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.