Snellen chart is a poster that serves to detect a person’s sharp vision. Because there is a difference between the measurement system used in Indonesia (as well as most other countries in the world) and the United States, this Snellen chart is also available in two numbers. One in the metric number and the other in the imperial number. Snellen chart metric is stated in comparison of 6 meters (6/6, 6/9, 6/12, and so on until 6/60).
While Snellen imperial chart is like the one in the picture below. The numbers are stated in the 20-foot comparison (20/20 to 20/200). Is 20 feet equal to 6 meters? Actually no: 20 feet is 6 meters more than 10 cm (exactly 609.6 cm). But of course the excess of 10 cm may be ignored.
Then what is the function of the fractional number next to each line? In visual inspection, the number that plays an important role is the number on the bottom line that can be read by the subject. For example the subject can only read until line 6/9. This means that people with sharp visual acuity can read the line at a distance of 9 meters. Meanwhile the new subject can read it at a distance of 6 meters. The higher the position of the bottom line that can be read by the subject, means the worse the sight is.
Subjects who cannot read up to lines 6/6 (or 20/20) may experience visual impairments due to organic diseases in the eye, or pure refraction disorders. Organic disease in the eye means there is a structural abnormality which results in decreased vision. For example there is damage to the cornea or turbidity in the lens (in cataracts). But in pure refraction disorders, there are no structural abnormalities found in the eye. To distinguish between them, a pinhole examination is used. Pinhole is a black screen with a small hole in the middle that is placed in front of the eye being examined. If visual acuity improves with the help of pinhole, it means that there are no structural abnormalities in the eye.
If someone cannot read the Snellen chart at all even with the help of a lens, the next check is counting the fingers (count fingers). Normal people can count fingers at a distance of 60 meters. So if a new subject can count fingers at a distance of 2 meters, it means sharp vision 2/60. The next examination is hand motion. Normal people can see waves at a distance of 300 meters. Just like finger counting, if a new subject can see a wave at a distance of 1 meter, it means that the sight is 1/300. The final examination is the presence or absence of light perception.