Stop Screen Dimming After 60 Seconds (MacBook running 12.04)

I'm running Ubuntu 12.04 on a MacBook air - everything works fine, apart from the fact that the screen dims after 60 seconds of activity. Is there any way to stop this?

My computer is on mains power. If I stop using the machine, after 30 seconds the keyboard backlight turns off (that's fine). After a minute, the screen dims by about 50% - this is annoying, especially when watching a video or reading a long page of text.

My "brightness and lock" settings seem fine:

I've tried

gsettings set org.gnome.settings-daemon.plugins.power idle-dim-time 600

But that doesn't make a difference - it still dims after 60 seconds, no matter what value I set it to.

I've also used gconf-editor but I don't appear to have /apps/gnome-power-manager/backlight

I'm not using Unity - just the standard Gnome desktop.

Is there any way to change this behaviour? I never want my screen to dim. I'm quite happy controlling it manually with the F1 and F2 keys.

EDIT

No - Caffeine doesn't work. The screen still dims.

No - changing the DPMS doesn't work.

No - Editing stuff in dconf doesn't work.

No - I don't have any other power manager software installed.

You could try using caffeine as a temporary fix. You can install caffeine by running the following command in your terminal (Ctrl+Alt+T)

sudo apt-get install caffeine

To find a fix you could do the following things:

Run the following command in your terminal:

xset -q

If you see the energy saving settings enabled (DPMS is enabled) you'll need to disable it.

You can do that by running:

xset s off
xset s noblank
xset -dpms

This should stop the screen from dimming. xset -dpms doesn't survive a reboot so you can add the command to either /etc/rc.local or to ~/.xprofile.

If this doesn't work of xset -q shows you that the energy saving options are disabled comment and I'll try the next method I know

check /org/gnome/settings-daemon/plugins/power with dconf-editor to check that idle-dim-time is set to 600 and that the "active" checkbox is checked if this does not work uncheck the "active" checkbox.
if that does not fix your problem you probably have a alternative powersaving program installed and if you can discover what that other program is I might be able to help you.

If you don't have any other energy saving software installed could you check if a live dvd has the same issue? If it doesn't you are most definitely running some energy saving option that doesn't come out of the box. If it does have the same issue, there is only one thing I can think of that might work.

Installing xbacklight and then running xbacklight -set 7 and xbacklight -time 500 should work (OP has confirmed this)

sudo apt-get install xbacklight