Factors affecting visual acuity

Factors affecting visual acuity Factors affecting Visual Acuity Visual acuity is measured in various different ways and that is dependent on the acuity task used. However factors affecting the visual acuity have to be taken into account. Visual acuity is affected by a number of ways such as diffraction, aberrations and photoreceptor density in the eye. There are a number of other factors too, which will be discussed more in depth in this article. Other factors that affect visual acuity include: Refractive error Size of the pupil Illumination Time exposure of target Area of retina stimulated State of adaptation of the eye Eye movements These factors will be discussed in detail below. Refractive Error Refractive error is one of the important factors that affect visual acuity. Refractive error affects visual acuity depending on the correction, either Hypermetropic or Myopic or an Emmetropic eye. Visual acuity is affected depending if it causes a defocus at the retina, in the case of a Hypermetropic or myopic eye this is the case. Defocus is when fine details, sharp edges and contrast detail is blurred out. In a Hypermetropic eye, the eye is too strong so the image is focused in front of the retina causing a defocus hence affecting visual acuity. The reverse occurs with a myopic eye, as the eye is too weak, the image is focused in front of the retina therefore again causing a defocus hence affecting visual acuity. On the other hand to obtain a perfect image, image should be focused sharply on the retina. This is the case in an emmetropic eye; no defocus hence no visual acuity problem due to refractive error. Having astigmatism also affects visual acuity. A phenomenon called the oblique effect states that oblique lines are seen less accurately, meaning that visual acuity is greater when lines are orientated horizontally or vertically. This is still the case even after a perfect correction of refractive error and astigmatism. Size of Pupil Eye takes time to see things. The nerve signals take a limited amount of time to travel along the optic nerve and instigate visual awareness. Therefore the size of pupil can determine how quickly or slow a signal can be stimulated thus determining how much visual acuity is affected. Large pupils let more light pass through into the eye so stimulating the retina more. However this does have a disadvantage; it affects resolution due to the aberrations that increase in the eye due to an increase in light. Small pupils reduce optical aberrations due to less light passing through into the eye however resolutions are diffraction limited. Therefore for good visual acuity a mid size pupil of approximately 3mm to 5mm is optimal as this is a compromise between the diffraction and aberration limits. Visual acuity also decreases with age due to decrease in pupil size, so a higher level of illumination is needed in older people to improve visual acuity. Illumination For recognition tasks, visual acuity increases with illumination. However at very high illumination levels, acuity may decrease because of loss of contrast between object and its background, or reflections on the surface of the object decreasing the contrast between the background and object. Higher the contrast between the object and the background the greater the acuity is. When contrast is reduced, it becomes more difficult to read it against a darker background therefore the text need to be made larger in order for the illumination levels to be correct for good visual acuity. Illumination levels greatly affect the older generation due to many people having cataracts. The cataract becomes a source of veiling glare within the eye therefore visual acuity decreases. Time Exposure of the Target To detect a small bright spot, detection is greatly dependent on the quantity of light rather than the exposure time. However to detect a line, the acuity is proportional to the exposure time. There is no simple acuity-exposure time relationship for the resolution of the target. Area of the Retina Stimulated Visual acuity is greatest at the centre of fixation, which in this case is at the fovea. Fovea has the greatest visual acuity due to the densely packed cones, so once the image is moved away from the centre of fixation visual acuity is affected. At a distance of 5 minutes of arc from the centre of fixation, there is a loss in visual acuity. At approximately 10 minutes of arc from fixation there is about 25% visual acuity loss. This shows that our vision is most accurate at the fovea and acuity falls rapidly as we move towards the peripheral retina. Therefore in order to achieve best visual acuity we must scan our visual field to place the fovea on the area of interest and to achieve this we must have excellent oculo-motor co-ordination. State of Adaptation of the Eye Highest level of acuity is achieved if the eye is tailored to the equivalent level as the test luminance of 34cd/m2 to 34,000cd/m2. Test luminance less than 34cd/m2 will achieve to some extent, better acuity. Under photopic conditions, the high density of cones at the fovea is accountable for the high levels of acuity. Under scotopic conditions it is a little altered, due to the Aii Amacrine cells appearing to limit resolution. Maximum scotopic acuity occurs at 5-15 degrees eccentricity which is corresponding to the cell density, while peak rod density occurs at about 15-20 degrees. Eye Movements In steady eye fixation, the eyes are in constant motion; therefore moving objects are more difficult to see than stationary objects thus affecting visual acuity. Visual acuity will be greater when point of interest is constant or little movement however visual acuity will be at a great loss when object is in constant movement. Conclusion: Dynamic visual acuity is lower than Static visual acuity. Conclusion Visual acuity is measured in a number of different procedures however it is dependant on many factors. Visual acuity cannot just be taken as a figure without considering all the factors that affect it. Such as the refractive error, size of pupil, illumination, area of retina stimulated, adaptation of the eye, eye movements and time exposure of the target. Therefore without an understanding of this visual acuity cannot be taken accurately. References Atchison D. A., Smith G., and Efron N. (1979) The effect of pupil size on visual acuity in uncorrected and corrected myopia. Am J Optom Physiol Opt. 56:315-32 http://employment.alberta.ca/documents/WHS/WHS-PUBeyeinjl.pdf http://webvision.med.utah.edu/KallSpatial.html W,Karwowski. Encyclopaedia of ergonomics and human factors. Volume 1.

