Do you still remember your school days? The blackboard was black or dark green, the chalk was white. The letters and numbers on it were visible.

Books and printed matter, on the other hand, are all printed in black on a white background; Black text on a white background also dominates the screens of PCs, smartphones or tablets. To the eye, however, black is not a color. It is a black hole and is difficult to see. For our vision, black is synonymous with darkness, the absence of light.

The white of the background can be easily perceived by the eye. A bright point can be seen better and, above all, faster in the dark than a dark point in the light. Because of economic and ecological aspects, it is not possible to do this with printed books. With modern PC screens or smartphones, on the other hand, it is easily possible to reverse these two colors. Almost all manufacturers now offer so-called "dark themes". These are designs in which white backgrounds are displayed in black and black fonts are displayed in white. The remaining colors are displayed either original or inverted, depending on the theme selected.

Andrea C. Aleman, Min Wang and Frank Schaeffel from the Research Institute for Ophthalmology at the University Hospital Tübingen discovered astonishing things in a study: When reading inverted, i.e. white writing on a black background, the growth in length of the eye could be slowed down. However, research on this is still at the very beginning. It is also planned to continue this study with school children to check the findings.

How does the retina of the eyes work?

While a digital camera processes all image points (pixels) in the same way, the retina of the eyes mainly "measures" the differences between neighboring "pixels". Only the difference in brightness between the center and the edge of the light-sensitive area is transmitted from the cell to the brain. A reduction in the visual information is necessary because the retina has 125 million "pixels", but the optic nerve can transmit significantly less. It “only” has about a million parallel connections. So there is a bottleneck in the transfer of information from the retina to the brain. This is avoided by simplifying the visual information described above.

There are two different types of these cells. One type compares whether the center is lighter and the surrounding area is darker in its light-sensitive area (so-called ON cells). The other type, in turn, compares whether the center is darker and the surroundings are lighter (so-called OFF cells). In natural vision, both types of cells are activated in a balanced manner.

But what about reading text? In today's digital everyday life you sit in front of screens or books all day. More OFF than ON signals are transmitted from the retina to the brain. The researchers were able to measure how the choroid - the layer behind the retina - behaves. It has long been known from other studies that the thickness of the choroid is related to the growth in length of the eye and the development of myopia.

The effects of inverted reading

After reading inverted text for only one hour, the test subjects showed a thinning of the choroid using optical coherence tomography (OCT). A thinner choroid helps reduce myopia. The researchers were also able to measure how the choroid became thicker after the test subjects had read black text on a white background for an hour. Black and white is the normal situation in our everyday life. The researchers concluded that inverted reading could stop progressive myopia.

Another side effect of inverted reading on the screen is that the monitor emits less LED light. Much of the area is virtually darkened.

How can you use inverted mode on your screen?

In principle, it doesn't matter whether you want to use a computer or smartphone screen in dark mode. Most of the time you will find one or more dark themes (also "dark theme" or dark mode or dark mode) in the settings of the respective device.

Depending on the operating system, not only are the font colors reversed, but in many cases images are also displayed inverted. Since this can be a hindrance to work, there are special extensions in some programs. In the Firefox web browser, for example, you install the add-on "Dark Background and Light Text". This has the effect that images are displayed in their original colors, only the text is inverted.

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