Fig 2 – Restoring of vision with convex lens
Astigmatism is a defect of the eye in which light rays are localized at different regions of the retina rather than being localized on a single focal point of the retina. Strictly speaking, there are two forms of astigmatism. The first is a third-order , which occurs for objects (or parts of objects) away from the . The second form of astigmatism occurs when the optical system is not symmetric about the optical axis. Problems persist when viewing lines placed at differing angles and the lines running in one direction appear sharp, whilst those in other directions appear blurred. Correction is achieved by wearing cylindrical lenses, which are placed in the out-of-focus axis.
Fig 3 – Showing the faint lines viewed by a person with astigmatism
1.3 Objective & subjective methods for measuring eye refractive power
Commonly employed methods to assess optical power are among subjective methods, although in the recent years objective strategies (e.g. nerve fiber analyzers) have been implemented which, however do not rely on patient responses.
Optical power is adversely affected by glaucoma, a major risk factor for vision loss, caused by the loss of retinal ganglion cells. Fiber analyzers are used to measure physical thickness of nerve fibers in the retina. As fiber layer thickness is a measure of glaucoma progression, thin thickness concludes the onset of glaucoma and thus impaired vision.
Subjective methods include the Snellen Chart Test, in which the optotype is placed twenty feet (6 meters) away from the subject. Some individuals may well wear spectacles in which case the examination is performed with the subject wearing them. The eye which is adversely impaired out of the two is examined first. Usually the examination commences by using large optotypes followed by the smaller ones. The subject is then prompted to recite the letters (or symbols) visible to them. This procedure is then repeated for the other eye.
Normally read at 60 metres.
Normally read at 36 metres.
Normally read at 18 metres.
Normally read at 12 metres.
Normally read at 9 metres.
Normally read at 6 metres.
Normally read at 5 metres.
Normally read at 4 metres.
Fig 4 – Showing distances at which letters can be read
The results from the snellen test are processed and denoted as fractions. For example 6/18 means that the third line down can be read from 6 meters away; 6/6 or 6/5 is considered to be normal distance vision. If no lines can be read from 6 meters then shorter distances are tried. For example, 3/36 means that the second line can be read from a distance of 3 meters away; 2/60 means that the top line can be read from 2 meters away1 (If the top letter cannot be read even with prescription lens or glasses then the subject is considered to be legally blind).
Alternatively a Lea test aimed at pre-school children may be used. In this case the optotypes denote an edible fruit (e.g. an apple). The Tumbling E chart (see visual acuity and its measurement) may also be used. In both cases the same principle applies as with the Snellen test.
Indirect subjective methods to access optical power include intraocular pressure (see below).
1.4 Intraocular Pressure & its measurements
As the name suggests, intraocular pressure is the pressure exerted by a fluid inside the eye. Fluid secretion may be triggered by genetic factors, side effects of medication, the inflammation of the eye or simply, during exercise.
Normal intraocular pressure lies between 10 mmHg and 21 mmHg. In spite of the optic nerve and visual field being intact, when the intraocular pressure is greater than normal, the condition is termed Ocular Hypertension. Ocular Hypertension is usually correlated with the increased incidence of glaucoma. When the intraocular pressure falls below the critical value (5 mmHg) the condition is termed Ocular Hypotony.
Intraocular pressure is typically measured by using a tonometer. Often eye drops are given to alleviate any pain. The procedure involves applying a dye (florescein) to the eye. This eases the examiners ability to see the cornea. The subject is then asked to stare at a bright-lighted slit lamp. The tonometer probe is then made to touch the
eye and subsequently the examiner notes down the tension dial which measures the intraocular pressure.
1.5 Color perception and its malfunctions
The trichromatic theory proposed by Young 1802, claims that any colour can be produced by a mixture of red, green and blue light. This infers that there only needs to be three types of cones – red (erythrolabes), blue (cyanolabes) and green (chlorolabes), with each maximally sensitive to one type of color. The cones respond to different degrees when exposed to light, with the brain synthesising this information to produce all other colors1.
Malfunctions of color perception include monochromacy, dichromacy, anomalous trichromacy and achromatopsia
Monochromacy, caused by the absence of two of the three cones, is the inability to distinguish between colors. Thus color vision is reduced to one dimension. There are two forms, rod and cone monochromacy respectively. Rod monochromacy, associated with light sensitivity (photophobia) is the absence or malfunction of the retinal cones. As a result the ability to distinguish colors is impaired. Cone monochromacy refers to color blindness which is accompanied by relatively normal vision.
