Right now you’re reading dark text on a bright background. This is called a positive-polarity display. Bright text on a dark background, like this, has a negative polarity. Positive polarity is much more common than negative polarity. Microsoft Word uses dark-on-bright text. This website uses it. And books use it, of course.
But a minority of people prefer it the other way around: bright-on-dark text. Negative polarity is particularly popular among software developers. For example, Atom is a programming editor that is developed by GitHub, the hippest of all hipster programmer communities. And Atom uses bright-on-dark text by default. Another example is OpenSesame, an editor for psychological experiments that I develop myself. By default, the editor component in OpenSesame uses negative polarity as well.
Examples of positive-polarity displays (left, dark-on-bright text), and negative-polarity displays (right, bright-on-dark) text.
So we have the nerds on one side, preferring bright-on-dark text, and the rest of the world on the other side, preferring good-old-fashioned dark-on-bright. So who’s right? Is this only a matter of taste? Or is one polarity really better than the other?
Well … There is a phenomenon called the positive-polarity advantage, which, as you might guess, refers to the fact that dark-on-bright text is better. In other words, Microsoft Word got it right, and GitHub and I got it wrong. But in what sense is positive polarity better? And why is it better?
One way to asses how well people can read text is through proofreading. In a proofreading experiment, participants have to spot mistakes in a text. As a measure of reading performance you can count the number of mistakes that participants spot, and the number of words they can read within a fixed time. And experiments show that reading performance is better for dark-on-bright text.
The explanation for this positive-polarity advantage is quite interesting. (This is only one of several explanations, but I find it the most convincing.) When you read from a bright background, there is a lot of light, and your pupils constrict (the pupillary light response). In contrast, when you read from a dark background, there is not a lot of light, and your pupils dilate. And because, as I explained in a previous post, you can see sharper with small pupils than with large pupils, it is easier to read from a bright background.
A video of changes in pupil size in response to light.
In other words, the positive-polarity advantage is really a small-pupil advantage. This explanation is supported by studies showing that reading performance increases with display brightness, irrespective of polarity: What matters is not whether the text is darker or brighter than the background, but simply how bright the background is.
In summary, experiments show quite unambiguously that dark-on-bright text is preferable over bright-on-dark text. In addition, you should make sure that your workplace is sufficiently illuminated. Not only will this keep you awake, but it will also reduce the size of your pupils and thus improve your reading performance.
Finally, a short note on visual fatigue: Many people feel that it’s tiring to stare at a bright screen all day long, and therefore prefer negative-polarity displays. There may be some truth to this, but remember: The best way to counteract visual fatigue is by getting away from your screen from time to time!
Buchner, A., Mayr, S., & Brandt, M. (2009). The advantage of positive text-background polarity is due to high display luminance. Ergonomics, 52(7), 882–886. doi:10.1080/00140130802641635
Piepenbrock, C., Mayr, S., & Buchner, A. (2014). Smaller pupil size and better proofreading performance with positive than with negative polarity displays. Ergonomics. doi:10.1080/00140139.2014.948496