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We are building a device based on a low-power CPU and we need to choose between a color back-lit LCD with a touchscreen and an "old-school" black and white LCD for the device. Obviously the user experience would be much better with the former, however, power consumption is a priority for us and in principle we can sacrifice user experience in case the black-white LCD saves us battery life considerably, i.e. for example 2 days vs. 5 days.

I'd appreciate if you could give me an insight into this. Am a software engineer myself.

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    \$\begingroup\$ Have you looked into OLED displays? Much lower power consumption than color LCD (no backlight), brilliant colors. \$\endgroup\$ – tcrosley Dec 17 '11 at 11:55
  • \$\begingroup\$ This sounds like the problem which Pixel Qi was invented to solve for 3rd-world-educational laptops, by having both color and low power monochrome modes. \$\endgroup\$ – Chris Stratton Dec 17 '11 at 17:29
  • \$\begingroup\$ I'd love to find an OLED display that wasn't a pain to use. I found one from OSRAM a couple years ago, and for all its 1.1" monochrome, it needed 3.3V, 5V, 9V, 12V, and 15V supplies, and the data sheet did not specify current. The demo unit I got had a PCB with just the power supplies that was bigger than the display. \$\endgroup\$ – Mike DeSimone Dec 17 '11 at 22:34
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By itself, an LCD being color or monochrome doesn't affect power consumption. Color LCDs are just monochrome LCDs with filters on the pixels to make them red, green, or blue. What will affect power consumption is:

  1. Is the display backlit, and if so, how bright? Even a small LED backlight drawing 10 mA can easily overshadow the rest of the power consumed in the display.
  2. How much computational effort are you using to keep the display refreshed? A high resolution graphical display may drive your system to have a faster processor, more memory, and higher clock speed than it would need for a simple character display. That increased performance comes with increased power consumption.

You have to look at the display options in the context of the system to make a determination of the ultimate effect on battery life.

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    \$\begingroup\$ Color LCDs have 3 or 4 times the pixels and need more backlight because those color filters (which are in addition to the polarizers) cost light amplitude. Therefore, I would be really surprised to see a color LCD use less power, all other factors being held equal. (Fun trick: to estimate how much light the backlight needs to illuminate the display, turn the backlight off and look at the powered-on display in sunlight. The darker it is, the more light you have to get from the backlight. This is one reason you don't see many sunlight-readable color LCDs.) \$\endgroup\$ – Mike DeSimone Dec 17 '11 at 22:38
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Based on your description, I assume you mean a passive black and white display without a back light or an active back-lit color LCD. In this case, power consumption will be order of magnitudes greater for the color LCD. Just think of how long a battery powered LCD watch can last on a small battery. Color LCDs require bright back-lights that draw significant power. Even if you want to back-light a monochrome display, you can use a much lower power back-light. The processing and memory power for a color display is also significantly greater.

But... do you need the display to be On constantly? You could use a monochrome (cheap as dirt) display for information that must be constantly displayed and provide a color display for a nice interface to menu systems, etc.

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