Ubuntu – How to temporarily disable the screen lock settings during video playback

Take a look at this AskUbuntu Question.

Basically, you should also disable screen locking in power manager settings

If you are not using power manager, you can try to comment out line LOCK_SCREEN=TRUE in /etc/default/acpi-support

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.