How to turn off the backlight?

[Pierre-Luc Drouin]

Hi,

I got a new Dell laptop (Precision M70). I would like to know how I can 
turn off the backlight of the LCD. The backlight doesn't turn off when I 
flip down the screen or when asked by xscreensaver.

I've also tried the different sleep states without success. S1 and S2 
don't seem to be supported and I get "acpi0: device_suspend failed" when 
I try S3 or S4.

Thanks

FreeBSD 5.4-RELEASE #0: Thu May 19 23:21:03 EDT 2005
...
acpi0: <DELL CPi R  > on motherboard
acpi_timer0: <24-bit timer at 3.579545MHz> port 0x1008-0x100b on acpi0
cpu0: <ACPI CPU (4 Cx states)> on acpi0
acpi_throttle0: <ACPI CPU Throttling> on cpu0
acpi_acad0: <AC Adapter> on acpi0
acpi_cmbat0: <Control Method Battery> on acpi0
acpi_cmbat1: <Control Method Battery> on acpi0
acpi_lid0: <Control Method Lid Switch> on acpi0
acpi_button0: <Power Button> on acpi0
acpi_button1: <Sleep Button> on acpi0
pcib0: <ACPI Host-PCI bridge> port 0xcf8-0xcff on acpi0
acpi_tz0: <Thermal Zone> on acpi0
atkbdc0: <Keyboard controller (i8042)> port 0x66,0x62,0x64,0x60 irq 1 on 
acpi0
sio0: <16550A-compatible COM port> port 0x3f8-0x3ff irq 4 flags 0x10 on 
acpi0
...
nvidia0: <Quadro FX Go1400> mem 
0xde000000-0xdeffffff,0xc0000000-0xcfffffff,0xdd000000-0xddffffff irq 16 
at device 0.0 on pci1