Vision Charts

[Janne Desir]

ASnellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862 as a measurement tool for the acuity formula developed by his professor Franciscus Cornelius Donders.[1][2] Many ophthalmologists and vision scientists now use an improved chart known as the LogMAR chart.

The normal Snellen chart is printed with eleven lines of block letters. The first lineconsists 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".

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:

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 (20 ft), 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 (9.8 ft), 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]

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 8.73 mm tall and the topmost (6/60) "E" should be 87.3 mm 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 w \displaystyle w is the optotype height or width (which are the same due to the optotype being on a square grid), d \displaystyle d is the distance from eye to chart, and θ \displaystyle \theta 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 17 mm model eye for calculations, and a letter which subtends 5 minutes of arc, gives a vertical height of the 20/20 letter to be 8.75 mm.[8]

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 DMV employee will ask you to read certain rows to find out how well you can recognize the characters on the chart. They can measure the sharpness of your eyesight based on the smallest row of letters you can read.

Improving your vision may be as simple as ordering a pair of prescription glasses. The doctor will measure your eyesight and provide you with an up-to-date vision prescription, along with different ways you can improve the quality of your eyesight.

Various types of eye charts are available. Some charts use pictures or patterns, while others use letters. Eye care providers might use certain charts for measuring distance vision and others for measuring near vision. Some eye charts are especially for children while others work for both children and adults. The Snellen eye chart, however, is the most common and the most recognizable.

You stand 20 feet away from the Snellen chart, and read from it without your glasses or contacts. You cover one eye and read out the smallest line of letters you can see. Then you cover the other eye and do it again. In some offices, you view the chart through a mirror. This means the test can be done with less than 20 feet of space. The results are the same whether or not a mirror is used.

The top number refers to the distance in feet that you stand from the chart. The bottom number is the distance at which a person with normal sight can read the same line you correctly read. A person with 20/20 vision can see what an average person can see on an eye chart when they are standing 20 feet away.

Dutch eye doctor Hermann Snellen developed the Snellen eye chart in the 1860s. He was a colleague of Dr. Franciscus Donders. Donders diagnosed vision problems by asking people to look at a chart on a wall and tell him what they could see. According to The New York Times, he asked Dr. Snellen to make the chart.

Your visual acuity is measured as part of an eye exam. The American Academy of Ophthalmology recommends that you get a baseline eye examination at age 40. This is when early signs of disease or changes in vision may happen. Eye charts do not help the eye doctor tell whether you have an eye disease such as glaucoma or a problem with your retina. They also do not measure other vision problems such as loss of peripheral (side) vision.

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During an eye exam, your eye doctor will ask you to find the smallest line of letters you can read, then ask you to read it. If you can read the bottom row of letters, your visual acuity (sharpness) is very good.

To evaluate your near vision, your eye doctor may use a small hand-held card called a Jaeger eye chart. The Jaeger chart consists of short blocks of text in various type sizes.

The font size on a modern Jaeger eye chart usually ranges from J10 (about 14-point type in Times New Roman font) to J1 (about 3-point type in Times New Roman). Some Jaeger charts have an additional paragraph labeled J1+ that may be even smaller than the J1 block of text.

Today Garmin is excited to bring HD content to our 2016 version of the BlueChart cartography product line, available in February. We have renamed our existing BlueChart g2 and g2 Vision charts to BlueChart g2 HD and BlueChart g2 Vision HD. The same features you enjoyed before are still included, with the addition of HD contours and improved Auto Guidance.

BlueChart g2 HD: The standard tier provides basic chart navigation, including up to 1-foot HD fishing contours, tides, currents, marine POIs, IALA symbols and perspective chart view; invaluable tools for the mariner. BlueChart g2 HD is ideal for handheld devices and for the value-oriented models of the Garmin chartplotter lineup. Available on microSD/SD card or as a download via Garmin.com.

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A growing number of tools enable users to make online data representations, like charts, that are accessible for people who are blind or have low vision. However, most tools require an existing visual chart that can then be converted into an accessible format.

Umwelt, an authoring environment designed for screen-reader users, incorporates an editor that allows someone to upload a dataset and create a customized representation, such as a scatterplot, that can include three modalities: visualization, textual description, and sonification. Sonification involves converting data into nonspeech audio.

The system, which can represent a variety of data types, includes a viewer that enables a blind or low-vision user to interactively explore a data representation, seamlessly switching between each modality to interact with data in a different way.

The researchers previously developed interactive interfaces that provide a richer experience for screen reader users as they explore accessible data representations. Through that work, they realized most tools for creating such representations involve converting existing visual charts.

For instance, a sighted user can see the overall pattern of a scatterplot and, at the same time, move their eyes to focus on different data points. But for someone listening to a sonification, the experience is linear since data are converted into tones that must be played back one at a time.

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