ABILITAZIONE OTTICO DISPENSA DI INGLESE

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Brian Pearson
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1 ABILITAZIONE OTTICO DISPENSA DI INGLESE

2 RETINA The vertebrate retina is a light-sensitive layer of tissue, lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina (through the cornea and lens), which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These are sent to various visual centres of the brain through the fibres of the optic nerve. In vertebrate embryonic development, the retina and the optic nerve originate as outgrowths of the developing brain, so the retina is considered part of the central nervous system (CNS) and is actually brain tissue. It is the only part of the CNS that can be visualized non-invasively. The retina is a layered structure with several layers of neurons interconnected by synapses. The only neurons that are directly sensitive to light are the photoreceptor cells. These are mainly of two types: the rods and cones. Rods function mainly in dim light and provide black-and-white vision, while cones support daytime vision and the perception of colour. A third, much rarer type of photoreceptor, the photosensitive ganglion cell, is important for reflexive responses to bright daylight. Neural signals from the rods and cones undergo processing by other neurons of the retina. The output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve. Several important features of visual perception can be traced to the retinal encoding and processing of light. Anatomy The vertebrate retina has ten distinct layers. From closest to farthest from the vitreous body - that is, from closest to the front exterior of the head towards the interior and back of the head: 1. Inner limiting membrane basement membrane elaborated by Müller cells 2. Nerve fibre layer axons of the ganglion cell nuclei (note that a thin layer of Müller cell footplates exists between this layer and the inner limiting membrane) 3. Ganglion cell layer contains nuclei of ganglion cells, the axons of which become the optic nerve fibres for messages and some displaced amacrine cells

3 4. Inner plexiform layer contains the synapse between the bipolar cell axons and the dendrites of the ganglion and amacrine cells. 5. Inner nuclear layer contains the nuclei and surrounding cell bodies (perikarya) of the bipolar cells. 6. Outer plexiform layer projections of rods and cones ending in the rod spherule and cone pedicle, respectively. These make synapses with dendrites of bipolar cells. In the macular region, this is known as the Fiber layer of Henle. 7. Outer nuclear layer cell bodies of rods and cones 8. External limiting membrane layer that separates the inner segment portions of the photoreceptors from their cell nucleus 9. Photoreceptor layer rods/cones 10. Retinal pigment epithelium - single layer of cuboidal cells (with extrusions not shown in diagram). These can be simplified into 4 main processing stages: photoreception, transmission to bipolar cells, transmission to ganglion cells which also contain photoreceptors, the photosensitive ganglion cells, and transmission along the optic nerve. At each synaptic stage there are also laterally connecting horizontal and amacrine cells. The optic nerve is a central tract of many axons of ganglion cells connecting primarily to the lateral geniculate body, a visual relay station in the diencephalon (the rear of the forebrain). It also projects to the superior colliculus, the suprachiasmatic nucleus, and the nucleus of the optic tract. It passes through the other layers creating the Optic disc in primates. Additional structures, not directly associated with vision, are found as outgrowths of the retina in some vertebrate groups. In birds, the pecten is a vascular structure of complex shape that projects from the retina into the vitreous humour; it supplies oxygen and nutrients to the eye, and may also aid in vision. Reptiles have a similar, but much simpler, structure.

4 CHOSING AN EYE DOCTOR Where do you go when you are having difficulty with your eyesight? Depending on the extent of the problem, your answer may vary. There are several different types of eye doctors and eye specialists you could see, including an ophthalmologist, optometrist, and optician. What Is an Ophthalmologist? Ophthalmologists are eye doctors that specialize in the medical and surgical care of the eyes and visual system, and also the prevention of eye disease and injury. They can be either doctors of medicine (MD) or doctors of osteopathy (DO). While medical doctors focus on disease-specific diagnosis and treatment, osteopaths concentrate on the loss of structure and function in different parts of the body due to disease, including the eye. An eye doctor who is an osteopath would give treatment based on the assumption that treating the parts of the visual system with the use of medicines, surgery, diet, and other therapies, will therefore treat the underlying eye problem. An ophthalmologist is an eye doctor who has completed four years of pre-medical undergraduate education, four years of medical school, one year of internship, and three or more years of specialized medical and surgical training in eye care. As a qualified specialist, an ophthalmologist is licensed by a state regulatory board to diagnose, treat, and manage conditions affecting the eye and visual system. An ophthalmologist is qualified to deliver total eye care, meaning vision services, eye examinations, medical and surgical eye care, diagnosis and treatment of disease, and visual complications that are caused by other conditions, like diabetes. What Is an Optometrist? Optometrists are eye doctors of optometry (OD). They are trained to examine, diagnose, treat, and manage some diseases and disorders of the eye and visual system. The optometrist has completed pre-professional undergraduate education and four years of professional education at an accredited college of optometry. In addition, some optometrists may have completed a one-year optional residency in a specialized area. Optometrists have not attended medical school. Like ophthalmologists, optometrists are trained to examine the internal and external structure of the eyes to detect diseases such as glaucoma, retinal detachment, and cataracts. Optometrists do not perform eye surgery and are not trained to care for and manage all diseases and disorders of the eyes. The optometrist is trained to diagnose

