Dimming the screen is controlling the backlight brightness. So, that was exactly the link I intended to post.
The level of brightness of the backlight can be adjusted from a range of "off" to very bright, depending on what the hardware supports (just because the fn+arrow doesn't let you turn it off doesn't mean the hardware doesn't support it).
An application created using the information at the MSDN page would work regardless of the brand of your display, and coded properly would be universally supported by all brands of displays, provided it actually supports the dimming levels desired. (which is more likely with a laptop display than a desktop display)
I am still hoping someone experienced in writing drivers and qualified to do this, will be able to use the information provided at the MSDN link and fill your request, as well as the previous request the blind guy made for a hotkey to turn his completely off, with a quality application that anyone could use to save battery power, and not specific to one brand of laptop or display.
I have discussed this request (and the other one) with various coders, and the possible things that could go wrong. They have all told me they would not feel comfortable enough playing around with this kind of stuff on their own laptop display to attempt to create an application like this. If they mess up in the process (and yes, coders do mess up all the time and see the worst of the bugs before you ever see an application), it would take far more than a simple reboot to fix it, as it would be controlling the hardware directly and not dependent on whether Windows was running or not. Whatever settings they change would survive a reboot. It would even survive formatting the hard drive. A worst case scenario mistake could mean a costly repair to the developer that is testing his work on his own hardware.