How to loop through all the frames of the current composition?

If the lasar light is always in the same position then sampleImage() will work. If the position is random, then you're in trouble because the sample area would have to be much larger and the percentage of change is soint to be a much smaller percentage of all of the pixrls. As far as I know there is no way to measure any property you cannot animate with a keyframe with an expression. I don't know of any property that reads every pixel in a frame so you can look for specific values. The lumetri scopes will tell you when a group of pixels exceeds a certain value, but there is no way I know of to examine all pixels in a frame and figure out when some random small group of pixels reaches a specific value.