Alejandro Bonilla Beeche wrote:
On Tue, 2005-08-16 at 23:44 -0400, Steven Rostedt wrote:
On Wed, 2005-08-17 at 13:29 +1000, Benjamin Herrenschmidt wrote:
On Wed, 2005-08-17 at 11:41 +1000, Benjamin Herrenschmidt wrote:
(I'm blind and I use a braille display. I use those functions to blank
my laptop's screen so people don't read it, and hopefully to conserve
power.)
At the OLS I learned that the backlight of a laptop (when the screen is
black, but still glows) actually spends more wattage than when the
screen is lit. So, unless you actually turn the laptop display off,
switching it to black will actually burn the battery quicker.
This sounds stupid. Who told you this? The actual brightness is the one
that consumes the most battery.
The backlight itself consume the same power wether the pixels are
transparent (white) or opaque (black). The pixels themselves consume
more power when black in some kinds of screens - regardless of what
the backlight is doing.
So a black screen may waste power - because the backlight is still on
behind those black pixels. Such a screen also get marginally hotter,
because wasted light from the backlight gets absorbed in black
pixels, but radiated into the room through white pixels.
If you want to save power:
1. turn the backlight off, if possible. If there is no other way,
close the lid and use an external keyboard. Or jam the lid switch with
something. A good laptop should definitely have a way to turn it
off though,
for example if the user leaves the machine alone for a long while with
the lid up.
2. For privacy (and power saving) blank all the pixels to white instead of
black - _if_ it is a tft display. There are of course designs where
the
white pixels use more power (all cases where light is produced in the
pixels themselves, instead of relying on backlight.) So there is no
universal solution.
Helge Hafting
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