What Is a Landolt Ring? The DIN 58220 Standard Explained
On the wall of almost every optician in Germany hangs a chart full of small rings, each with a gap in its edge. You may know them as the circles you point at during the eye test, but they have a precise name, a long history and a German standard of their own. This is the Landolt ring, and the Landolt ring DIN 58220 standard defines exactly how big it must be, how it must be printed, and how the chart that carries it must be built.
This article explains what a Landolt ring is, the DIN 58220 standard behind it, the ring sizes used in the German driving licence eye test, and the history of the ring from its invention in 1888 to today. It is about the ring itself and the standard, not about how to read the openings. If you want to know how the openings are tested, our guide to Landolt rings in the eye test covers that.
What Is a Landolt Ring?
A Landolt ring is a circle with a gap in it, printed as a black ring on a white background. The ring is not a letter. It is a symbol that tests your ability to see detail, and its design is set by strict proportions. The outer diameter of the ring is five times the width of the gap, and the thickness of the ring is exactly the same as the width of the gap. In other words, if the gap is one unit wide, the ring is one unit thick and five units across, with a hole of three units in the middle.
Those proportions are the heart of the ring. Because the gap and the ring are the same width, the ring measures one thing only: the smallest detail your eye can resolve. You can see the gap only if your eye can separate two points one gap-width apart. The design makes the ring a pure test of resolution, without any help from the shape of a letter or the memory of a word.
The gap can point in eight directions: up, down, left, right, and the four diagonals between them. On a printed chart, the rings are arranged in rows, with each row smaller than the one above it, and the gap direction varies from ring to ring so that no two rings in a row are the same. Each ring is the same shape as the others, just a different size.
The History: Edmund Landolt and 1888
The ring was invented by Edmund Landolt, a Swiss ophthalmologist who lived from 1846 to 1926 and worked in Paris. Landolt presented the ring in 1888, at a time when eye tests used letters from the alphabet, which worked only for people who could read, and only in languages whose letters they knew. A child who had not yet learned to read, or a person who spoke a different language, could not be tested fairly with letters.
Landolt’s idea was simple: use a symbol that anyone can understand, whatever their language or education. A ring with a gap needs no reading, no letters, and no words. A person only has to say where the gap is. Landolt presented the ring to his fellow eye doctors, and it spread quickly, because it solved a real problem. Within a few years, eye doctors across Europe were using the ring, and it became the standard symbol for measuring visual acuity.
The ring is also called the Landolt C in English, after its shape, and the Landolt ring in German. Landolt himself was a respected figure in ophthalmology, and his ring is his best known contribution, although he also worked on eye surgery and on the measurement of the eye. The fact that his ring is still in use more than a hundred years after it was invented says a great deal about the quality of the design.
DIN 58220: The German Standard
DIN 58220 is the German standard for eye test symbols, known in German as Sehzeichen. DIN stands for the Deutsches Institut fuer Normung, the German Institute for Standardisation, which writes the technical standards used across German industry. DIN 58220 sets out the design of the Landolt ring and the rules for building eye test charts from it, including the sizes of the rings, the spacing between them, the contrast of the printed symbols, and the lighting conditions under which the chart must be used.
The standard exists because an eye test is only useful if the charts are the same everywhere. If one optician printed rings of the wrong size, or used a chart with too little contrast, the result would be different from the test in the next town. DIN 58220 makes the test repeatable. The rings on a chart made to the standard are the same size, printed with the same black on white, and tested at the same distance, so that a result in Hamburg can be compared with a result in Munich.
The standard also defines the test distance. A distance chart built to DIN 58220 is normally used at five metres, and the sizes of the rings on that chart are calculated for that distance. A smaller chart for a shorter distance can be made to the same standard, but the rings on it must be scaled by the same factor, so that the angles stay the same. The standard is the reason that the chart on the optician wall is not just a poster. It is a measuring instrument.
Ring Sizes and the Five Arcminute Rule
The size of a Landolt ring is tied to the angle it covers in your field of vision. Visual acuity, called Visus in German, is defined by the smallest angle that your eye can still separate. For a Visus of 1.0, the gap in the ring covers one arcminute, which is one sixtieth of one degree. Because the ring is five times as wide as the gap, the whole ring covers five arcminutes. That angle, five arcminutes at Visus 1.0, is the basic unit of the whole chart.
At the usual test distance of five metres, those angles give real sizes in millimetres. A ring that tests a Visus of 1.0 has an outer diameter of about 7.3 millimetres, and its gap is about 1.5 millimetres wide. A ring for a lower Visus is larger, in the same proportion. The ring that tests the driving licence minimum of 0.7 is about 10.4 millimetres across, with a gap of about 2.1 millimetres, because its angle is one divided by 0.7 times the angle for Visus 1.0. Each row of the chart is bigger than the one below it by a fixed factor, so that the steps between the Visus values are even, and the smallest row is the one that tests the highest Visus.
That is why the Landolt ring is more than just a symbol. Its size in millimetres is a direct measure of your eyesight, and the chart is built so that the size on paper is tied to a precise angle. If a chart is printed at the wrong scale, the results it gives are wrong too, which is why the standard also covers the printing of the chart and the way it is lit. A ring that is even slightly too large or too small would give a wrong reading of your vision.
