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Color affects the additional energy consumption of a GUI in several ways. Our first experiment measures the energy of iPAQ1 when the CPU is idle for one second with screens of different colors. Since each pixel consists of three color components, R, G, and B, we perform measurement for colors containing different combinations of these three components. We also perform measurements with the LCD turned off to obtain the energy consumption of the LCD only. Table VII summarizes the results under energy consumed by the LCD and the non-LCD energy. It also shows the percentage energy increase compared with pure white. R, G, B, and RG refer to red, green, blue, and orange, respectively. They have R, G, B, and R and G component(s) deactivated, respectively. ``Grey" refers to the color obtained when the corresponding originally activated component(s) is (are) half-activated. For example, when all three components are fully activated, the color is ``Full" black. When they are half-activated, the color is ``Grey" black.

Table VII: Energy breakdown for presenting screens of different colors
Color Non-LCD (EU) LCD (EU) % Increase
Black Full 36,100 18,700 3.3
Grey 36,100 19,100 5.5
Red Full 36,100 18,500 2.2
Grey 36,100 18,700 3.3
Green Full 36,100 18,500 2.2
Grey 36,100 18,700 3.3
Blue Full 36,100 18,600 2.8
Grey 36,100 18,700 3.3
RG 36,100 18,300 1.1
White 36,100 18,100 0

The energy difference disappears after the LCD is turned off, which demonstrates that it is the LCD that makes the difference. There are two observations one can make. First, the more color components activated, the more the energy consumption. In a reflective TFT LCD, when one color component is deactivated, the corresponding liquid crystals are fully unpolarized, and there is no need to repeatedly charge the polarizers. For instance, ``White" has all three color components deactivated and consumes the least energy, while ``Black" has all three activated and consumes more energy. This observation is the same as that made in [3] for a transmissive TFT LCD. More interestingly, the second observation is that a half-activated component consumes more energy than an activated component, as the ``Grey" ones consume more energy than their ``Full" counterparts. As mentioned in Section III, each color component has a TFT to charge a storage capacitor, which maintains the appropriate voltage between the polarizers. When a component is half-activated, the capacitor puts the TFT into a state that draws a higher amount of current, which contributes to higher energy consumption. On Zaurus, users can choose different colors for GUI themes. Table VIII gives the system energy for presenting the ``Application" window with different colors for the QPE theme for one second. It confirms the observation made in the iPAQ1 experiment. It also shows the percentage system energy increase when compared with that of the ``Bright" theme color.

Table VIII: Energy of different colors for the QPE theme on Zaurus
Theme color Energy (Joule) Over Bright(%)
Bright 0.416 0
Purple 0.417 0.4
Desert 0.422 1.5
Grey 0.423 1.6

next up previous
Next: Color sequence Up: Window properties Previous: Size
Lin Zhong 2003-10-13