Screen dimming

[maizychicken at earthlink.net]

Is it normal for the ibook screen to go ever so slightly dimmer after about 15 seconds of inactivity?  My screen then goes completely black after about another 10-15 seconds.  It brightens immediately when I touch the trackpad, but it is a bit annoying to have it happen so soon.  I have checked my settings in the energy saver control panel, and as far as I can tell nothing should be happening with the screen in less than 10 minutes of inactivity.  

Thanks for help,
Maizy