Are you asking for some sort of photodetector? What do you mean by "touching stroke."
Why I ask is that one could have a photodetector embedded in an object that would respond whenever a light beam hit it. In an alternative, one could put a photodetector near the source of light and detect when the light reflects off of an object in the vicinity. Yet another alternative would detect breaking of a light beam.
Rather than trying to address all of the options, it would be best if you described a little more what you mean and are trying to do.
John