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## Spherical Lens

Discuss about 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.

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

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.

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

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:

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.

## Toric Lens physical characteristics

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

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.

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.

## Difference in focus produced by a spherical and toric lens

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.

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).

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.

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.

## Improving the field of view for hypermetropic patients

A 5-year-old child requires spectacles. Their Rx is +4.00DS R & L. No patching is required.

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.

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.

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.

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