حدة الإبصار

Typical Snellen chart used for visual acuity testing.

حدة الإبصار(VA) هوالحدة أووضوح الرؤية، والذي يعتمد على حدة هجريز الشبكية داخل العين.

التاريخ

Year	Event
1843	German treatise advocating the need for standardized vision tests and developed a set of three charts.
1854	Eduard von Jaeger published a set of reading samples to document functional vision. He published samples in German, French, English and other languages. He used fonts that were available in the State Printing House in Vienna in 1854 and labeled them with the numbers from that printing house catalogue.
1861	Franciscus Donders coined the term visual acuity to describe the “sharpness of vision” and defined it as the ratio between a subject's VA and a standard VA.
1862	Hermann Snellen published his famous letter chart. His most significant decision was not to use existing typefaces but to design special targets, which he called optotypes. He based it on a 5x5 grid. This was crucial because it was a physical standard measure to reproduce the chart. Snellen defined “standard vision” as the ability to recognize one of his optotypes when it subtendedخمسة minutes of arc, thus the optotype can be recognized only if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc (one element of the grid).
1868	John Green of St. Louis, who had worked with Donders and Snellen, proposed a chart with a geometric progression of letter sizes and proportional spacing between letters. At that time, Green’s proposals were not accepted. A century later, his principles would be incorporated in international standards.
1875	Snellen changed from using feet to meters (from 20/20 to 6/6 respectively). Today, the 20-foot distance prevails in the United States,ستة meters prevails in Britain,خمسة orستة meters are used in continental Europe. Also in 1875 Ferdinand Monoyer proposed replacing the fractional Snellen notation with its decimal equivalent (e.g., 20/40 = 0.5, 6/12 = 0.5, 5/10 = 0.5). Decimal notation makes it simple to compare visual acuity values, regardless of the original measurement distance.
1888	Edmund Landolt proposed the Landolt C, a symbol that has only one element of detail and varies only in its orientation. The broken ring symbol is made with a "C" like figure in aخمسة xخمسة grid that, in the 20/20 optotype, subtendsخمسة minutes of arc and has an opening (oriented in the top, bottom, right or left) measuring 1 minute of arc. This proposal was based in the fact that not all of Snellen's optotypes were equally recognizable. This chart is actually the preferred visual acuity measurement symbol for laboratory experiments but gained only limited acceptance in clinical use.
1898	Marius Tscherning reported the inadequacy of 20/20 (1 minute of arc) as a norm value of VA and explained the Snellen’s mistake who referred to a normal observer using this wrong value. Tscherning’s opinion is echoed by many modern investigators who have found that Snellen’s criterion does not represent the normal limits of vision. Many observers are capable of producing results that surpass the limit of the supposed 20/20 standard for visual acuity. Surprisingly, the 20/20 myth still continues today.
1923	Soviet ophthalmologists Sergei Golovin and D. A. Sivtsev developed the table for testing visual acuity. Later this table became known as Golovin-Sivtsev Table.
1959	Louise Sloan designed a new optotype set ofعشرة letters, all to be shown in each and every line tested, in order to avoid the problem that not all letters are equally recognizable. The larger letter sizes thus required more than one physical line. Louise Sloan also proposed a new letter size notation using the SI system stating that standard acuity (1.0, 20/20) represents the ability to recognize a standard letter size (1 M-bunit) at a standard distance (1 meter).
1976	Ian Bailey and Jan Lovie published a new chart featuring a new layout with five letters on each row and spacing between letters and rows equal to the letter size. This layout was created to standardize the crowding effect and the number of errors that could be made on each line, so letter size became the only variable between the acuity levels measured. These charts have the shape of an inverted triangle and are much wider at the top than traditional charts. The character size change is a logarithmic progression of letter sizes. These charts are also known as Log-MAR charts. Lea Hyvärinen created a chart, the Lea chart, using outlines of figures (an apple, a house, a circle and a square) to measure visual acuity in preschool children. Hugh Taylor used these design principles for a "Tumbling E Chart" for illiterates, later used to study the visual acuity of Australian Aborigines.
1982	Rick Ferris et al. of the National Eye Institute chose the Bailey-Lovie layout, implemented with Sloan letters, to establish a standardized method of visual acuity measurement for the Early Treatment of Diabetic Retinopathy Study (ETDRS). These charts were used in all subsequent clinical studies, and did much to familiarize the profession with the new layout and progression. Data from the ETDRS were used to select letter combinations that give each line the same average difficulty, without using all letters on each line.
1984	The International Council of Ophthalmology approved a new 'Visual Acuity Measurement Standard', also incorporating the above features.

