A light-collection system often accompanies the basic LArTPC as a means of extracting more information from an event by scintillation light. It can also play an important role in triggering, because it collects scintillation light only nanoseconds after the particle passes through the detector. This is comparatively (on the order of 1000 times) shorter than the time taken by the freed electrons to drift to the wire planes, so it is often sufficient to demarcate the collection time of scintillation photons as a trigger time ("t"0) for an event. With this trigger time, one can then find electron drift times, which enables three-dimensional reconstruction of an event. While such systems are not the only means by which a LArTPC can identify a trigger time, they are necessary for studying phenomena like supernovae and proton decay, where the particles undergoing decay or interaction are not produced in a human-made accelerator and the timing of a beam of particles is therefore not known. Photomultiplier tubes, light guides, and silicon photomultipliers are examples of instruments used to collect this light. These are typically positioned just outside the drift volume.