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Model: MacBook Pro (Retina, 13-inch, Mid 2014) OS: 10.13.6

I connect an external monitor to the macbook, set the Arrangement to be Mirror Displays and then use the keyboard F1 to dim the built-in display to black. It seems it's off and even the back apple log is black (since it's using the same light as the monitor).

So, can I consider the built-in display it's off? (means it will not use any hardware/software resources such as CPU/RAM etc..). Thanks.

  • My guess is that the display is still using resources, just not the LED to light the screen. The info is still going to the screen, you just can't see it. – fsb Nov 30 '18 at 12:47
  • @fsb, yeah, you are right. the below answer shows a way to use Digital Color Meter to prove it's using resources. very smart. – sgon00 Nov 30 '18 at 14:18
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The display is still using resources. You can prove this by opening the Digital Colour Meter. Just hover your mouse to the built-in display which is currently dark. You will still see the content in the Digital Colour Meter on your second screen.

  • Yeah, you are right. I never knew Digital color meter before. This is so cool. It definitely proves the display is still using resources. Thanks a lot. – sgon00 Nov 30 '18 at 13:58
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    This test is inconclusive. You could do the same with an external VGA monitor (VGA to render the Mac unable know whether the monitor is actually turned on): connect the monitor, press its Power button to turn it off, try getting some colors from the part of desktop which is supposed to be on the external monitor: you'll still get some non-zero values. But in this case the monitor is off by construction of the experiment. – Ruslan Dec 1 '18 at 15:30
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Even the physical LCD is still active with the backlight off. On older laptops, you can shine a light (i.e. phone flashlight) through the apple on the back of the display and see a spot of the display, but the new ones are lame and don't have a glowing apple*.
* Sarcasm... I think

  • the new ones are lame and don't have a glowing apple* (*not sarcasm XD). Also the polarizing filters are never perfect.. with a strong enough flashlight, you can shine it at the front, and enough light will make it through to see that the LCD is still displaying an image. – Aaron Nov 30 '18 at 19:06
  • Yep, although my screen protector (privacy filter) makes that slightly harder, so the apple is easier. – Dev Nov 30 '18 at 20:51
  • Oh, the glowing apple is actually using the LCD backlight? That's smart! I always thought they just had another light in there controlled by software. +1! – SilverWolf Dec 1 '18 at 17:19
  • You'd have to make the display thicker for that, lol! I also think it's cool, sad the newer laptops don't have it though. – Dev Dec 1 '18 at 23:55
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No, the display can still be running and taking power even if the backlight is off.

The display has three major hardware components: a backlight, an array of colour filters, and an array of LCDs.

The backlight takes a lot of power, but is conceptually very simple: it just emits white light evenly across its area.  The colour filters are complex (with red, green, and blue areas for each pixel) but fixed and don't take any power. The LCD array is equally complex, but also takes power: it has to filter the light differently for each pixel, letting all the light through for white pixels, blocking it for black pixels, and in between for other colours and shades.

So when the display is running, the LCD cells are working and taking power, even if the backlight is providing little or no light.  (This also means that the graphics card and all the associated circuitry is running too.)

I can verify this from personal experience: I had the backlight die on a 2015 MacBook Pro.  At first, I thought the entire machine had died.  But after a while I thought of shining a torch through the apple-shaped translucent panel: and sure enough, it revealed a small patch of desktop background and the edge of a window — I could even see the pointer when I moved it across that area.  That indicated it was just the backlight that had died.

So no, the LCD array, graphics card, and all the other display-related hardware and software &c can still be running and taking power even without the backlight.

This probably applies across most or all types of LCD display.  (It's possible that some combinations of hardware and software may automatically shut down those things when you turn the brightness down to zero.  But they don't in my case.)

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The display itself still takes power (see gidde's answer).

Since you are mirroring, your Mac does relatively little extra work. It has to generate images anyway for the second monitor, and that's the biggest chunk of work the computer does, and is not duplicated.

The Mac then needs to send the contents of its video memory to two connectors - normally it would be one, but since you are mirroring, the same video memory is sent to two connectors. So some extra work is done, but not very much.

All in all, it will use few resources, but not none at all.

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