Low visual acuity is one of the most common eye problems people face worldwide. An eye chart is the most common diagnostic tool used to test the visual acuity. In fact, there’s no other better way to test visual acuity and diagnose vision errors than through eye chart
Interested in knowing more about eye chart? Here’s a 5-minute read on some of the amazing facts about eye chart.
1) Invention of eye chart
The first eye chart was invented by French ophthalmologist, Ferdinand Monoyer who is also credited for the introduction of a lens’ measurement unit of optical power, dioptre. The eye chart that he invented was the Monoyer chart which was to be used to test visual acuity.
The chart includes his name – his first name ‘Ferdinand’ can be seen on the right as it reads upwards. Similarly, his surname can be seen on the left which reads upwards as well. In both the cases, ignore the last two letters ‘Z’ and ‘U’.
Here’s what a Monoyer chart looks like
2) Who uses eye chart?
Health care professionals such as optometrists, ophthalmologists, physicians, or nurses use an eye chart to screen patients, mainly for vision impairment. Specialized eye care professionals such as physicians and ophthalmologists also use the eye chart to keep track of the changes in visual acuity caused by various therapies such as surgery or medication.
3) How often is eye chart used?
As mentioned above, eye care professionals use eye chart not just for determining refractive errors but also for monitoring visual acuity when patients undergo medical treatment.
As previously mentioned, visual acuity can only be determined only through an eye chart test, which is why it is used by eye care professionals all over the world.
4) How is visual acuity tested using eye chart?
There are several rows of ototypes (standardized symbols used for vision test) on an eye chart. The ototypes can vary as letters, geometric symbols, or numbers. In most cases, eye chart with letters (aka Snellen chart) is used.
During the visual acuity test, the patient is required to sit at a standardized distance away from the chart. The patient is then required to identify the letters. Generally, the patient is required to start reading the larger letters and then gradually move on to smaller ones.
As such, there may come a moment when the patient is unable to identify the progressively smaller alphabets. The smallest alphabet that the patient can identify marks his/her visual acuity. The eye care professional can then determine the refractive error and prescribe glasses, contact lenses, medication, or surgery accordingly.
5) There are different types of eye charts
Yes, you read it right. There’s not just one but different types of eye charts and all are used to test vision. These include Snellen Chart, LogMAR Chart, Jaeger Chart, E Chart, and Landolt C Chart.
These charts are described thoroughly below.
6) Snellen chart
It was developed by Dutch ophthalmologist Herman Snellen in 1862 and thus is named after him. Snellen used symbols based on a 5×5 unit grid while developing the charts. In 1861, Snellen used abstract symbols while developing the experimental charts. The letters used in the original chart include 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.
In a standard Snellen chart, there are eleven lines of block letters. The patient, while taking the test, is required to cover one eye and read aloud the letters from a distance of 20 feet. The common Snellen chart uses ten Sloan letters (ototypes that are specifically designed for visual acuity tests). These ten Sloan letters include C, D, E, F, L, N, O, P, T, and Z.
7) LogMAR chart
LogMAR chart was developed at the National Vision Research Institute of Australia. It was designed by Jan E Lovie-Kitchin and Ian Bailey. Logarithm of the Minimum Angle or Resolution is used as a reference when scoring visual acuity using the LogMAR chart.
The principles used in LogMAR chart is similar to Monoyer and Snellen chart. Observers can score LogMAR 0 when they are able to resolve 1 minute of visual angle (refer Visual Angle and Minute and second of an arc). Interpretations such as LogMAR 0.3 denote reduced visual acuity.
8) Landolt C
Swiss-born ophthalmologist Edmund Landolt developed the Landolt C. Also called Japanese Vision Test, the eye chart uses a Landolt broken ring (or simply Landolt ring) which is an ototype.
The ototype used in the chart is a ring that has a small gap which makes it look similar to the letter C. In the chart, the set of ototype is placed such that the gaps point in different directions (generally right, left, top, bottom, and positions spanning 45° angle).
During the vision test, a patient is required to determine the sides that the gaps face. Similar to other eye charts such as Snellen and LogMAR, the ototypes progressively shrink in size downwards. The ototype used in this eye charts is considered to have inherent problems as the gap appears closed near to the resolution at a 6 o’clock position.
