To filter 200 events from 40 million every second is the job of the trigger. It involves a three step filtering process, the level one trigger, the level two trigger and the event filter—each level conducting a finer analysis than the previous. Srini Rajagopalan: "The level one has to look at the 40 million pictures see which ones are interesting, and select about 75 thousand pictures. That is now given to this Level-2. The Level-2 gives about 2,000 of those pictures to the event filter. The event filter then takes those and writes out about 200 of those pictures." The level one trigger has 2.5 millionths of a second to decide whether to send an event to the next level. The detectors used to make this decision are the electromagnetic calorimeter and the muon spectrometer. Calorimeters can be thought of as thermometers but instead of measuring temperature, they measure the energy deposited by particles produced in a collision. The electromagnetic calorimeter is specifically designed to measure the energy of particles that interact via the electromagnetic force: electrons and photons.