After being pointed into light sensor things I have found myself here asking if anyone has some example code for use with dimming the screen of an android phone by using the ambient light around it.
This blog shows you how to get light sensor readings, http://android-coding.blogspot.com/2011/10/using-android-device-build-in-light.html
and this SO answer tells you how to adjust the brightness of the screen: Android 2.2: Adjusting screen brightness
Make sure to add any required permissions to your Android manifest for reading sensors.