Automatic brightness script for use with ACPI Ambient Light sensor and xbrightness. Other sources of ambient brightness are also possible of course.

auto_brightness.sh

#!/bin/bash

ODB=$(xbacklight -get)

while :
do
  BR=`cat /sys/class/als/acpi_als0/illuminance`
  echo -n $BR
  [ $BR -lt 8 ] && BR=0
  DB=`echo "l($BR+1)*l($BR+1)" | bc -l`
  echo " - $DB"
  [ $DB != $ODB ] && xbacklight -steps `echo "sqrt(($DB-$ODB)^2)*2+1" | bc | cut -d. -f1` -time `echo "sqrt(($DB-$ODB)^2)*20" | bc | cut -d. -f1` -set $DB%
  sleep 0.5
  ODB=$DB
done