Understanding Spherical, Toric, Polarized Lenses and Aspheric Lenses

This article covers the physical characteristics of spherical and toric lenses, how polarized lenses are manufactured, and the advantages of aspheric lenses. It also provides troubleshooting steps for progressive lenses and explains lens and frame choices for children. The article is informative and helpful for anyone looking to understand more about lenses.

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Describe the physical characteristics of a spherical lens?
Spherical lenses are classified as either concave or convex due to the curving of their spherical
surfaces. Convex lenses are thick at the middle and thin at the edges. When light passes through
the convex lens, it converges thus referred to as a converging lens. Concave lenses are thin at the
middle and thick at the edges. Concave lenses diverge light passing through it thus are diverging
lenses.
Describe the physical characteristics of a toric lens and state the names given to the
principal meridians on the back surface?
Combines a spherical curve and cylindrical curve thus referred to as spherocylindrical
Base curve - a principal meridian on the back surface with a minimum curvature
What is the difference in the focus produced by a spherical and toric lenses?
In a spherical lens, parallel light rays diverge from a singular focal point in concave lenses and
converge in a singular focal point in convex lenses while in toric lenses, focus is at different
focal points from different meridians
How can you improve the field of view for hypermetropic patients?
Patients who have a long sightedness disorder are referred to as hypermetropic patients or rather
suffer from hypermetropia. This means, when parallel light rays pass through their eyes, objects
are focused beyond their eye's retina. First, troubleshoot clinically the level of hypermetropia in
order to give the right prescription. Some stages may require a prescription of spectacles with
convex lenses so that focus of rays is on the retina or contact lenses prescription to eliminate

prismatic effects. Other advanced stages may require surgical procedures but consult with the
patient further if a surgical procedure is the only method that will improve their field of view.
Describe two methods of reducing the finished centre and edge thickness of a pair of
spectacles?
Flattening the lens’s base curve
Using aspheric lens designs that generally use small sizes of spectacle frames, therefore,
reducing the edge thickness of the spectacles
What does minus cylinder form mean how is it used in lens design?
Minus cylinder is a notation used to describe the cylinder power for the few diopters whose
divergence is greater than the sphere component. The plus cylinder and minus cylinder differ by
way of the different shapes produced for the ophthalmic lens used in managing the thickness and
frame fitness during the lens design. Therefore, in lens design, a minus cylinder is used to
manage the thickness of a lens and fitness of a lens into the frame.
Polarization and polarized lenses:
How are polarized lenses manufactured and how does this differ from non-polarized tinted
lenses?
Manufacture of polarized lenses involves stretching a material made of needle-like crystals of
herapathite and a transparent polymer film of nitrocellulose, through the application of magnetic
or electric fields to make the material dichroic. Polarized lenses and non-polarized tinted lenses
differ by way of how they work in protecting an individual if worn as sunglasses. Polarized
lenses for instance protect an individual’s eyes from the sun’s UV rays as well as reflections and

glares that cause irritation. On the other hand, non-polarized tinted lenses used in sunglasses do
not offer UV protection and instead block sunlight from above in gradient tinted lenses or
everywhere if they are double gradient tinted.
Explain how sunlight from the environment is polarized?
The Rayleigh sky model explains how sunlight from the environment is polarized by a way of a
celestial pattern consisting of molecules from air, aerosols, water and dust and geometrically
represented by a celestial triangle with Zenith, The sun and the scattering point of the sun rays as
the apices. The pattern depends solely on the position of the sun in the solar system. Rays of light
are maximally polarized at a circular band in an angle of 90 degrees from its source which is the
sun and as the earth rotates around the sun, the degree of sunlight polarization changes too.
(Ropars, Lakshminarayanan, & Le Floch, 2014).
How do polarized sunglasses lenses filter plane polarized light?
When plane-polarized light passes through the polarized lenses of the sunglasses, the light waves
vibrating parallel and horizontally from the source of light travel in a traverse form and enter the
sunglasses. The lenses filter the waves by blocking red light which causes glares and allows blue
light which is less glaring. This gives way to blue light waves vibrating vertically from the
source of light
What is the advantage of filtering polarized light?
When used in cameras for photography purposes, filtering polarized light leads to the production
of an image with increased image contrast by reducing the glare.

Reflections on high minus lenses are reduced by an anti-reflective coating. Explain how the
antireflective coating does this?
Lenses with anti-reflective coating are manufactured using an oxide coat which blocks light
reflections which would otherwise cause glares, halos and reflections around light. High minus
lenses with anti-reflective coating when hit with light, the reflections made usually look like
rings which make the lenses appear thick thus reducing external and internal reflections of light.
List the troubleshooting steps for a progressive lens wearer complaining of a narrow
reading range and distorted peripheral vision?
Take a square mirror about 7 inches square and make a dot at the center of the mirror using a
marking pen
Place the mirror between you and the patient on the dispensing table so that it is at the patient’s
working distance.
Place all necessary markings on the PAL lenses. Make a good adjustment after putting them back
to the patient.
With his head straight and a reading position, let the patient take a look at the marked dot made
on the mirror
Look back and forth into the mirror and the patient as they look at the dot.
Observe if the patient is looking via the point circle or otherwise. The highest probability is they
are not.
With great care, mark with the use of the marking pen on the lenses where the patient is looking
so that you can make PD requirements. (Bentley, Trevaskis, Woods, Guest, & Watt, 2017).

