Sorry, I meant is that package installed on your system? You can verify in Synaptic or please report what the following command shows in terminal:
Code:
dpkg-query -W -f '${status} ${package} ${version}\n' | grep xfce4-power
 
install ok installed xfce4-power-manager 1.2.0-3ubuntu4.1
install ok installed xfce4-power-manager-data 1.2.0-3ubuntu4.1
install ok installed xfce4-power-manager-plugins 1.2.0-3ubuntu4.1
 
I found something...When I right click on the task bar I clicked on "+add new items"...in that menu there is a brightness plugin....I added it to the taskbar but when I hover over the icon I get "no device found"...Dan
 
Did the xbacklight command that I posted yesterday work for you? Just trying to track down the issue.
 
I assigned keys ctrl+1 to xbacklight +10 and ctrl+2 to xbacklight -10.  Neither key combinations worked. As for the %'s I did not do that. I'd rather have a control working properly as it does in Xfce Mint 17.3 on this machine. Dan
 
I understand, however I am trying to diagnose if this is X server or hardware related. If you don't mind, please run xbacklight -set 50 to reduce the backlight. If it works, simply run xbacklight -set 100 to set it back to 100%. 
Please also post the results of this command:
Code:
for interface in /sys/class/backlight/*; do echo -e "\n $interface"; cat $interface/{brightness,max_brightness,actual_brightness}; done
 
I get this.....dan@dan-HP-G60-Notebook-PC:~$ xbacklight -set 50
No outputs have backlight property
dan@dan-HP-G60-Notebook-PC:~$ 
 
Interesting. What about the second command?
 
dan@dan-HP-G60-Notebook-PC:~$ for interface in /sys/class/backlight/*; do echo -e "\n $interface"; c
> 
 
Whoops, it doesn't look like you pasted the whole command. The easiest thing to do is to click on the Select link above the code to highlight the entire command. Then right-click and paste into the terminal.