5 and treat vision conditions such as nearsightedness, farsightedness, astigmatism, and presbyopia. They may also test a person's ability to focus and coordinate the eyes and see depth and colors accurately. Optometrists are licensed by states to examine the eyes to determine the presence of vision problems and visual acuity, prescribe eyeglasses, contact lenses, eye exercises, low vision aids, vision therapy, and medications to treat eye diseases. What Is an Optician? Opticians are eye healthcare professionals who work with ophthalmologists and optometrists to provide vision services related to the diagnosis and treatment of vision problems and eye disease. They assist optometrists and ophthalmologists in providing complete patient care before, during, and after exams, procedures, and surgeries. With a two-year technical degree, opticians analyze and interpret eye prescriptions; determine the lenses that best meet a persons needs; oversee ordering and verification of eye-related products from start to finish; dispense, replace, adjust, repair, and reproduce previously ordered contact lenses, eyeglasses, and frames. Eye health is the result of a working partnership between you and your eye healthcare provider. Ophthalmologists, optometrists, and opticians work collectively and with the patient to ensure good eye health and life-long vision. You should visit your eye doctor - either an optometrist or ophthalmologist -- for an eye exam at least once a year.

6 LENS ( OPTICS ) A lens is an optical device which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element. Lenses are typically made of glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called lenses: for instance, a microwave lens can be made from paraffin wax. The variant spelling lense is sometimes seen. While it is listed as an alternative spelling in some dictionaries, most mainstream dictionaries do not list it as acceptable. Construction of simple lenses Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres, with the lens axis ideally perpendicular to both surfaces. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens. Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes. They have a different focal power in different meridians. This is a form of deliberate astigmatism. More complex are aspheric lenses. These are lenses where one or both surfaces have a shape that is neither spherical nor cylindrical. Such lenses can produce images with much less aberration than standard simple lenses. These in turn evolved into freeform (digital/adaptive/corrected curve) spectacle lenses, where up to 20,000 ray paths are calculated from the eye to the image taking into account the position of the eye and the differing back vertex distance of the lens surface and its pantoscopic tilt and face form angle. The lens surface(s) are digitally adapted at nanometre levels (normally by a diamond stylus) to eliminate spherical aberration, coma and oblique astigmatism. This type of lens design almost completely fulfills the sagittal and tangential image shell requirements first described by Tscherning in 1925 and further described by Wollaston and Ostwalt. These advanced designs of spectacle lens can improve the visual performance by up to 70% particularly in the periphery.

7 Types of simple lenses Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex (or double convex, or just convex) if both surfaces are convex. If both surfaces have the same radius of curvature, the lens is equiconvex. A lens with two concave surfaces is biconcave (or just concave). If one of the surfaces is flat, the lens is planoconvex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is convex-concave or meniscus. It is this type of lens that is most commonly used in corrective lenses. If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens will be converged (or focused) to a spot behind the lens. In this case, the lens is called a positive or converging lens. The distance from the lens to the spot is the focal length of the lens, which is commonly abbreviated f in diagrams and equations. If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens. Convex-concave (meniscus) lenses can be either positive or negative, depending on the relative curvatures of the two surfaces. A negative meniscus lens has a steeper concave surface and will be thinner at the centre than at the periphery. Conversely, a positive meniscus lens has a steeper convex surface and will be thicker at the centre than at the periphery. An ideal thin lens with two surfaces of equal curvature would have zero optical power, meaning that it would neither converge nor diverge light. All real lenses have nonzero thickness, however, which causes a real lens with identical curved surfaces to be slightly positive. To obtain exactly zero optical power, a meniscus lens must have slightly unequal curvatures to account for the effect of the lens' thickness.

