spaces that light passes through
on its way to the retina. Cataracts, macula changes, cloudy vitreous, too small
a pupil, all reduce contrast and acuity.
The iris helps preserve contrast by forming an opening, the pupil, which
acts as a lens stop. The lens stop reduces the amount of aberration, which
enters the eye. As the eye ages, pupil size tends to decrease, which assists in
preserving the visual quality of the image to the eye.
Surgeries like laser eye surgery can add to the higher order aberrations of
the ocular system. As a result, wavefront-optimized laser eye surgery was
developed to reduce the amount of higher order aberrations that might be
caused by the operation.
To summarize, every eye has an inherent ability to perceive contrast, called
Contrast Sensitivity Function. Each optical structure, within the eye, contributes to the translated contrast of the original object reaching the retina.
CONTRAST AND SPECTACLE LENSES
When spectacle lenses are placed in front of the eye, they become part of the
optical system. Any aberrations within lenses will impact the ability of the
eye to perceive contrast.
Lack of anti-reflective treatment on eyeglass lenses reduces contrast. The
veiling glare created blur and distortion that reduces contrast.
Lens aberrations also create blur that reduce contrast. As we said, fewer high
order single-vision lenses have uniform curvature so they produce aberrations
and as a result, have less effect on contrast sensitivity. Progressive addition
lenses are not rotationally symmetrical because of the add power in the lower
portion of the lens. The changes in curvature required to provide a seamless
transition from far to near power create higher-order aberrations across the
surface of the lens, even in areas, which are thought to be “spherical.”
Changing curvatures across the surface of a PAL creates unwanted astigmatism and also higher order aberrations, like coma, which can have a significant impact on contrast sensitivity. Manufacture and processing of the
surfaces can further create aberrations that affect the wearer’s sensitivity to
contrast and crispness of vision. Therefore, the design of the lens must consider these conditions to provide the crispest vision. The production of
prescription progressive lenses must also be precise to replicate the intended design.
WHAT DO PATIENTS SAY?
 The symptoms of decreased contrast sensitivity
include difficulty with nighttime vision, vision that
“doesn’t seem sharp” and trouble reading in dim
illumination. Additionally, many patients with
decreased contrast sensitivity may fail to report
symptoms since they have become used to the
decrease in contrast perception. Patients complaining of not seeing clearly at night or in low light situations,
like driving at dusk or at night, may have contrast sensitivity issues.
Patients
that complain of vision that “doesn’t seem sharp” even though their visual
acuity may be 20/20, may also have contrast sensitivity challenges.
Emmetropes and low prescription patients may especially notice this. Also,
patients that complain about poorer vision with eyeglasses than their contact
lenses are candidates.
Generally speaking, difficulty seeing contrast when there is not enough
light also means problems during the day. How much are your patients missing? There could be an entire world of color and detail that they’re not seeing. So how can you make patients see better?
NEW TECHNOLOGY TO DELIVER INCREASED
CONTRAST SENSITIVITY
Given the vital role contrast sensitivity plays in quality vision, Essilor engineers have created new technology to control higher order aberrations in
progressive surfaces. Essilor created advances in three areas of design technology to deliver improved contrast sensitivity:
- Optical surface measurement of higher order aberrations.
- A calculation engine capable of controlling detected aberrations.
- A manufacturing process capable of transferring precise designs to a finished optical surface.
To accurately measure the wavefront produced by an ophthalmic lens,
Essilor developed patented technology, which can simultaneously analyze
both surfaces of a lens. Simultaneous analysis of both lens surfaces allows
engineers to measure the wavefront produced by the lens to identify specific aberrations present in the lens. Using this instrument, design engineers are
able to measure the higher order aberrations of a wavefront produced by
any ophthalmic surface. Once higher order aberrations have been identified, a calculation engine
iterates a surface to eliminate and control
aberrations. This is called W.A.V.E. Technology: Wavefront Advanced Vision Enhancement. It is the design engine/process created
to control aberrations.
W.A.V.E TECHNOLOGY
W.A.V.E Technology enabled Varilux engineers
to move past prior limitations in the design
process. This advanced engineering dramatically reduces or eliminates higher order aberrations, providing measurably increased contrast sensitivity for all light conditions. This new technology delivers an improvement in the contrast
sensitivity delivered to the wearer of up to 30 percent versus progressive
lenses without W.A.V.E. Technology.
PREPARE PRECISION SURFACES USING DIGITAL SURFACING
Finally, measurement and control of these aberrations would have little real
world impact without a manufacturing technology capable of producing the
sophisticated design on a finished lens.
Traditional methods of progressive mold manufacture are incapable of creating the precision needed for this kind of surface control. As a result, single-point diamond turning, digital surfacing, a new mold and lens surface manufacturing process, is used. It replicates the design exactly. Digital surfacing
produces the accuracy required (1/10th micron) to create the molds for Varilux Physio and the optimized back surface of Varilux Physio 360°.
Varilux Physio 360° is the result of W.A.V.E. Technology and the optimization of individual prescriptions. Using proprietary Digital Surfacing, the Varilux Physio 360° and the Varilux Ellipse 360° create a unique backside design
based on the individual wearer’s prescription to deliver optimal performance.
This fully integrated design provides enhanced fields
of vision, optimized optics and optimal sharpness for
every patient, regardless of prescription.
DESIGN EQUALS PERFORMANCE
Using digital surfacing, progressives can deliver
exactly what is required to optimize vision and provide patients with a better experience for them personally. However, one must start with the right
design. Just being able to digitally surface lenses
does not automatically mean a better lens. Starting
with the right design is critical, one that is the result of good vision science
and wearer tests that confirm its efficacy.
Varilux Physio was designed and tested with more than 2,000 wearers
before its launch in 2006; it is one of the most-tested lens designs ever produced by Essilor. Results of these
tests consistently indicate superior wearer performance.
In one 81 patient double blind
clinical study, visual quality of Varilux Physio and another digitally
surfaced lens was measured. It
demonstrates that the combination of good vision science, design
measurement and manufacturing
technologies could produce a preferred lens i.e., one that is perceived to
improve the wearer’s experience.
CONCLUSION
Contrast Sensitivity is a vital
component of perceived
visual quality. Although a
patient may have acceptable resolution (i.e., may
see “20/20”), decreased
contrast sensitivity will negatively impact their vision.Although patients may
grow accustomed to a loss
of contrast sensitivity, some
will not recognize the
decrease or verbalize this
loss. They should not be left with less than the best vision.
W.A.V.E. Technology allows Varilux Physio to produce images with greater
contrast resulting in greater visual comfort and satisfaction. ■
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