Scrims are metal screens mounted in a metal frame. They are placed in front of a light to reduce light intensity.
Arri makes four different types of scrims.
Full single- this scrim covers the entire lightface and reduces the intensity of the entire light beam.
Full double - this scrim covers the whole face of the light. It has two metal screens in it, reducing light intensity by two times the amount of the full single.
Half single - this scrim only covers half of the lightface reducing half of the light beams intensity.
Half double - same as the half single but reduces light intensity twice as much as the half single.