8 NEARSIGHTEDNESS (MYOPIA) Definition Nearsightedness (myopia) is a common vision condition in which you can see objects near to you clearly, but objects farther away are blurry. The degree of your nearsightedness determines your focusing ability. People with severe nearsightedness can see clearly only objects just a few inches away, while those with mild nearsightedness may clearly see objects several yards away. Nearsightedness may develop gradually or rapidly, often worsening during childhood and adolescence. Nearsightedness tends to run in families. A basic eye exam can confirm nearsightedness. You can easily correct the condition with eyeglasses or contact lenses. Another treatment option for nearsightedness is surgery. Symptoms Being nearsighted may mean: Distant objects appear blurry You need to squint to see clearly You have headaches caused by excessive eyestrain Nearsightedness is often first detected during childhood, from early school years through the later teens. A child with nearsightedness may: Persistently squint Sit very close to the television, movie screen or chalkboard Hold books very close while reading Seem to be unaware of distant objects

9 ASTIGMATISM Astigmatism is a mild and easily treatable imperfection in the curvature of your eye. The condition can cause blurred vision. Astigmatism occurs when the front surface of your eye (cornea) or the lens, inside your eye, has a slightly different surface curvature in one direction from the other. Instead of being even and smooth in all directions, the surface may have some areas that are flatter or steeper. When the cornea has a distorted shape, you have corneal astigmatism. When the lens is distorted, you have lenticular astigmatism. Either type of astigmatism can cause blurred vision. Astigmatism blurs your vision at all distances. Astigmatism is often present at birth and may occur in combination with nearsightedness or farsightedness. The condition tends to remain constant, neither improving nor deteriorating much over time. Astigmatism may occur in combination with other refractive errors. In most instances, astigmatism is present at birth. Sometimes, astigmatism develops after an eye injury, disease or surgery. Astigmatism isn't caused or made worse by reading in poor light, sitting too close to the television or squinting.

10 LENSMETER A lensmeter or lensometer, also known as a focimeter or vertometer, is an ophthalmic instrument. It is mainly used by optometrists and opticians to verify the correct prescription in a pair of eyeglasses, to properly orient and mark uncut lenses, and to confirm the correct mounting of lenses in spectacle frames. Lensmeters can also verify the power of contact lenses, if a special lens support is used. The parameters appraised by a lensmeter are the values specified by an ophthalmologist or optometrist on the patient's prescription: sphere, cylinder, axis, add, and in some cases, prism. The lensmeter is also used to check the accuracy of progressive lenses, and is often capable of marking the lens center and various other measurements critical to proper performance of the lens. It may also be used prior to an eye examination to obtain the last prescription the patient was given, in order to expedite the subsequent examination. History In 1848, Antoine Claudet produced the photographometer, an instrument designed to measure the intensity of photogenic rays; and in 1849 he brought out the focimeter, for securing a perfect focus in photographic portraiture. In 1876, Hermann Snellen introduced a phakometer which was a similar set up to an optical bench which could measure the power and find the optical centre of a convex lens. Troppman went a step further in 1912, introducing the first direct measuring instrument. In 1922, a patent was filed for the first projection lensmeter, which has a similar system to the standard lensmeter pictured above, but projects the measuring target onto a screen eliminating the need for correction of the observer's refractive error in the instrument itself and reducing the requirement to peer down a small telescope into the instrument. Despite these advantages the above design is still predominant in the optical world.