9) Jaeger chart
The Jaeger chart is a card in which several paragraphs of texts is used to test visual acuity. In a normal Jaeger chart, the text size increases from 0.37mm to 2.5mm. During the visual acuity test, the patient is required to hold the card at a fixed distance. The smallest print that the patient can read determines the visual acuity.
The original Jaeger chart, which was developed in 1867, contained seven paragraphs along with a corresponding seven-point scale. Different variations of the test letters of Jaeger card are used which render them incomparable. Therefore, the eye chart is not standardized.
10) E chart
E Chart, also known as Tumbling E chart, is similar to Snellen chart except all the letters used are E. In the chart, the letter E is placed at different spatial alignments with a difference of 90°.
During the visual acuity test, the patient is required to look at the chart from a standardized distance and point the face of the limbs of the E. Visual acuity is determined by how far the patient can differentiate the spatial alignments of the E. The working principle of the E chart is similar to that of the Snellen chart.
11) Lea Test
Lea Test (colloquially LEA Vision Test System) is a set of vision tests that are designed for children who don’t have the ability to read standard letters, the ones that are used in standardized eye charts.
There are several types of LEA tests for determining visual acuity and other vision problems such as motion perception, colour vision, visual adaptation, visual field, and contrast sensitivity.
LEA Symbols Test and LEA Numbers Test are the two types of LEA tests designed to determine visual acuity. LEA Symbols Test uses four symbols which include outlines of an apple, a circle, a square, and a pentagon.
Image of LEA Symbols Test:
LEA Numbers Test is similar to Snellen chart. Apart from older children, this test can also be used to determine the visual acuity of adults. The numbers used in the LEA Numbers Test are calibrated against the Landolt C chart.
12) Golovin-Sivtsev table
Golovin-Sivtsev table was developed by Soviet ophthalmologists Sergei Golovin and D. A. Sivtsev, hence the name “Golovin-Sivtsev table.” The table is a standardized medical diagnostic tool for testing visual acuity.
The table used a series of Cyrillic letters including, Ш, Б, М, Н, К, Ы, and И. The layout of these Cyrillic letters is similar to that of the Snellen chart.
13) Different types of eye chart are used to test different vision
Distant and near vision are the two types of vision that determine correct visual acuity or 20/20 vision. The different types of eye charts are designed to test these two types of vision accordingly.
Snellen chart is generally used to test distance vision. Near vision can be tested by the Jaeger chart. Visual acuity of young children and illiterate are generally assessed by the Tumbling E Chart. Landolt C is generally used in laboratories.
14) Eye charts that are mostly used
Snellen and LogMAR charts are the ones that are widely and mostly used. There are several controversies about the charts being superior to the other. However, the LogMAR chart is considered as an improved version of the Snellen chart.
The LogMAR chart is designed to estimate visual acuity more precisely and therefore, it is used more for research purpose. When it comes to diagnosing children, disabled, and illiterate, LEA Test(s) and E Chart are commonly used.
15) Ototypes (symbols) are much more than just an ordinary typographic font
The symbols used in eye charts are designed to be read as letters. However, they are much more than that as they are designed using simple geometry – the height and the width of the ototype are five times thicker than the line, and the thickness of the white spaces and lines is equal.
16) What do the numbers on the side of the ototype denote?
The numbers represented on the side of the ototypes are acuity ratios which the eye care professionals use to determine visual acuity. For instance, the ototypes designated 20/20 or 6/6 are the smallest letters that a patient with normal visual acuity can read at a distance of 20 ft (6 metres).
The otoypes designated 20/40 are of twice the height of the ones designated 20/20. If a patient cannot read letters smaller than the ones designated 20/40, it means that he/she has a visual acuity of 20/40. To interpret it further, the patient having 20/40 acuity needs to view the chart at 20 feet while the same can be done by a patient having 20/20 acuity at 40 feet.
17) Alternatives to traditional eye charts
Computer-based alternatives to eye chart test such as online vision test are garnering much attention. But according to WebMD, online eye tests cannot replace visits to the eye doctor.
Online tests can only determine vision problems and do not check the overall health of the eyes. Whereas an eye doctor can diagnose any symptoms of eye diseases such as glaucoma, cataract, diabetic retinopathy, and macular degeneration.
Get your vision tested at a leading eye hospital in Kolkata
If you have any concerns regarding your vision and overall health, then get your eyes tested by our experienced eye care professionals at Disha Eye Hospital.