brightness and lock doesn't control brightness on external monitor

[fugounashi]

1) The release of Ubuntu you are using, via 'lsb_release -rd' or System -> About Ubuntu

Release: 16.04

2) The version of the package you are using, via 'apt-cache policy pkgname' or by checking in Software Center

unity-control-center:
  Installed: 15.04.0+16.04.20160413-0ubuntu1
  Candidate: 15.04.0+16.04.20160413-0ubuntu1
  Version table:
 *** 15.04.0+16.04.20160413-0ubuntu1 500
        500 http://jp.archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        100 /var/lib/dpkg/status

3) What you expected to happen

When moving the brightness slider the monitor brightness should change.

4) What happened instead

Nothing.

5) Other comments

This could be implemented using DDC/CI.

6) Workaround

https://askubuntu.com/questions/220886/control-external-monitor-brightness-via-software