Factors affecting visual acuity

Factors modify optic edgeFactors bear on Visual AcuityVisual sharp-sightedness is measured in various different ways and that is pendent on the keenness task used. provided factors affecting the ocular raciness direct to be taken into account. Visual visual alertness is affected by a piece of ways such as diffraction, aberrations and photoreceptor dumbness in the center field. at that place ar a number of other factors in any case, which will be discussed to a greater extent in depth in this article.Other factors that affect visual visual acuity includerefractile errorSize of the pupilIlluminationmagazine exposure of targetArea of retina stimulated conjure up of adaptation of the eye centre of attention movementsThese factors will be discussed in detail below.Refractive f aloneacyRefractive 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 Emm etropic eye. Visual acuity is affected depending if it causes a defocus at the retina, in the case of a Hypermetropic or improvident eye this is the case. Defocus is when fine details, sharp edges and secernate detail is blurred out.In a Hypermetropic eye, the eye is too strong so the watch is focused in front of the retina create a defocus and so 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 go past 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 collectible to refractive error.Having astigmatism also affects visual acuity. A phenomenon called the oblique effect states that oblique lines are seen microscopical accurately, meaning that visual acuity is greater when lines are orientated horizontally or vertic ally. This is still the case even after a perfect correction of refractive error and astigmatism.Size of PupilEye takes time to see things. The nerve signals take a limited come 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 therefore 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 overdue to the aberrations that increase in the eye due to an increase in light. Small pupils reduce optic aberrations due to less light passing through into the eye however resolutions are diffraction limited. Therefore for nice visual acuity a mid(prenominal) size pupil of approximately 3mm to 5mm is optimal as this is a via media between the diffraction and aberration limits. Visual acuity also decreases with age due to decrease in pupil s ize, so a higher direct of illumination is needed in older people to improve visual acuity.IlluminationFor recognition tasks, visual acuity increases with illumination. However at very high illumination levels, acuity may decrease because of loss of contrast between object glass and its background, or reflections on the surface of the object diminish 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 arduous to involve it against a darker background therefore the text need to be do 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 deep down the eye therefore visual acuity decreases.Time Exposure of the guideTo get wind a small bright spot, detection is greatly dependent on the quanti ty of light rather than the exposure time. However to detect a line, the acuity is proportional to the exposure time. There is no elemental acuity-exposure time relationship for the resolution of the target.Area of the Retina StimulatedVisual 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 curl from the centre of fixation, there is a loss in visual acuity. At approximately 10 minutes of arc from fixation there is nearly 25% visual acuity loss. This shows that our vision is most accurate at the fovea and acuity falls rapidly as we move towards the off-base 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 A daptation of the EyeHighest level of acuity is achieved if the eye is tailored to the analogous 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 alike to the cell density, while peak rod density occurs at to the highest degree 15-20 degrees.Eye MovementsIn steady eye fixation, the eyes are in uniform 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 l ower than Static visual acuity.ConclusionVisual 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.ReferencesAtchison 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. 56315-32http//employment.alberta.ca/documents/WHS/WHS-PUBeyeinjl.pdfhttp//webvision.med.utah.edu/KallSpatial.htmlW,Karwowski. Encyclopaedia of ergonomics and human factors. Volume 1.