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I had a Dell 21.5" monitor, which was previously connected to a Windows PC via its DVI port, which has no problem with DDC/CI which allowed me to control brightness and Contrast of the attached display via Windows.

My question is: through Apple's adapter, can I control a non-Apple display within System Preferences?

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For me, a similar setup does not enables me to control the brightness. –  Max Ried Aug 17 '12 at 9:09
@MaxRied wow, that's encouraging LOL –  Shane Hsu Aug 17 '12 at 9:13
Is DDC/CI listed as a feature on your Dell Display? –  MrDaniel Aug 17 '12 at 13:53
@MrDaniel Yes, it is listed as a feature. –  Shane Hsu Aug 17 '12 at 15:19

2 Answers 2

up vote 12 down vote accepted

I wish it were possible too, but the short answer is no, you can't use System Preferences to control the brightness of an external display via DDC/CI.

It might be feasible to write a program that can adjust the brightness of your external display, but it's by no means a sure thing. Read on if you want the gory details…

After doing a fair bit of research, I'm pretty sure that Apple doesn't use DDC/CI to control the brightness of displays, either internal or external.

  1. The brightness of Apple Cinema Displays can only be adjusted when the USB interface is plugged in. With DDC/CI this would not be necessary.

  2. Apple provides (though apparently does not document) an interface to display brightness through their IOKit library, specifically in ioGraphicsLib:

    …but this interface does not seem to work reliably for external displays:

    ( I read up on this a few years ago and while I can't now find a supporting link, I seem to recall that display brightness (at least for internal displays) may be delegated to Apple's System Management Controller (SMC), which has responsibility for all power-related functions: fan speed, monitoring battery level, etc. I wouldn't be surprised if Apple had implemented their own I²C control interface between the SMC and the internal display - but this is pure speculation on my part. )

  3. It is possible to communicate with/control external displays using DDC/CI and custom software, but support for this has been broken more than once in the OS X era. DDC/CI support is also dependent on the graphics chipset and drivers used.

    From http://update.necdisplay.com/spectraview/readmemac_v1_1_12_1.html:

    As of Mac OS 10.8.0, support for monitor connunications[sic] via DDC/CI is not functioning on some Macs, including systems with DVI video outputs, and when using a miniDisplayPort to DVI cable or adapter. NEC is working with Apple to fix this issue in a future Mac OS update.

  4. DDC/CI works over I²C. Apple's IOKit library includes an I²C interface which should make it possible to write a display control program, but there are a few reports that this does not work reliably (specifically, values can be written to the monitor but not read, so you can't tell what the current brightness is):

So that's about the size of it, as far as I can tell.

If you are (or you know) an Objective-C developer, I did find a project on GitHub that you could have a look at tinkering with – however as mentioned above, even assuming the code is OK, it might not work for your combination of monitor, Mac and display drivers.

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I appreciate the gory details. Thanks for doing so much organizing and link curation on this. –  bmike Nov 18 '12 at 22:36
Thank you so much! –  Shane Hsu Nov 23 '12 at 1:52
Thank you so much for the github project who worked amazingly well! I’ve looked this for years! I’ve got a Dell u2311h on a 2011 MacBook Pro with OS X 10.8.4. –  user53454 Jul 17 '13 at 20:06
@user53454 -- How did you get it to work? –  Ze'ev Feb 9 at 4:13

You can also try this app: https://github.com/superduper/BrightnessMenulet, though it allows to adjust brightness only on main display(one where menu bar appears).

I've tested it on my MacBook Pro + Dell U2412M, worked fine.

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Looks promising, but for the record nothing happens when I launch it on OSX 10.7.5 on MBP '12 with a HP ZR2440w over Mini Displayport. –  Erika Jan 14 '13 at 9:02
Doesn't do anything for me either. Doesn't even appear in menubar. 10.7.4. @Victor -- what OS are you running? –  Ze'ev Feb 20 '13 at 7:18
Unfortunately this froze up my machine (mini, 10.8.current, dell 24" monitor via displayport). –  Dan Pritts Jun 18 '13 at 3:41
This froze my Mac. No offense, but I suggest any future reader take extreme caution when trying this out. It's not malicious of course, but save your work before opening this. –  Shane Hsu Jan 12 '14 at 17:11
Any news on this? –  Ze'ev Feb 5 at 20:16

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