FWIW, iio-sensor-proxy gives you a user-accessible dbus API for ambient light sensors. GNOME has had a feature to auto-adjust screen brightness based on it for ages (though their implementation leave much to be desired and hasn't changed despite of several attempts to rework it over the years).

  $ monitor-sensor
      Waiting for iio-sensor-proxy to appear
  +++ iio-sensor-proxy appeared
  === Has accelerometer (orientation: normal)
  === Has ambient light sensor (value: 56.200000, unit: lux)
  === Has proximity sensor (near: 0)
      Light changed: 56.200000 (lux)
      Light changed: 55.100000 (lux)
      Light changed: 58.500000 (lux)
      Light changed: 48.800000 (lux)