Beam Gas Ionization monitors (BGI), called also Ionization Profile Monitors (IPM) are devices which measure the transverse size of the beam by detecting position of products of rest gas ionization.
The figure explains the working principle of LHC and SPS BGI. Electrons from ionized gas are extracted towards a multi-channel plate (MCP) by electric field.
They movement is confined by external magnetic field, which reduces transverse spread of the trajectories (electrons are spiraling).
On the MCP they are multiplied and then converted to light on the phosphor screen. The light is extracted from vacuum through an optical system and registered by a camera.
A projection of the registered image gives the beam size in one plane.
This page is for Beam Gas Ionization monitors at CERN, in particular for SPS and LHC models. It contains information about hardware elements, controls, software, physics etc.
For the project of PS BGI go to https://twiki.cern.ch/twiki/bin/viewauth/PSBGI/WebHome