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I have two laptops, one Macbook and one Lenovo. If I press the Brightness Down button repeatedly, eventually the backlight is turned off completely. The Lenovo also has a shortcut key to turn off the backlight (Fn+F9).

Without the backlight, both monitors of the laptops are completely invisible even in a very dark room. So, I thought the monitors were turned off and laptop manufactures had put this feature to conserve energy by turning off the monitor when not needed like when listening to music.

But when I placed a flashlight near the screens, I could see the screen on both laptops. So, the LCDs were actually turned on but only the backlights were turned off.

Surely, no user would try to see the screen this way, and surely LCD would consume some power if it still animates pixels even without the backlight. So, I am wondering why the manufactures did not completely turn off the screen but just turned off the backlight instead? What advantages are there to keep the LCD on when there is no backlight?

When I used an application to turn off the monitor on the Lenovo, the LCD was completely turned off and I could not see anything on the screen even with a flashlight.

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    That's a question I have often thought of - although when disconnected and reconnecting a physical laptop screen from it's chassis, it can occasionally completely garble the output, unlike vga cabled screens, which carry on from where they left off... – JohnnyVegas Nov 20 '15 at 23:50
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    Simple.. it is the backlight that hogs most of the power; running the lcd itself takes hardly anything, so you would not gain much by shutting it off. – psusi Nov 21 '15 at 0:32
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Typical laptop LCD (without backlight) uses really low power (maybe 0.1W?).

If you turn off LCD completely you will see latency after turning on. Graphics card and operating system has to reinitialize everything.

I guess that reinitialization could use more energy than you could save by turning off LCD completely for a while and this may be the reason.

  • I don't think reinitialization would use that much energy. But it would be very intrusive to the user. Users want an "instant on" behavior after the backlight turns off. – Jamie Hanrahan Oct 7 '18 at 14:52

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