Dichromacy constitutes the absence or malfunctioning of one of the three cones, thus limiting vision to two dimensions. It may be passed on to the offspring genetically (i.e. sex linked), in particular having a predilection in the male population. As with monochromacy, this defect comprises two forms, (a congenital sex linked color vision defect caused by the absence of the red retinal photoreceptors) and (red-green color blindness resulting from the loss of function of medium wavelength cones or ‘’M-cones’’)
Anomalous trichromacy is a congenital color vision deficiency, referring to the relatively low quantity of one of the three types of cone photo-pigments. The condition is thought to occur when one of the three cone pigments are altered, but trichromacy or normal three dimensional color vision is not fully impaired.
Achromatopsia is congenital or inherited deficiency of color perception. It is caused by the absence of cone cells or severe defect in those initially present. Individuals with this condition typically perceive the world as being gray, black and white2.
1.6 Binocular 3D Vision
Binocular vision yields a wider scope of vision. Most objects in our visual world have texture. The acquisition of two eyes (binocular vision) as oppose to one (monocular vision) makes the grain of texture appear finer as we move from one region to another. This gives binocular summation, in which the ability to detect faint objects is enhanced.
The perception of depth is the ability to perceive the world in three dimensions1. Binocular disparity arises as each eye has a slightly different perspective of the object being viewed2. Therefore the closer the object, the more disparate the image. Thus binocular disparity is used as a binocular depth cue. Other binocular depth cues include t (the process in visual perception leading to perception of the depth or distance of objects)3 and binocular convergence (the simultaneous inward movement of both toward each other, usually in an effort to maintain single binocular vision when viewing an object) 4.
Because of binocular disparity, light entering one eye can alter papillary diameter in the other closed eye upon opening. It may also affect the process of accommodation (focusing of the eye) as the accommodation of the closed eye, upon opening, will inevitably be equal to that of the first eye.
Fig 5 – 3D processing of the brain
The picture on the previous page contains two images of a chair, one red and one blue, from two slightly different angles. When wearing two different lenses, one lens will filter out the blue color and the other the red color. The result is each eye is only receiving one of the two images on the page. Just as if you were looking at a real chair from two different angles, the brain forms these images into one three-dimensional image (hence the term binocular 3D vision arises).
1.7 Devices for night vision
Night vision devices are best appreciated by deciding what you intend to use them for.
Of the numerous devices available, night glasses are typical preferred amongst others, possibly because of their primitive outlook. Their large lenses can accumulate light and subsequently project it through the exit pupil of 7nm or more, and into the individual’s eye, thus enhancing vision in hours of darkness.
Thermal vision is a fairly modern exploitation of science in which a device (e.g. security camera equipment) constructs an image in response to microwaves or sound waves, which are transmitted from the source. Thermal vision devices are generally not considered to be night vision devices as they construct images with mechanisms substantially different from the methods used to sense visible light.
Amplification of visible light from an image can be achieved by making use of an image intensifier. This allows the image to be viewed by the naked eye.
References
1.1:http://en.wikipedia.org/wiki/Visual_acuity
Further acknowledgements:
1 –
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12809155&dopt=Abstract
1.2:http://www.chemsoc.org/exemplarchem/entries/2002/upton/defects_of_the_eye.htm
Further acknowledgements:
1 –
Chapters from Biophysics; ISBN:80-239-8173-0
1.3:http://www.agingeye.net/glaucoma/glaucomaeyeexam.php
Further acknowledgements:
1 –
http://freespace.virgin.net/amanda.gibson/snellen.htm
1.4:http://en.wikipedia.org/wiki/Intraocular_pressure
1.5:http://en.wikipedia.org/wiki/Color_vision_deficiency
Further acknowledgements:
1 -
Psychology A2: The exam companion; ISBN: 0-7487-9262-7
2 -
http://www.visionchannel.net/colorvision/types.shtml
1.6:http://en.wikipedia.org/wiki/Binocular_vision
Further acknowledgements:
1 -
http://en.wikipedia.org/wiki/Depth_perception
2-
http://ahsmail.uwaterloo.ca/kin356/cues/bcues.htm
3 -
http://en.wikipedia.org/wiki/Depth_perception
4 -
http://en.wikipedia.org/wiki/Convergence
1.7:http://en.wikipedia.org/wiki/Night_vision
