Figure 2.

Sequence of photolysis in various areas. The shaded area represents a mask that is in a plane equivalent to the image of the mask, a slit with a hole. When the experiment begins, light can only reach the sample through the circle. Once polymers have formed in this incubation circle, the mask is moved to the right, unblocking the illumination of the channel. After some preset time, the mask is again moved to the right, extinguishing the channel, and leaving a tiny region. This detection area is observed for a period to determine whether polymers have propagated this far while the channel was on.