Exhilaration station

While poking around for a controllable LED on my computer¹, I saw the entries:

/sys/class/backlight/acpi_video0/
/sys/class/backlight/intel_backlight/

I wonder what’s in those…

actual_brightness
bl_power
brightness
max_brightness
power
type
uevent

Oh! Could controlling the LCD backlight be that simple?

I read the value stored in brightness under both the acpi_video0 and intel_backlight folders, while changing the backlight brightness using the keys on the keyboard. Turns out, it is that simple - the values in these files changes with the brightness of the display backlight.

acpi_video0/brightness holds values from 0 (dimmest), to 15 (brightest).

At the same time:

intel_backlight/brightness varied from 47 (dimmest), to 3000 (brightest).

47.

Hmm. 47 is not 0.

Could I turn it off completely? And is intel_backlight/brightness my lever?

I tested writing to the file.

Wrote 3000 to the file, all good… 2000, fine, 1000, getting dimmer, 100, 80, 50, getting low².

Poking the dragon

Uncautiously proceeding, (with caution):

It would be pretty foolish if you locked yourself out from a perfectly functioning latop because you simply turned the backlight off and couldn’t see anymore. Buuut I need to know, and researching an answer hardly sounds entertaining…

40, 30, 20. Yup, it can go below 47. 10? Yup. 5, 4…

The tension kept building, so I made a quick script to venture further; Set it as low as I dared, then right back up (twice).

#!/bin/bash
echo 3 | sudo tee /sys/class/backlight/intel_backlight/brightness
sleep 3
echo 2 | sudo tee /sys/class/backlight/intel_backlight/brightness
sleep 3
echo 1 | sudo tee /sys/class/backlight/intel_backlight/brightness
sleep 3
echo 0 | sudo tee /sys/class/backlight/intel_backlight/brightness # AHHHH!
sleep 3
echo 3000 | sudo tee /sys/class/backlight/intel_backlight/brightness # Phew
sleep 1
echo 3000 | sudo tee /sys/class/backlight/intel_backlight/brightness

3, 2, 1, 0 (oh snap)… and it came back!

walks away whistling while the laptop screen strobes frantically.