Usually has an 800 frame roll of film.
When a vehicle exceeds a preset speed threshold, the device is activated taking two photographs half a second apart. The photographs show the date and time of the offence, together with the speed recorded. The unit takes between three and four hundred readings of a single vehicle as it passes through the beam. There must not be a greater speed variation on all of these readings of more than two miles per hour or the device will abort the test
These can also use the TruVelo system or the inductive loop system.