brightness applescript

so as of 10.5.7 for some reason i cant get this script to work

any input?

property brightness_level : 0.5

tell application "System Preferences"
activate
set current pane to pane "com.apple.preference.displays"
tell application "System Events"
set value of slider 1 of group 2 of tab group 1 of window "iMac" of process "System Preferences" to brightness_level
end tell
quit
end tell