Have you noticed that many business intelligence (BI) software companies have introduced black screens as the standard for mobile devices? Is this because mobile devices work better with black screens? If you look for the research, as I have, it isn’t likely that you’ll find any. Just as when dashboards were new and someone came up with the bright idea of using graphs that looked like speedometers and fuel gauges on cars and everyone else followed suit, the practice of black screens on mobile devices has been adopted for the same reason: someone did it and others followed.
Few software vendors in the BI space do research or even read relevant research by others. A software developer does something on a whim and others copy it. Vendors find this approach faster and cheaper than research. The results, however, are costly to those who use the software.
I recently gave a keynote presentation at one of Actuate’s events and also led a separate smaller session to discuss data visualization best practices. During the smaller session, the issue of black screens on mobile devices arose—a practice that Actuate emulates. A fellow from Actuate made a passionate case for black screens by taking a bright flashlight, shining it directly into his face, and proclaiming, “This is what happens when you use a white screen on a mobile device.” His point was dramatic, but erroneous. The amount of light that is emitted from mobile devices with white screens is no greater than that emitted by laptop or desktop screens. It is not like staring into blazing light.
The same vendors that advocate black screens for mobile devices when graphics are involved, such as in a dashboard display, conversely advocate white screens for applications that involve reading, such as e-books. Illustrated below, RoamBI uses a black background for its analytics applications and a white background for its reports application.
No one seems to question the efficacy of light backgrounds for reading text. Why the difference? Text and graphics both involve objects that are constructed of lines and filled in areas of color. Do they differ in a way that demands a different background color? I don’t think so.
If you’ve worked with computers as long as I have, you probably remember that the original CRT displays used black backgrounds (i.e., the absence of light) and projected green phosphor pixels to form text. I remember the first time that I saw a screen that didn’t render text in green but did so in orange, which was a delight. Those old screens were hard on the eyes. As display technologies improved, light backgrounds became the norm for most applications. As it turns out, there was a reason for this. White or slightly off-white screens provide a better background against which information, whether in the form of text or graphics, stands out clearly and is easy on the eyes.
When I’ve asked vendors and others about the emergence of black screens on mobile devices, I’ve encountered the following arguments for their use:
- Black is the absence of light, so a screen with a black background uses less energy, which extends the time that a mobile device can run without having to recharge the battery
- Mobile devices are used in a broader range of lighting conditions, including direct sunlight, and black screens are less reflective than white and thus easier to read in direct light
- White screens emit too much light and are therefore hard on the eyes (the explanation provided by the fellow from Actuate)
- Bright colors on black screens look cool
- Everyone else is doing this, so there must be a reason
If we’re interested in effectiveness—the users ability to see information as clearly as possible—the last three arguments above can be dismissed without further consideration. The first argument, regarding the preservation of the battery’s charge, sounds plausible, but it might be based on a misunderstanding of the display technology. When I have questions about uses of color in data visualization or about display technologies, I usually consult my friend and colleague Maureen Stone, the author of A Field Guide to Digital Color and long-time researcher in the field of information visualization. When I asked Maureen about the battery life issue, she said the following:
On an LCD display, the pixels fundamentally act as shutters for the backlight, which is on all the time whether the screen is black or white. If you want to save power, turn down the brightness.
Even if some forms of mobile displays operate in a way that preserves battery life when the screen is black, does it make sense to sacrifice usability for the sake of a little extra time between charges? Clearly, makers of e-book readers wouldn’t consider this a worthwhile compromise.
This leaves us with argument number 2 above regarding lighting conditions. To test this informally, I took my new iPad outdoors to view it in different lighting conditions, including direct sunlight, and didn’t find that a black background was less annoyingly reflective. In fact, the opposite appeared to be true. The black background acts like a mirror, providing a surface that is reflective enough for checking my teeth for stray flecks of green after a meal. I didn’t want to rely on informal observation alone, however, so I consulted Maureen about this. Here’s her response:
The glass is reflecting the ambient light. It doesn’t matter what the screen is emitting. The value that reaches your eyes is the sum of the two. The variation in the reflected light is much more obvious and distracting on a black screen than a light one because the contrast between the images created by the reflected light and the background is less when the background is white.
What Maureen described in the last sentence is the mirror-effect that I noticed when I informally observed the glare on my iPad.
As an expert in this field, Maureen shared a few general thoughts on the topic as well:
In general, I recommend a light or white background because that gives your visual system a constant place to focus and also controls your white adaptation. A fundamental part of color vision is perceiving colors relative to the current “white.” If you have colored text floating on a black background, it can feel less stable and in theory, you can get some differences in color perception as your adaptation shifts. You can also create focus problems: An extreme case is intense red letters on black vs. intense blue ones on black. You can’t really get both in focus at once because of chromatic aberration in the eye’s optical system. This is easy to create on a CRT, less so on an LCD.
Another way to describe it is that a white background gives you more a feel of a constantly colored surface with text and figures on it, more like reading off of paper, and our visual system is more designed for this sort of viewing. I generally use this as the basis for my recommendation to use a light background.
Maureen did mention two situations when a dark background makes sense.
However, if you are using a display in a dark environment, it’s better to use a dark background as it lets you keep your eyes dark adapted. That’s why controls for airplanes and GPS units for cars switch to a dark background at night. Usually, however, the results don’t look like symbols and text floating in the darkness of space…there’s still a sense of there being a dark surface to ground the view. So the concerns above are somewhat mitigated.
I’ve seen recommendations that white on black is better for aging eyes, and for people with low vision because it reduces the amount of light scattering and distorting the image.
She completed her thoughts with the following:
Modern displays are now bright enough that they can be uncomfortably bright. But that effect will be seen in a dark room, not in daylight. Our visual system overall adapts (adjusts its sensitivity) to the ambient brightness by several orders of magnitude. That’s how we see in a dim room and in full sunlight, and that adjustment is the reason you are momentarily blind when you walk into a dark theater after being out of doors, or painfully squint on the way out. If your display is bright enough to be significantly brighter than anything else around it, then you’ll find it uncomfortable. But we cover this by saying you should use a dark background when in the dark. And turning down the brightness remains an option for leaving the background white. Many mobile devices automatically adjust the brightness as a function of the ambient light…brighter for brighter rooms, darker for darker rooms. Saves both battery and your eyes.
There might be more to it than this. Perhaps other conditions besides those that Maureen described can benefit from dark screens, but we should determine this through research rather than following the latest fad. Visual perception can be tested using the methods of hard science. Put your trust in best practices that are based on scientific evidence. Until evidence exists, trust your own eyes. There’s a lot that you can tell just be opening them.