Color sensing (as well as detecting empty spaces) settled on IR-based sensors commonly used in line-following robots. You can watch the clock in action in the video embedded below just under the ...

RTX feels that advanced electro-optical/infrared (EO/IR) sensors are coming of age for detecting unmanned air vehicles, ...

[CCrome] admits it may well be the cheapest and worst IR camera available ... printer for the project anyway. Don’t expect the results you would get from a real thermal sensor.