How do I change the backlight brightness on an iMac 12,2?

Question

Problem: I can't adjust the brightness of my iMac and it's killing me. (My eyes! The goggles do nothing!).

Has anyone gotten this to work?

-Booting with rEFIt 0.14

-Product Name: iMac12,2

-Kernel: 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

I've tried this fix: https://wiki.ubuntu.com/Kernel/AppleGmuxBacklight

But it doesn't seem to work since the kernel module doesn't seem to be loading. (dmesg | grep gmux returns nothing)

I've also tried doing this:

xrandr --output LVDS --brightness 0.3

and it dims the screen, but the mouse cursor still blazes away at full brightness.

Any thoughts? Thanks!

UPDATE:

Performing ls /sys/class/backlight/ returns acpi_video0

-The brightness slider in "System Settings" has no effect.

-The "brightness" function keys on my apple bluetooth keyboard don't have any effect.

Thanks!

Solution

Have you tried adjusting the acpi_video0 with a terminal?

To check max value you can use run:

cat /sys/class/backlight/acpi_video0

To check current brightness:

cat /sys/class/backlight/acpi_video0/brightness

To change brightness (input between 0 and your max_brightness):

echo YOUR_BRIGHTNESS_HERE | sudo tee /sys/class/backlight/acpi_video0/brightness

If this doesn't work perhaps your bootloader is messing something up or this video driver doesn't work on your machine.

Comments

[Admin]

Can you ls /sys/class/backlight/

[Admin]

If I perform that, I get: acpi_video0 That's it.

[Admin]

You are not using external monitor, right?