Problem controlling brightness

[Liyu,She]

Hello folks,
    I am using FreeBSD 7.0 Beta3 on Acer Aspire 4710Z.But soon after the
boot menu I completely lose control of display brightness. I cannot control
the brightness under both console and x.
    I also tried to modify the keyboard section into this(I can't find a
exact match so I chose acer_tm_800 & airkey & acer_300 & acer_ferrari4k
instead. None of them worked.)

Section "InputDevice"
    Identifier  "Keyboard0"
    Driver      "kbd"
    Option      "AutoRepeat"    "200 50"
    Option "XkbRules"   "xorg"
    Option "XkbModel"   "acer_tm_800"
    Option "XkbLayout"  "us"
EndSection

    Here's the keyboard layout:
             http://www.xiyuit.com/picture/article/7/6685.jpg

    I did a lot of search but none of them works. I don't get it. The
function keys are working fine before boot menu.
    Could anybody please tell me how to set the brightness? Thank you very
much.

        Liyu