(1) The light passing through each filter is detected by the photocell , where it is converted to an electrical pulse.(2) In the presence of a magnetic field, the amount of laser light that is absorbed by the atoms changes and this is detected by a photocell .(3) The receiver uses a photocell or photodiode to detect the light.(4) Single flies were loaded into glass tubes and each glass tube was clamped between the diodes of the photocells .(5) We used a battery of twenty photocells and lasers, enough to register an object as small a fly passing by.