Snellen developed charts using symbols based in a 5×5 unit grid. The experimental charts developed in 1861 used abstract symbols.[3] Snellen's charts published in 1862 used alphanumeric capitals in the 5×5 grid. The original chart shows A, C, E, G, L, N, P, R, T, 5, V, Z, B, D, 4, F, H, K, O, S, 3, U, Y, A, C, E, G, L, 2.[2]
Description
The normal Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, which may be one of several letters, for example E, H, or N. Subsequent rows have increasing numbers of letters that decrease in size. A person taking the test covers one eye from 6 metres or 20 feet away, and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that specific eye. The symbols on an acuity chart are formally known as "optotypes".
Variation of Snellen chart with another letter sequence. Australia, 1946
In the case of the traditional Snellen chart, the optotypes have the appearance of block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font. They have a particular, simple geometry in which:
the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter "C"
the height and width of the optotype (letter) is five times the thickness of the line.
Only the nine letters C, D, E, F, L, O, P, T, Z are used in the common Snellen chart. The perception of five out of six letters (or similar ratio) is judged to be the Snellen fraction.[4] Wall-mounted Snellen charts are inexpensive and are sometimes used for approximate assessment of vision, e.g. in a primary-care physician's office. Whenever acuity must be assessed carefully (as in an eye doctor's examination), or where there is a possibility that the examinee might attempt to deceive the examiner (as in a motor vehicle license office), equipment is used that can present the letters in a variety of randomized patterns. BS 4274-1:1968 (British Standards Institution) "Specification for test charts for determining distance visual acuity" was replaced by BS 4274-1:2003 "Test charts for clinical determination of distance visual acuity — Specification". It states that "the luminance of the presentation shall be uniform and not less than 120 cd/m2. Any variation across the test chart shall not exceed 20%." According to BS 4274-1:2003 only the letters C, D, E, F, H, K, N, P, R, U, V, and Z should be used for the testing of vision based upon equal legibility of the letters.[citation needed]
Snellen fraction
Visual acuity is the distance at which test is made / distance at which the smallest optotype identified subtends an angle of five arcminutes and the critical distinguishing features of the optotype subtend an angle of one arcminute.[5][2]
Snellen defined "standard vision" as the ability to recognize one of his optotypes when it subtended 5 minutes of arc. Thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of one minute of arc.
Outside the United States, the standard chart distance is 6 metres (20ft), and normal acuity is designated "6/6". Other acuities are expressed as ratios with a numerator of 6. Some clinics do not have 6-metre eye lanes available, and either a half-size chart subtending the same angles at 3 metres (9.8ft), or a reversed chart projected and viewed by a mirror is used to achieve the correct sized letters.
In the most familiar acuity test, a Snellen chart is placed at a standard distance: 6 metres. At this distance, the symbols on the line representing "normal" acuity subtend an angle of five minutes of arc, and the thickness of the lines and of the spaces between the lines subtends one minute of arc. This line, designated 6/6 (or 20/20), is the smallest line that a person with normal acuity can read at a distance of 6 metres. This definition is arbitrary, since human eyes typically have higher acuity, as Tscherning writes, "We have found also that the best eyes have a visual acuity which approaches 2, and we can be almost certain that if, with a good illumination, the acuity is only equal to 1, the eye presents defects sufficiently pronounced to be easily established."[6]
Three lines above, the letters have twice the height of those letters on the 6/6 (or 20/20 in the US) line. If this is the smallest line a person can read, the person's acuity is "6/12" ("20/40"), meaning that this person needs to approach to a distance of 6 metres (20ft) to read letters that a person with normal acuity could read at 12 metres (39ft). In an even more approximate manner, this person could be said to have "half" the normal acuity of 6/6.
At exactly 6 metres' distance from the patient, the letters on the 6/6 line shall subtend 5 minutes of arc (such that the individual limbs of the letters subtend 1 minute of arc), which means that the chart should be sized such that these letters are 8.73mm tall and the topmost (6/60) "E" should be 87.3mm tall. Putting it another way, the eye should be at a distance 68.76 times the height of the top (6/60) letter. The formula is
where is the optotype height or width (which are the same due to the optotype being on a square grid), is the distance from eye to chart, and is the angle subtended by the optotype, which is 5 arcminutes as specified by Snellen.[7] Another calculation for United States clinics using 20-foot chart distances (slightly more than 6 m), and using a 17mm model eye for calculations, and a letter which subtends 5 minutes of arc, gives a vertical height of the 20/20 letter to be 8.75mm.[8]
Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription.
The largest letter on an eye chart often represents an acuity of 6/60 (20/200), the value that is considered "legally blind" in the US. Many individuals with high myopia cannot read the large E without glasses, but can read the 6/6 (20/20) line or 6/4.5 (20/15) line with glasses. By contrast, legally blind individuals have a visual acuity of 6/60 (20/200) or less when using the best corrective lens.
Electronic chart
To ensure adequate illumination of the Snellen charts, various medical device manufacturers had developed Snellen chart products with backlight or projection.
Digital chart
Since computer monitors typically have good lighting for reading and LCD/LED monitors have high DPI (between 96 and 480) they are suitable for displaying optotypes. Commonly digital chart products support randomizing optotypes displayed to prevent patients from memorizing lines they have previously read. In Google Play and App Store (iOS), there are Snellen chart apps for smart phones and tablets.
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol ′, is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, or complete rotation, one arcminute is 1/21600 of a turn. The nautical mile (nmi) was originally defined as the arc length of a minute of latitude on a spherical Earth, so the actual Earth circumference is very near 21600 nmi. A minute of arc is π/10800 of a radian.
Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye influence the sharpness of an image on its retina. Neural factors include the health and functioning of the retina, of the neural pathways to the brain, and of the interpretative faculty of the brain.
An eye chart is a chart used to measure visual acuity comprising lines of optotypes in ranges of sizes. Optotypes are the letters or symbols shown on an eye chart. Eye charts are often used by health care professionals, such as optometrists, physicians and nurses, to screen persons for vision impairment. Ophthalmologists, physicians who specialize in the eye, also use eye charts to monitor the visual acuity of their patients in response to various therapies such as medications or surgery.
An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, ophthalmologist, or an orthoptist. Health care professionals often recommend that all people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are asymptomatic.
Herman Snellen was a Dutch ophthalmologist who introduced the Snellen chart to study visual acuity (1862). He took over directorship of the Netherlands Hospital for Eye Patients, after Franciscus Donders. He was elected an International Member of the American Philosophical Society in 1894.
The Landolt C, also known as a Landolt ring, Landolt broken ring, or Japanese vision test, is an optotype: a standardized symbol used for testing vision. It was developed by the Swiss-born ophthalmologist Edmund Landolt.
Visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc. It also is called the object's angular size.
The LEA Vision Test System is a series of pediatric vision tests designed specifically for children who do not know how to read the letters of the alphabet that are typically used in eye charts. There are numerous variants of the LEA test which can be used to assess the visual capabilities of near vision and distance vision, as well as several other aspects of occupational health, such as contrast sensitivity, visual field, color vision, visual adaptation, motion perception, and ocular function and accommodation (eye).
The cranial nerve exam is a type of neurological examination. It is used to identify problems with the cranial nerves by physical examination. It has nine components. Each test is designed to assess the status of one or more of the twelve cranial nerves (I-XII). These components correspond to testing the sense of smell (I), visual fields and acuity (II), eye movements and pupils, sensory function of face (V), strength of facial (VII) and shoulder girdle muscles (XI), hearing and balance, taste, pharyngeal movement and reflex, tongue movements (XII).
The Golovin–Sivtsev table is a standardized table for testing visual acuity, which was developed in 1923 by Soviet ophthalmologists Sergei Golovin and D. A. Sivtsev. In the USSR, it was the most common table of its kind, and as of 2008 its use is still widespread in several post-Soviet states.
Sloan letters, designed by Louise Sloan in 1959, are a set of optotypes used to test visual acuity generally used in Snellen charts and logMAR charts.
Optimum HDTV viewing distance is the distance that provides the viewer with the optimum immersive visual HDTV experience.
A logMAR chart is a chart consisting of rows of letters that is used by ophthalmologists, orthoptists, optometrists, and vision scientists to estimate visual acuity. The chart was developed at the National Vision Research Institute of Australia in 1976, and is designed to enable a more accurate estimate of acuity than do other charts. For this reason, the LogMAR chart is recommended, particularly in a research setting.
Stereoscopic acuity, also stereoacuity, is the smallest detectable depth difference that can be seen in binocular vision.
The prism cover test (PCT) is an objective measurement and the gold standard in measuring strabismus, i.e. ocular misalignment, or a deviation of the eye. It is used by ophthalmologists and orthoptists in order to measure the vertical and horizontal deviation and includes both manifest and latent components. Manifest is defined by the eye deviating constantly or intermittently, whereas latent is where the deviation is normally controlled but becomes present when the eyes are dissociated. A PCT reveals the total deviation and cannot distinguish between latent and manifest strabismus as you are using an alternate cover test.
Subjective Refraction is a technique to determine the combination of lenses that will provide the best corrected visual acuity (BCVA). It is a clinical examination used by orthoptists, optometrists and ophthalmologists to determine a patient's need for refractive correction, in the form of glasses or contact lenses. The aim is to improve current unaided vision or vision with current glasses. Glasses must also be comfortable visually. The sharpest final refraction is not always the final script the patient wears comfortably.
The Jaeger chart is an eye chart used in testing near vision acuity. It is a card on which paragraphs of text are printed, with the text sizes increasing from 0.37 mm to 2.5 mm. This card is to be held by a patient at a fixed distance from the eye dependent on the J size being read. The smallest print that the patient can read determines their visual acuity. The original 1867 chart had a text containing seven paragraphs and a corresponding seven-point scale.
Louise Littig Sloan was an American ophthalmologist and vision scientist. She is credited for being a pioneer of the sub-division of clinical vision research, contributing more than 100 scientific articles in which she either authored or co-authored. Her most notable work was in the area of visual acuity testing where she developed and improved equipment. Sloan received her Ph.D. from Bryn Mawr College in experimental psychology. She spent a short period of time in both Bryn Mawr's experimental psychology program as well as the Department of Ophthalmology at Harvard Medical School. The majority of her career, however, was spent at Johns Hopkins Wilmer Eye Institute where she directed the Wilmer Laboratory of Physiological Optics for 44 years. In 1971, Sloan was the second woman awarded the prestigious Edgar D. Tillyer Award by Optica (formerly Optical Society for her many achievements in the field of vision.
Photostress recovery time (PSRT) is the time taken for visual acuity to return to normal levels after the retina has been bleached by a bright light source. Photostress recovery time measurement procedure is known as photostress test. Normal recovery time is about 15–30 seconds.
The MNREAD acuity chart or Minnesota low vision reading chart is a text based chart used to measure near visual acuity in people with normal or low vision. It can also be used to measure maximum reading speed, critical print size and the reading accessibility index of a person. Digital and printed types of charts are available.
References
↑ H. Snellen, Probebuchstaben zur Bestimmung der Sehschärfe, Utrecht 1862.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