Imported Medicines: A Strategic Shift in Human Thinking Essay

Internet and online businesses gradually change our lives. What used to be impossible two years ago now has turned into a matter of daily performance. With Internet, we are no longer limited in our activities and can broaden the scope of all major operations that have previously been unachievable. Healthcare and medicines are not exceptions, and where Internet works to provide us with additional opportunities for communication, we have finally been given a chance to reconsider the costs of all healthcare services and medical preparations. Online medical markets have produced an irreversible change in the structure of healthcare services and human thinking. With the advent of the new technological age and the growing globalization, customers have become more attentive to what they buy and how much they pay for it. Imported medicines purchased online offer a competitive alternative to brand-name drugs. Simultaneously, imported drugs are not always licensed and may create serious health risks. In this situation, the state should actively work to impose severe price restrictions on the major brand-name drugs and to develop effective monitoring systems with regard to the imported drugs that are sold online, to guarantee that customers are given a viable choice and are not limited in their access to quality healthcare services and medical preparations. True, the growing customer commitment to imported medicines is the result of the national pharmaceutical markets’ being largely non-regulated. Americans typically pay the highest prices for brand-name drugs because, unlike Canada and many other countries, the United States does not impose price controls on pharmaceutical companies† (Colliver). The growing number of online pharmacies reflects the misbalanced nature of the price strategies, to which present day pharmaceutical companies adhere. Despite the fact that brand-name drugs are sold with significant discounts, the majority of healthcare insurance plans does not cover them and does not make them affordable for consumers. The current economic crisis makes Americans more attentive toward their expenditures, and online pharmaceuticals offer an excellent opportunity to obtain the needed list of medical preparations for a much lower price. â€Å"If the government doesn’t get its act together, and with 47 million who are uninsured, there is a marketplace for alternative supply of some of these drugs† (Colliver). It appears that in the current system of healthcare services customers are not given any relevant choice. With the absence of price restrictions, pharmaceutical companies feel free to establish prices as high as they deem necessary, making customers seek effective alternatives elsewhere. That is why imported drugs as such, and online imported drugs in particular have caused a strategic shift in human thinking, giving people a chance to resolve their health issues at a lower cost. With the growing number of Internet users and given the cost-effectiveness of imported drugs, customers are no longer willing to stick to traditional purchasing practices and choose a technological way of resolving their health issues. More and more people realize the benefits of online pharmaceutical marketing. Many of them enjoy the freedom of choice and convenience they are offered by online pharmaceuticals. â€Å"More Americans appear willing to take a calculated risk in the absence of prescription drug coverage† (Colliver). In other words, when health and life are at stake, customer will not think twice before ordering an affordable imported medical preparation from online suppliers. For many of them, imported medicals offer the most attractive and acceptable option; that is why online companies gradually grow to cover larger shares of online markets even without any specific advertising. Unfortunately, ordering imported drugs online is not always safe. Despite the fact that technology has forever changed the direction of human thinking and has made the process of purchasing imported drugs fast and convenient, imported drugs are associated with substantial risks. Customers cannot always be confident that they buy an authentic drug from an authentic foreign manufacturer. Some Web sites hawk controlled substances without a prescription or falsely purport to get drugs from a Canadian pharmacy or other more trustworthy source. While operators try to separate themselves from such operations, it’s tough to tell the difference† (Colliver). Even if the government imposes strict limitations on pharmaceutical prices, the shift that has already occurred in customer attitudes will require time to be reversed. The problem is not in imported drugs; given the growing globalization trends, imported drugs will keep conquering larger shares of pharmaceutical markets. The problem is that for customers to purchase imported products is a matter of â€Å"no other choice†. Many of them are no longer willing to return to conventional medical practices. As a result, and taking into account that online businesses will further work to conquer the minds and spirits of the American customers, the state should develop effective monitoring systems that will impose price restrictions and will control imported drugs sold online, to provide customers with a reasonable choice and to give them another chance for recovery. Conclusion Imported drugs and Internet have produced an irreversible change in customer attitudes toward healthcare. Beyond broader access to imported medical products, customers are given an opportunity to reduce their costs and to purchase medical products in fast and convenient manner. Given the threats which imported non-licensed products may produce on human health, and taking into account the growing customer commitment to purchasing imported medicines online, it is the state’s responsibility to develop effective regulatory and monitoring strategies that will not limit customer choice but will guarantee safety of all imported medical products.

Johannes Kepler Essay - 991 Words

Johannes Kepler Johannes Kepler is now remembered for discovering the three laws of planetary motion, and writing about them in books that were published in 1609 and 1619. He also did important work in optics, discovered two new regular polyhedra, gave the first mathematical treatment of close packing of equal spheres, gave the first proof of how logarithms worked, and devised a method of finding the volumes of solids of revolution. This can be seen as contributing to the development of calculus. Not only did he help the development of calculus, but he calculated the most exact astronomical tables known today. This accuracy did much to establish the truth of heliocentric astronomy, which states that†¦show more content†¦The curriculum included geocentric astronomy. This is the study of how all seven planets at the time - Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn - moved around the Earth. Their positions to the stars were being calculated by combining circular motions. This system was what worked with the current physics of the time, although there were certain difficulties. However, these astronomers, who also saw themselves as mathematicians, were content to carry on calculating positions of planets. They figured they would leave it to natural philosophers to worry about whether the mathematical figures added up to the physical ones. Kepler, however, did not take like this attitude. His earliest published work in 1956 says that people should consider the actual paths of the planets, and not just the circles used to construct them. At Tubingen, Kepler studied Greek and Hebrew as well as mathematics. Ironically, at the end of his first year Kepler got As in every subject except mathematics. It is supposed that Maestlin was trying to tell him he could do better, because Kepler was one of the select pupils that Maestlin chose to teach more advanced astronomy to. 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Pbs Breaking The Silence Essay - 823 Words

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