Physiology of visual acuity

Visual acuity expression

Visual acuity scales
Foot	Metre	Decimal	LogMAR
20/200	6/60	0.10	1.00
20/160	6/48	0.125	0.90
20/125	6/38	0.16	0.80
20/100	6/30	0.20	0.70
20/80	6/24	0.25	0.60
20/63	6/19	0.32	0.50
20/50	6/15	0.40	0.40
20/40	6/12	0.50	0.30
20/32	6/9.5	0.63	0.20
20/25	6/7.5	0.80	0.10
20/20	6/6	1.00	0.00
20/16	6/4.8	1.25	-0.10
20/12.5	6/3.8	1.60	-0.20
20/10	6/3	2.00	-0.30

Name	Abbreviation	Definition
Counting Fingers	CF	Ability to count fingers at a given distance.
Hand Motion	HM	Ability to distinguish a hand if it is moving or not in front of the patient's face.
Light Perception	LP	Ability to perceive any light.
No Light Perception	NLP	Inability to see any light. Total blindness.

Legal Definitions

A person's visual acuity is registered documenting the following: whether the test was for distant or near vision, the eye(s) evaluated and whether corrective lenses (i.e. glasses or contact lenses) were used:

Distance from the chart
- D (distant) for the evaluation done at 20 قدم (6.1 م).
- N (near) for the evaluation done at 15.7 بوصة (40 cم).
Eye evaluated
- OD (Latin oculus dexter) for the right eye.
- OS (Latin oculus sinister) for the left eye.
- OU (Latin oculi uterque) for both eyes.
Usage of spectacles during the test
- cc (Latin cum correctore) with correctors.
- sc: (Latin sine correctore) without correctors.
Pinhole occluder
- The abbreviation PH is followed by the visual acuity as measured with a pinhole occluder, which temporarily corrects for refractive errors such as myopia or astigmatism.

So, distant visual acuity of 20/60 and 20/25 with pinhole in the right eye will be:
DscOD 20/60 PH 20/25

Distant visual acuity of count fingers and 20/50 with pinhole in the left eye will be:
DscOS CF PH 20/50

Near visual acuity of 20/25 with pinhole remaining at 20/25 in both eyes with spectacles will be:
NccOU 20/25 PH 20/25

Measurement considerations

Visual acuity testing in children

Other measures of visual acuity

انظر أيضاً

Dioptre
Eye examination
Optical resolution
Pediatric ophthalmology
Refractive error
Strabismus
Troxler's fading
Retinal summation
Hyperacuity (scientific term)

المصادر

^ Herman Snellen (www.whonamedit.com)
^ http://www.ski.org/Colenbrander/Images/Measuring_Vis_Duane01.pdf

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قراءات أخرى

Duane's Clinical Ophthalmology, V.1 C.5, V.1 C.33, V.2 C.2, V.2 C.4, V.5 C.49, V.5 C.51, V.8 C.17, Lippincott Williams & Wilkins, 2004.
(بالروسية) Головин С.С. Сивцев Д.А. Таблица для исследования остроты зрения. ثلاثة изд. М., 1927

ةصلات خارجية

How Visual Acuity is Measured
Visual Acuity of the Human Eye
Golovin-Sivtsev Table for testing visual acuity - used in the USSR and post-Soviet states in different formats (as pdf or cdr files)
Visual Acuity Chapter from the Webvision reference, University of Utah