Don't have a light sensor? Use VirtualLightSensor.exe from the SDK!
What is the Virtual Light Sensor tool? This tool allows the user to simulate an ALS (Ambient Light Sensor) device. The main purposes for simulating ALS devices would be to either experiment with the new Windows 7 Adaptive Brightness feature, or to test sensor applications that implement light-awareness for readability optimizations, or use ALS in some other way (night/day themes, etc).
This tool is included in the Windows 7 SDK, so once you install the SDK, you'll have this tool.
Fore more detail and install directions, see: