In today’s tough
marketplace, it is important to
differentiate the professional optical dispensary from one
in which just orders are taken. One way to do this is to
offer premium products to the patient. In order for this to
happen, one must be knowledgeable in lens design and
technology. Aspheric lenses incorporate some of the most
effective and efficient lens designs.
Spherical Lenses
Most lenses are spherical lenses. A basic lens in terms
of design, spherical lenses have the same curvature
throughout the front surface, or base curve, of the lens. To
create the prescribed power, the back surface has layers
ground off of the lens, resulting in correction needed by
the patient. Spherical lenses are the easiest way a
transparent material can be utilized to aid in vision
through the process of grinding and polishing. It does have
some issues though.
One such issue with spherical lenses is an optical
phenomenon called spherical aberration. Spherical
aberrations occur because not all points of the lens refract
the incoming light the same amount due to the continuous
curvature of the lens. The optical center of the lens has an
imaginary axis that passes through it and is at right angles
to the lens surface. This axis is called the optical axis.
It is the point on the lens where the incoming light is not
deviated. However, as the lens curves, light rays entering
the periphery of the lens are bent more than rays closer to
the optical axis. This causes the rays to come to focus at
multiple points sooner than the location that they would be
expected to focus based upon the prescription. This problem
results in a blurred image. Since this is a wide beam
aberration, and the pupil filters some of the distortion
from reaching the retina, it is not as much of a concern to
lens designers as the narrow beam aberration, marginal
astigmatism. However, in higher prescription powers
spherical aberration is still a considerable issue and is
the reason that aspheric lenses were designed initially.
Marginal astigmatism, also known as oblique and radial
astigmatism, is the result of narrow parallel beams of light
passing obliquely through the lens. Because the rays are
narrow, the pupil does not filter them as well and lens
design becomes more of a factor. The narrow oblique beams,
entering the lens at opposing meridians, cause the rays to
come to focus at two different points. The difference
between the two points is the amount of marginal astigmatism
that is found in the lens.
If the power of the lens is minimal, the mind overcomes
the distortion. If the power of the lens is higher, the more
marginal astigmatism becomes an issue. This is because
marginal astigmatism is heavily affected by the relationship
between the front curve and back curve of the lens. The
effect of marginal astigmatism is also greater the farther
away from the optical axis the eye travels. This is one
reason why a patient wearing single vision lenses may
complain of peripheral distortion when glancing out of the
side of his or her glasses.
When using spherical lenses, lens designer handle the
issue of marginal astigmatism by selecting the appropriate
base curve utilizing Tscherning’s Ellipse. It uses plot
points to determine the best curves for a lens. Tsherning’s
system prefers fairly steep base curve to eliminate the most
marginal astigmatism but the flat base curve is more
cosmetically appealing. The lenses designed with this
process are called best form lenses because they are the
most optically and cosmetically appealing lens available
using spherical curves. Unfortunately, these lenses are also
thick, heavy and steeply curved for many prescriptions. To
remedy this, lens designers introduced aspheric lenses to
routine prescriptions in 1989. Previously, aspheric curves
were only available on high plus aphakic lenses.
Aspheric Lenses
Aspheric lenses addressed the two main issues lens
designers have when developing a lens: marginal astigmatism
and appearance. Aspheric, or “non-spherical” lenses, are
lenses in which the front surface powers gradually change
from the center of the lens to the edge of the lens. The
resulting minor changes in lens curvature minimize, or may
even eliminate, the blurry images created by spherical
aberration when the eye rotates away from the optical center
of the lens. The asphericity of the lens also allows the
lens to be flatter without compromising optical quality. The
result is a lens that:
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Fits closer to the eye.
-
Is flatter and thinner.
-
Fits better in most frame designs.
-
Has reduced magnification and/or minification of the
eyes in higher powers.
-
Has reduced magnification and/or minification of the
retinal image size in higher powers.
Aspheric Lens designs
Computer programs are needed to create the gradually
changing, multi-curve surface of an aspheric lens. These
computer programs use a polynomial, mathematical equation to
determine the rate of flattening the lens surface will have.
The reason each lens manufacturer has a different
asphericity in their lens design is because each one uses
different coefficients in their polynomial equation. Most
aspheric lenses are either a true aspheric or semi-aspheric.
In a true aspheric lens the change in curvature begins at
the optical center. The curvature can begin steep or it can
begin at a more gradual rate. In a semi-aspheric lens, the
lens has a spherical center in widths varying from 10mm to
45mm. Although a semi-aspheric lens is not as cosmetically
appealing as the aspheric lens, it is still thinner than a
spherical lens and is more forgiving if the fit is off.
Fitting Aspherics
Although accuracy is important when fitting any patient,
aspheric lenses are not as forgiving as some other lens
designs. Therefore, it is important that the ECP brush up on
his or her fitting skills when designing a pair of glasses
using aspheric lenses.
Rules to fitting aspheric lenses include:
Following the manufacturer’s
recommendations- Because
each manufacturer designs their own proprietary computer
systems to create their lenses, each one has unique fitting
guidelines. Know and utilize the guidelines for each
aspheric lens that you use.
Use a pupilometer- Take monocular PDs.
Center the eye- The frame selected should have the eye in
the center of the lens, both vertically and horizontally, as
much as possible. Be sure that the faceform and pantoscopic
tilt match the eye’s location in the frame. The closer the
eye is to the center of the frame, the less face form and
tilt that is needed.
Adjust, adjust, adjust- Good frame adjustment is key,
both at the fitting and the dispensing. Ensure that the
frame fits properly, is comfortable and has the correct
amount of pantoscopic tilt and face wrap according to the
placement of the eye in the frame.
Measure the optical center- This is the point most ECPs
either forget to do or perform improperly. First, dot the
pupil just as one would do when measuring a progressive
lens. Measure, and then drop the height 1mm for every 2
degrees of tilt, but do not exceed 5 mm. If it exceeds 5mm,
refit the patient with a better frame for him or her. The
alternative method is to have the patient raise his or her
chin until the glasses are perpendicular to the floor, have
the patient look at the ECP and then dot the pupil while the
patient remains in that position. Both of these methods are
done to help reduce the amount of induced spherical or
cylindrical power in the lens from unneeded tilt.
Use Anti-reflective Coating- Most aspheric lenses are a
created with a high index material (which has higher amounts
of reflections) so the use of AR coating can bring the
amount of light transmission into the eye to 99.5%. Also, AR
coating reduces the amount of reflections that can occur
from the flatter back surface of the lens.
Conclusion
Patients go to a practice because they expect the best,
and it is the job of the eye care professional to give it to
them. It is imperative that the patient be offered the best
in lens technology to fit his or her needs. Aspherics are an
excellent tool for the ECP to use to give the patient the
thin, lightweight and visually superior lens he or she
desires.
