Screens versus SEM grids for single pass measurements in SPS, LEP and LHC

When the transfer channels of the SPS and LEP were designed in the 70¹s and early 80¹s, it was foreseen to use screens to observe qualitatively the beam positions and shapes and Secondary Emission Grids and Split Foils to perform precision profile and position measurements. Foils covering the whole aperture were installed for measuring the beam intensities. The original screens were 1 mm thick, which blows up the low energy beams and limits the number of screens which can be used simultaneously. With the use of different screen materials, image acquisition hardware and processing software, the TV screens are now competing with the SEM Grids for precision measurements. The screens are simpler in construction, have a large spatial resolution, typically 105 points, and are more sensitive. The use of fast luminescent material and thin Optical Transition Radiation screens are extending the screen monitor field to bunch length and time structure measurements. Limitations of Secondary Emission Monitors and results obtained with screens with hadrons and leptons are reported. Future applications are considered.