WHEN SHINOBOU ISHIHARA JOINED THE JAPANESE IMPERIAL ARMY IN 1907, it was badly in need of eye specialists. Ishihara’s superiors sent him to study with the country’s first professor of ophthalmology in Tokyo. There, Ishihara had the chance to document Japan’s first case of a patient who was completely color blind.

Color blindness affects about one in 12 men and one in 200 women in the world. A normal eye has three types of cone-shaped photoreceptor cells, each of which handles a different color—red, green or blue. Genetic anomalies, most often affecting the red and green cones, can affect color perception. Some people are unable to distinguish subtle tones, and others see a world entirely without color.

In 1913 the army sent Ishihara to study in Germany, home to some of the era’s most prominent ophthalmologists. German researcher Jakob Stilling had developed an ingenious test for color blindness after Sweden’s infamous Lagerlunda train wreck in 1875. Two trains had collided, possibly because an engineer had been unable to distinguish between red and green signal lights. Stilling presented his patients with dots in juxtaposed colors that were difficult for people with red-green color blindness to distinguish from one another.

The outbreak of World War I in 1914 sent Ishihara back to Japan. Ishihara had long thought he could improve on Stilling’s design and had been painting his own images in watercolor. A physician in the ophthalmology department happened to be color blind, and the two men entertained themselves devising different pages on which one of them could see a figure, while the other saw only pebbly dots.

When the army asked Ishihara for a screening tool for color blindness, he went to work. People with color blindness saw no figures in Stilling’s images. But Ishihara added three new types of dotted images, some of which only the color blind could see. In others, people with a defect in their red cones saw one figure, and those with a green deficit saw another. Often the image was a simple Japanese character.

The army soon put the test to use with recruits, reportedly with great success. The test was easy to administer, and its results were simple to interpret. By then, Ishihara had achieved his M.D., and by the following year he had adapted the test for civilian use. He sent 90 copies to ophthalmologists all over the world, and Scandinavian countries were the first to embrace the test. Demand slowly grew.

About 300 million people live with some form of color blindness today. While there is a push to find new treatments—gene therapy offers some promise for severe cases—identifying people with the condition is still the first step. And for that, Ishihara’s simple tests are still the most widely used initial tool.