Why Rings Instead of Letters?
Letters were the first symbols used to test vision, but they have two weaknesses. The first is language. A letter chart works only for people who know the alphabet, and the letters a German uses are not the same as the ones a person from another country knows. The Landolt ring needs no alphabet. It is a shape, and anyone can point at it, whatever language they speak. That is why the ring is used not only in Germany but in eye tests across the world.
The second weakness is memory. After a few visits, a person can learn a letter chart by heart, and the test stops measuring their vision. The Landolt ring cannot be learned in the same way, because the gap direction changes from ring to ring, and the tester can present a new set of rings each time. The chart itself has thousands of possible combinations, so there is nothing to memorise. That is also why the ring is valued in the driving licence eye test, where it is important that the result measures what you can see, not what you can remember.
The ring is also used for children, for people with a learning disability, and for patients who cannot speak or read. Those people could not be tested fairly with letters. In that sense, the Landolt ring made the eye test fairer, and it is one of the reasons the ring is still used today, even when digital screens and projectors are available.
The Landolt Ring in the German Driving Licence Test
In the German driving licence eye test, the Landolt ring is the standard symbol. The tester shows you rings of decreasing size on a chart built to the DIN 58220 standard, at a distance of five metres. The smallest row you can read reliably gives the result. For a car licence, the minimum is a Visus of 0.7 with both eyes together, which corresponds to a ring of about 10.4 millimetres on the test chart. Our guide to measuring visual acuity explains what the Visus numbers mean in plain language, and our guide to visual acuity 0.7 explains the 0.7 minimum for the driving licence in detail.
The test is carried out in a brightly lit room, with the chart well lit, because the ring test depends on the contrast between the black ring and the white paper. Under dim light, the rings are harder to see, and the result can be worse than it should be. A good tester checks the light before starting, and will use a light box or a daylight lamp if the room light is not good enough.
The fact that the ring is standardised also helps people who do not pass on the first attempt. If you take the test again at another optician, the chart will be built to the same standard, so the result should be the same. You are not tested against one shop test, but against a standard that applies to every shop in Germany.
ISO 8596 and the International Standard
Germany is not the only country to use the Landolt ring. The ring is also the international standard optotype, defined by ISO 8596, the international standard for visual acuity testing. ISO 8596 was written to give eye tests the same basis everywhere, so that a result measured in Germany can be compared with a result measured in Japan, in Brazil or in the United States. The ring, its proportions and its sizes are the same in both standards, and DIN 58220 matches ISO 8596 in the essentials.
The two standards differ mainly in small details, such as the number of rings in a row or the way a chart is labelled. The ring itself, the five to one proportion, and the five arcminute rule are the same. That is why a result measured on a German chart can be compared with an international result without a conversion table.
If you come from a country that uses letters, you may be surprised by the rings on the German chart. There is nothing to learn before the test. You only need to know that the gap can point in eight directions, and that the tester will ask you where it is. The rings are simple to understand, and they are there to measure your vision fairly, whatever your language.
Conclusion
The Landolt ring is a small symbol with a precise design and a long history. Invented by Edmund Landolt in 1888, the ring measures visual acuity with a gap that is exactly one fifth of its diameter, and the whole ring covers five arcminutes at a Visus of 1.0. The German standard DIN 58220 sets the ring sizes, the printing and the test conditions so that every chart in Germany measures the same thing, and the international standard ISO 8596 does the same worldwide. In the driving licence eye test, the ring that tests the 0.7 minimum is about 10.4 millimetres wide at five metres. The ring is simple, but it is also a measuring instrument, and the standard is what makes it work.
Frequently Asked Questions
What is a Landolt ring?
A Landolt ring is a black ring with a gap in it, used to measure visual acuity. The gap is one fifth as wide as the ring, and the ring is five times as wide as the gap. The whole ring covers five arcminutes of your field of vision at a Visus of 1.0.
What does DIN 58220 cover?
DIN 58220 is the German standard for eye test symbols. It defines the design of the Landolt ring, the sizes of the rings, the layout of the chart, the contrast of the printed symbols and the lighting conditions for the test, so that charts across Germany measure the same thing.
Who invented the Landolt ring?
Edmund Landolt, a Swiss ophthalmologist, presented the ring in 1888. He wanted a symbol that did not depend on reading or language, so that children and people from other countries could be tested fairly.
How big is a Landolt ring?
At the usual test distance of five metres, the ring that tests a Visus of 1.0 is about 7.3 millimetres across, with a gap of about 1.5 millimetres. The ring for the driving licence minimum of 0.7 is about 10.4 millimetres across.
Why are Landolt rings used instead of letters?
Because they work for anyone, whatever their language or education, and because they cannot be memorised like a letter chart. The gap direction changes from ring to ring, so the test measures what you can see, not what you remember.
Is the Landolt ring used outside Germany?
Yes. It is the international standard optotype, defined by ISO 8596, so a result measured in Germany can be compared with a result measured in other countries.