Dark screens save energy!!

My friend Anisha just changed her blog background to white. Why did you do that Anne? Little does she realize what effects it has on the environment. Hence this post.

In the early days the computers displayed black text on top of a gray background. Now almost all display defaults are black text and white background. Why has such a trend been popular in the last 10 years? I don't know for sure but I'm guessing it is to make people feel more comfortable. Everyone is used to white standard paper, off-white newspaper, white background in magazines, most books, and so on. Printing black on white is just technically easier and cheaper.

Why do dark backgrounds matter? A minor reason is personal preference, which is what most people are thinking at this point. But that's just a small concern. I'll discuss the main reasons.

The first is your eyes. Yes, those things you are reading this with. Talking engineering, think of the background as "noise", and the text as a signal. Your eyes are trying to read the signal and ignore the noise. From an energy perspective let's simplify and say dark colors are low energy and light are high. So with a light background you are flooding your eyes with a high intensity wall of noise and trying to pick out the low intensity dark signal. With a dark background you instead change the important text to the high intensity signal. If that didn't make sense then think of it as a looking at the stars. At night your eyes open up and you can pick out the faint signals of a star from light years away. An then try to imagine trying to pick out black eagles flying high in the sky on a bright sunny day.

A second reason also associated with your eyes is eye strain. This can come from at least two sources. The first is the effort to pick out the weaker dark text signals as described above. The second is viewing such displays in dark environments. Try using a regular word processors with white background and black text in a pitch black room. Now try with a black background and white text. Your eyes try to adjust to both the room environment and to the screen you are looking at. To those that prefer or must work in bright environments this is less of a concern. For those who are in dark environments, it is almost impossible to handle the brightness without turning on a background room light. In some cases, such as with portable devices, this is not possible and the only option is turning down the display back light which lowers the contrast and readability between background and foreground significantly.

The third reason dark backgrounds are important is energy. It takes a lot of energy to make pixels bright. In general, a text interface is using many fewer pixels to display the important text information than the background. So a dark background is much more energy efficient. This is extremely important for battery powered displays in notebook computers, PDAs, mobile phones, digital cameras, and so on. Even on desktops the power required to drive a large display with a full white background is much higher than with a dark background. If you don't believe me, buy a cheap in line power meter and test it yourself. Lower energy will save you money, stop power companies from needing to supply you, and in the end, save the environment. You do love the earth don't you?

Google.com realized it a couple of years ago, and released a dark-screen version of itself called "Blackle". And just that could save the world a lot of energy. Lets do the math. an all white web page uses about 74 watts to display, while an all black page uses only 59 watts, a difference of 15 watts. Google.com gets about 200 million queries a day. Let's assume each query is displayed for about 10 seconds; that means Google is running for about 550,000 hours every day on some screen all over the world. On a CRT monitor, the shift to a black background will save a total of 15 watts. That turns into a global savings of 8.3 Megawatt-hours per day, or about 3000 Megawatt-hours a year. Not bad just for changing a few color codes, isn't it?