12 OPTICIAN An optician, or dispensing optician, is a technical practitioner who designs, fits and dispenses corrective lenses for the correction of a person's vision. Opticians determine the specifications of various ophthalmic appliances that will give the necessary correction to a person's eyesight. Some registered or licensed opticians also design and fit special appliances to correct cosmetic, traumatic or anatomical defects. These devices are called shells or artificial eyes. Other registered or licensed opticians manufacture lenses to their own specifications and design and manufacture spectacle frames and other devices. Corrective ophthalmic appliances may be contact lenses, spectacles lenses, low vision aids or ophthalmic prosthetics to those who are partially sighted. The appliances are mounted either on the eye as contact lenses or mounted in a frame or holder in front of the eye as spectacles or as a monocle. Opticians may work in any variety of settings such as joint practice, hospitals, laboratories, eye care centers or retail stores. However, registered opticians have to meet standards of practice and training, commit to ongoing education, hold professional liability insurance and are held to these standards by their respective regulating bodies. A fully credentialed optician in the United States is college educated in Optical Science and is known as an Ophthalmic Optician (O.O.) and they are credentialed by the Society to Advance Opticianry (SAO). To achieve this nationally registered title an optician must achieve a combination of a college education, American Board of Opticianry and National Contact Lens Examiners advanced certifications, or maintain their state license in both eyewear dispensing and contact lens fitting when applicable. In the United Kingdom, an ophthalmic optician is also known as an optometrist and is regulated by the General Optical Council under the Opticians Act Like many health care providers, opticians are regulated professionals in certain countries. The profession is often regulated by optician-specific agencies, as in Canada and some states of the U.S., or jointly with optometry such as the New Zealand Optometrist and Dispensing Opticians Board or the United Kingdom General Optical Council. Opticians may work independently or dependently with an optometrist or ophthalmologist although some opticians may work in an optical laboratory as a laboratory technical optician. Opticians convert a prescription for the correction of a refractive error into an ophthalmic lens or some other device, such as reading aids or telescopic lenses.

13 Equipment Opticians use a variety of equipment to fit, adjust and dispense eyewear, contact lenses and low vision aids. The dispensing of eyewear requires the use of a focimeter, or lensometer, to verify the correct prescription in a pair of eyeglasses, properly orient and mark uncut lenses, and to confirm the correct mounting of lenses in spectacle frames. Certain lensometers also have the ability to examine contact lenses. The parameters appraised by a lensometer are the sphere, cylinder, axis, add, and in some cases, prism. The lensometer is also used to check the accuracy of progressive lenses, and is often capable of marking the lens center and various other measurements critical to proper performance of the lens. Another indispensable piece of equipment is a pupilometer. A pupilometer is a tool for more accurately measuring interpupillary distance (IPD or PD). It is used for fitting eyeglasses so that the lenses are centered in the visual axis. This is the most common nomenclature. A pupilometer may be manually operated, or may be digital. Pupilometers may also be used to verify a PD measurement taken by hand with a millimeter ruler. A pupilometer is best suited for better assurance in fitting progressive lenses, and other specialty lenses, since even tiny errors cause eye strain. Pupilometer apps have also been developed for smart phones and tablets. The fitting and dispensing of contact lenses requires the use of additional equipment, all with very specific purposes. A keratometer is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of astigmatism. It was invented by the French ophthalmologist Samuel Hankins in Opticians, like ophthalmologists and optometrists, also use a slit-lamp/bio-microscope to examine the anterior segment, or frontal structures and posterior segment, of the human eye, which includes the eyelid, sclera, conjunctiva, iris, natural crystalline lens, and cornea. The binocular slit-lamp examination provides stereoscopic magnified view of the eye structures in detail, enabling anatomical diagnoses to be made for a variety of eye conditions. While a patient is seated in the examination chair, he rests his chin and forehead on a support to steady the head. Using the biomicroscope, the optician then proceeds to examine the patient's eye. A fine strip of paper, stained with fluorescein, a fluorescent dye, may be touched to the side of the eye; this stains the tear film on the surface of the eye to aid examination. The dye is naturally rinsed out of the eye by tears. Adults need no special preparation for the test; however children may need some preparation, depending on age, previous experiences, and level of trust.

14 The list of equipment used by an optician is extensive and is often specified in jurisdiction specific Professional Standards of Practice. The standards of the College of Opticians of British Columbia serve as an example.

Sensory receptors External internal stimulus change detectable energy transduce action potential different strengths different frequencies

Sensory receptors External internal stimulus change detectable energy transduce action potential different strengths different frequencies General aspects Sensory receptors ; respond to changes in the environment. External or internal environment. A stimulus is a change in the environmental condition which is detectable by a sensory receptor