The mirror test is a useful diagnostic tool for progressive lenses.
Describe the process of using it including the equipment needed?
Using a square mirror that is about 7 square inches, make a dot at the center of the mirror using a
marking pen
Place the mirror between you and the patient on the dispensing table so that it is at the patient’s
working distance.
Place all necessary markings on the PAL lenses. Make a good adjustment after putting them back
to the patient.
With the patient’s head straight and at a reading position, ask them to take a look at the mirror’s
marked dot.
Look back and forth into the mirror and the patient as they look at the dot.
Observe if the patient is looking via the point circle or otherwise. The highest probability is the
patient is not looking through the near point circle
Carefully mark using the marking pen on the lenses where the patient is looking in order for you
to make PD requirements.
The requirements for this procedure is a mirror, lenses for making new markings on, and a
marking pen. (Bentley, Trevaskis, Woods, Guest, & Watt, 2017).
Explain why it is considered more accurate than just checking the mono distance PD’s for
near vision alignment?

The ophthalmologist has the ability to make keen observations of the eye and thereafter make
conclusive PD requirements that totally minimize the margin of error if they use the mirror test.
This is in contrast to the mono distance PD for near vision alignment whose margin of error is
higher. This is because the ophthalmologist is neither able to observe the pupil directions nor the
position of light on the pupil.
The significant advantage of aspheric lenses
Aspheric lenses eliminate unwanted magnification appearance that is common in the
conventional spherical lenses used by hypermetropic patients. Among myopic patients, aspheric
lenses eliminate minification, which is the converse of the magnification appearance of spherical
lenses used by hypermetropic patients.
What measurements must be taken when dispensing aspheric lenses and why is this
necessary?
A dispenser records all information provided by the lens manufacturer just in case the lens has to
be remade in the occasion that problems arise. To avoid the off-axis power errors that are mainly
caused by improper fitting, the ray of the optical axis has to go via the eye’s center of rotation.
This should be done by the dispenser placing the optical axis on the surface of the aspheric lens.
Therefore, in every 2 degrees of pantoscopic tilt, there should be a 1mm drop in the lens whereby
the optical centers of the lenses are placed vertically and horizontally but in a monocular way.
A 5-year-old child requires spectacles. Their Rx is +4.00DS R & L. No patching is required.
Explain some of the difficulties you may face in meeting the needs of the child taking into
account their wants as well as those of the parent?

The parents may have the unfounded belief that wearing spectacles weakens eyes. This may be
as a result of past experiences of friends or relatives who had complained after having a
spectacles prescription from quacks or unexperienced ophthalmologists and optometrists. This
should be a greater cause of alarm for the parents since reading spectacles prescription for a 5-
year-old is quite rare, unless the child is myopic or hypermetropic. Studies have also shown that
Chinese children have this belief too, that wearing spectacles contribute to the weakening of
eyes. Care, therefore, has to be taken in order not to prescribe erred spectacles which lead to
ocular disorders
The child may complain of the weight of the spectacles. Even though the weight of spectacles is
usually negligible, for a 5-year-old child, the weight might be overwhelming. However, they
should adapt to wearing the spectacles after a while.
The choice of the frame that the child or the parents may prefer prior to visiting the clinic may
not be available. Since dissatisfaction is one of the leading causes for the negative attitude
towards a certain type of spectacles prescribed, it is important to meet their needs by giving them
a future date of when to collect their preferred spectacles with the specified frame of choice.
The child may prefer spectacles that are beyond the cost that the parents are able to meet. Since
children can’t understand cost setbacks the parents are incurring, it would be very hard
convincing the child to choose another cheaper type.
What lens and frame choice will be suitable for the child given that they are only 5 years of
age?

The spectacles need to have a small frame in order for the lens thickness too to be as small as
possible. Lens that are less thick tends to reduce the risk of peripheral vision that is blurred or
distorted.
Plastic frames have for long been the most suitable frames for children due to the fact that they
are more durable than their metal counterparts. Plastic frames with spring hinges would be more
advantageous as they reduce regularly adjustments that would be made as children are not very
careful with spectacles.
Given that the child will be reviewed every 12 months by the ophthalmologist, why is it
necessary to remake the spectacles even if the Rx doesn’t change?
The pupillary distance will have changed since the child will be growing up, therefore, the need
to remake the spectacles. If not corrected or remade, the spectacles would be sliding down their
nose as they would not be fitting well with the child’s developing nose.
Due to the playful nature of children at this age, the lens may have scratches or the frame lose
due to cracks, therefore the need to remake the spectacles if repairs and adjustments are not
possible for the old spectacles.

References
Ropars, G., Lakshminarayanan, V., & Le Floch, A. (2014). The sunstone and polarised skylight:
ancient Viking navigational tools? Contemporary Physics, 55(4), 302-317.
Bentley, S. A., Trevaskis, J. E., Woods, C. A., Guest, D., & Watt, K. G. (2017). Impact of
supervised student optometry consultations on the patient experience. Clinical and
Experimental Optometry.