Apple Cinema Display Brightness no longer works in Mavericks or Yosemite So when you have a ~5-7 year old Apple Cinema Display monitor that looks something like this:



and you upgrade to Mavericks (MacOS 10.9) or Yosemite (MacOS 10.10), you will lose the ability to adjust the brightness on the thing via the "Display" Preference Pane. In other words, the brightness will be stuck at whatever level it was at when you were on Mountain Lion.

And this is because of a conscious decision by Apple to obsolete these things to motivate you to throw perfectly good monitors into the trash or recycling bin :-( (and buy new Apple products, but of course).

So, how to get your brightness controls back in Mavericks?
There are various threads on discussions.apple.com, but the solution I found on MacRumors I will not repost the solution there, since it's likely to be deleted since it's definitely a bit of a hack. 

But one that will extend the useful life of your Cinema Display for at least a few more months (if not a year or two): 


  I got the following .kext files from a computer running mountain lion.
  
  AppleBackligt.kext AppleBacklightExpert.kext AppleUSBDisplays.kext
  
  The files are placed in /System/Extensions
  
  Then i placed the kextfiles on the desktop, and ran a tool called
  "KextBeast". This app installs every kext file on the desktop. Then i
  rebooted and voilá..
  
  
  Get kext files
  Run KextBeast
  Reboot
  


"Running KextBeast" requires registering (i.e. potential spam) to download the KextBeast app.  

Here is how to do the same thing in the Terminal:

 # make certain you have the ability to run as root or admin (so you can "sudo")
 cd /System/Library/Extensions/

 # move the old kernel extensions out of the way
 sudo mv AppleBacklight.kext AppleBacklightOLD.kext
 sudo mv AppleBacklightExpert.kext AppleBacklightExpertOLD.kext 
 sudo mv AppleUSBDisplays.kext AppleUSBDisplaysOLD.kext

 # move the Mountain Lion kexts into position
 sudo mv ~/Desktop/AppleBacklight.kext .
 sudo mv ~/Desktop/AppleBacklightExpert.kext .
 sudo mv ~/Desktop/AppleUSBDisplays.kext .

 # set proper permissions and ownership
 sudo chown -R root:wheel AppleBacklight.kext
 sudo chmod -R 755 AppleBacklight.kext
 sudo chown -R root:wheel AppleBacklightExpert.kext
 sudo chmod -R 755 AppleBacklightExpert.kext
 sudo chown -R root:wheel AppleUSBDisplays.kext 
 sudo chmod -R 755 AppleUSBDisplays.kext

 # mv the OLD kernel extensions out of the way 
 # (or put them in the trash via the Finder)
 sudo mv *OLD* ~/Desktop


Reboot and voila!  You will have brightness controls with your "Vintage" Cinema Display.  

I hope this helps somebody else out.